Red Hole Gamma-Ray Bursts A New Gravitational Collapse Paradigm Explains the Peak Energy Di
在海拔5000米以上地区利用单粒子方法探测γ暴实验构想--基于水切伦科夫技术
在海拔5000米以上地区利用单粒子方法探测γ暴实验构想--基于水切伦科夫技术刘茂元;厉海金;扎西桑珠;周毅【摘要】Ground extensive air shower experiment is powerless for detecting cosmic ray particles of tens GeV en⁃ergy renge in the GRBs (Gamma Ray Burst) so far, because of its threshold energy. The experimental altitude needs to be increased in order to achieve more effective observation. In the present paper, setting up a water Che⁃renkov detector array at 5200m altitude in Tibet was proposed and the idea of ground experiments on multi-GRB and tens of GeV photon observing can be achieved by using single-particle technology, and also can supportpre⁃dicting for large-scale experiments.%目前，对于伽玛射线暴（Gamma Ray Burst, GRB）的探测，地面广延大气簇射实验由于阈能原因，对几十GeV能区的宇宙线粒子探测无能为力，只有提高实验海拔才能实现更有效的观测。
文章描述了在海拔5000m以上地区建造水切伦科夫(WCD)探测器阵列，利用单粒子技术，来实现地面实验多GRB几十GeV光子的正观测设想，为大规模实验提供预言支持。
小学上册第十次英语第1单元真题试卷(有答案)
小学上册英语第1单元真题试卷(有答案)英语试题一、综合题(本题有100小题，每小题1分，共100分.每小题不选、错误，均不给分)1.My brother loves __________ (学习新技能).2.The process of converting liquid to vapor is called ______.3.I have a toy _______ that can race with my friends.4. A goldfish can live for many ________________ (年) if properly cared for.5.The transition from a solid to a liquid is called _______.6.What do we call a device that takes pictures?A. CameraB. ProjectorC. TelevisionD. Computer答案:A7.The cat is ________ (懒洋洋的).8.The process of extracting copper from ore is called _______.9.What is the capital of the UK?A. LondonB. ParisC. BerlinD. Madrid答案: A10.The bear catches _______ in the river.11.I have a collection of ________.12.What is the term for a baby pig?A. CalfB. PigletC. ChickD. Lamb答案:B13.When I help others, I feel ______ (满足). It’s important to be kind and ______ (乐于助人).14.This boy, ______ (这个男孩), enjoys playing chess with friends.15.What do you call the center of an atom?A. ElectronB. NeutronC. ProtonD. Nucleus答案:D16.What is the name of the famous explorer who sailed the ocean blue in 1492?A. Christopher ColumbusB. Ferdinand MagellanC. Vasco da GamaD. Marco Polo答案: A17.The sunset is ______ and beautiful. (colorful)18.The peacock spreads its _______ (羽毛) to attract mates.19.The train is _______ (准时到达).20.How many sides does a square have?A. ThreeB. FourC. FiveD. Six答案:B21.What do you call the process of a solid becoming a liquid?A. FreezingB. MeltingC. EvaporatingD. Condensing答案: B22.The _____ (飞盘) is great for playing outside.23.I like to decorate my ________ (玩具名) with stickers and paint.24. A _______ (小蜈蚣) is often found in gardens.25.根据文意和图示选单词,并将单词规范地书写在四线格内。
What Gamma-Ray Bursts Explode Into
a r X i v :0706.0461v 1 [a s t r o -p h ] 4 J u n 2007What Gamma-Ray Bursts Explode IntoRoger A.ChevalierDept.of Astronomy,University of Virginia,P.O.Box 400325,Charlottesville,VA22904,USA1IntroductionObservations of the long duration gamma-ray bursts (GRBs),which are the primary topic here,suggest that they are associated with the deaths of massive stars.One lineFruchter et al.(2006)found that the positions of GRBs on galaxies are more concentrated to the brightest pixels than are core collapse super-novae,suggesting that the GRBs are associated with more massive stars than are most core collapse super-novae.These properties imply that the pro-genitors of most explosions are Wolf-Rayet(WR)stars.Expectations for the surroundings of WR stars are discussed in Section2.Normal SNe Ib/c(without a GRB connection) are also expected to interact with the surroundings of WR stars.The prop-erties of the interaction,as observed at radio and X-ray wavelengths,can provide insight into the GRB case (Section3).The SNe Ib/c that are as-sociated with nearby,low luminosity GRBs distinguish themselves from the normal SNe Ib/c and are dis-cussed in Section4.Afterglow emis-sion gives us a prime method of de-termining the surrounding medium and has been used since the discov-ery of afterglows to infer the density. This issue is examined in the light of recent observations in Section5.A frequent deduction from the analysis of afterglows is that the surroundings have a constant density.In this pa-per,GRB is used to refer to the long duration GRBs.The short bursts are discussed in relation to the long bursts in Section 6.Possible ways of producing a constant density sur-roundings are discussed in Section 7.Absorption lines in optical spec-tra provide another possible window on the immediate surrounding of GRBs and are treated in Section8. The various issues related to the sur-roundings of GRBs are summarized in Section9.2The Surroundings of Wolf-Rayet StarsIn this section,I consider the sur-rounding medium created by the free wind from a WR star,as it is likely to provide the immediate environment for the GRB.At some point the wind is expected to transition to a region that results from interaction with the surroundings;this possibility will be considered in Section7.Typical parameters for a WR star wind are a mass loss rate˙M=10−5M⊙yr−1 and a wind velocity v w=103km s−1.The wind densityρw=Ar−2, where A=˙M/4πv w,is the crit-ical parameter for a high veloc-ity interaction and I characterize it by A∗=A/5×1011g cm−1= (˙M/10−5M⊙yr−1)(103km s−1/v w). For Galactic stars,Nugis&Lamers (2000)listed mass loss parameters for64WR stars,yielding an A∗range of0.07to7.4.The lowest density winds are produced by WO stars, because of their high values of v w, up to5500km s−1.These results are based on Nugis et al.(1998),who determined clumping-correcting ra-dio mass loss rates,noting that the effect of clumping is small at the stellar surface,grows to a maximum at∼5−10R∗,and again becomes small in the outer wind because of the expansion of clumps at the local sound speed.A difference of the progenitors of GRBs with Galactic WR stars isthat the GRB progenitors probablyhave lower metallicity.Modjaz et al. (2007)found that the nearby GRB/SNeare in regions that are systematically more metal poor than the regionscontaining core collapse supernovae.Metallicities were determined from emission line regions close to the ex-plosions.The metallicities could be afactor of∼6smaller than solar.For more distant GRBs,it is not clearwhether the GRBs have lower metal-licities compared to other galaxies ata similar redshift z,but metallicitiesZ∼0.05−0.5Z⊙are indicated.In recent years,it has been found thatheavy elements around the Fe peak play a role in driving the winds fromWR stars,so that their mass lossrates are Z dependent.In this metal-licity range,mass loss rates from WCstars vary as∼Z−0.6(Crowther,2006),suggesting that values of˙M for GRB progenitor stars are lowerthan the rates for Galactic WR stars by a factor of2−3,and the valuesof A∗are lower by a similar factor. Another issue is the possible asym-metry of the stellar wind.Polar-ization studies of Galactic WR stars have generally shown an un-detectable amount of polarization, although∼20%show a polarization >∼0.3%that can be interpreted as a density contrast of a factor of a 2–3(Harries et al.,1998).However,asymmetry could be a significant fac-tor for the small percentage of WRstars that become GRBs.A plau-sible distinguishing feature of the GRB WR stars is rapid rotation,so that the central core is rapidly rotat-ing.For radiation driven winds,the higher gravitational acceleration on the polar axis can lead to a higher radiativeflux and mass loss rate on this axis,although a lower tempera-ture and higher opacity on the equa-tor favors equatorial mass loss(e.g., Maeder,2002);the density contrast from pole to equator can be a factor of a few.Meynet&Maeder(2007) have suggested that higher mass loss along the polar axis is needed so that the WR progenitor does not lose too much angular momentum through its wind.Overall,the additional effects to con-sider for GRB progenitors compared to Galactic WR stars do not have a substantial effect on the expected wind density when compared to the large possible range of densities.3Circumstellar Interaction of Normal Type Ib/c Super-novaeThe interaction of normal SNe Ib/c with their surroundings provides an interesting case of comparison for the GRB case because the driving force is generally understood for super-novae and there is synchrotron emis-sion resulting from the interaction as in the the GRB case.In a normal su-pernova,the supernova shock wave accelerates through the steep density profile at the outer edge of the star; the acceleration stops when radia-tion can stream freely from the star. The radiation accelerates the outer gas and the radiation dominated shock front disappears.The shock wave re-forms as a viscous shock in the surrounding stellar wind.Thereis a shocked region bounded by a reverse shock on the inside and a forward shock on the outside.Thereverse shock is in the steep outer power law portion of the supernova density profile.The shock fronts are plausible sites of particle accelera-tion;however,the material entering the reverse shock front has a very low magneticfield because of the su-pernova expansion,so there is some question of the efficiency of particle acceleration at that site.The deceleration of the ejecta by the surrounding medium gives rise to a Rayleigh-Taylor instability and a turbulent region in the shocked layer.The magneticfield can be built up in this region,although it is not clear that the efficiencyis high.In numerical simulations, Jun&Norman(1996)found that thefield is strongest on the smallest scales.The energy density in thefield was limited to∼0.3%of the turbu-lent energy density,but the result was limited by the numerical reso-lution.Another possible source of magnetic amplification is related to instabilities in the collisionless shock waves(e.g.,Bell,2004).The basic hydrodynamic model of steep power law ejecta driving an interaction shell into a surrounding stellar wind can reproduce the ob-served radio emission from SNe Ib/c if some fraction of the postshock energy density goes into relativistic particles and magneticfields,and synchrotron self-absorption(SSA)is important at early times(Chevalier, 1998;Chevalier&Fransson,2006). In this situation,the peakflux of Fig.1.Peak radio luminosity and cor-responding age for well-observed core collapse supernovae.The dashed lines give curves of constant expansion veloc-ity,assuming synchrotron self-absorp-tion at early times(updated version of Fig.4in Chevalier(1998)).the radio emission gives information on the radius,and thus the velocity, of the radio emitting region.Figure 1shows observed peakfluxes and ages of radio supernovae.The dashed lines give the velocities of the radio emitting regions if the peak is due to SSA.If another absorption process, such as free-free absorption,is dom-inant,the velocity inferred from the turn-on is lower than the actual ve-locity.In Fig.1,it can be seen that the SNe II have lower inferred veloc-ities,which is both because of the relatively low velocities in SNe II and the importance of free-free absorp-tion.There is a large range in peak luminosity for SNe II,which can be primarily attributed to a range in cir-cumstellar density.Multiwavelength observations confirm the large range in density.The normal SNe Ib/c show systemat-ically higher velocities than the SNe II(Fig.1),which can be attributed to some combination of higher peak velocities at the time of shock break-out,higher mean ejecta velocities because of lower ejecta mass,and lower deceleration because of lower circumstellar density.There is again a large range in peak luminosity; this may be due to a range in cir-cumstellar density,but in this case there is no independent evidence for a range in density.If the super-novae have similar efficiencies for the production of synchrotron radiation and a range of circumstellar density that is comparable to that around Galactic WR stars,the observed lu-minosity range can approximately be reproduced ifǫB≈ǫe≈0.1 (Chevalier&Fransson,2006).The required magneticfield is high and cannot be produced by compres-sion of the wind magneticfield(this would require a wind energyflux that was completely dominated by the magneticfield).There are a number of X-ray obser-vations of SNe Ib/c,but the data are much less extensive than at radio wavelengths(Chevalier&Fransson, 2006,and references therein).The observed luminosities are higher than expected from thermal emis-sion from interaction with a normal WR star wind,so that a nonther-mal mechanism is indicated.Near maximum light,inverse Compton scattering of photospheric photons with relativistic electrons is a possi-bility(Bj¨o rnsson&Fransson,2004; Chevalier&Fransson,2006).At later times,inverse Compton emis-sion fades because of the low super-nova luminosity,and synchrotron emission is the most plausible non-thermal mechanism.However,an ex-trapolation of the radio synchrotron emission falls below the observed X-ray emission,especially when synchrotron cooling of the radiat-ing electrons is taken into account. Chevalier&Fransson(2006)sug-gested a model of particle accelera-tion in a cosmic ray dominated shock front so that the particle spectrum flattens to high energy.At low en-ergy,the particle spectrum is rela-tively steep,with energy index p≈3, in accord with radio observations of SNe Ib/c.At high energies,the spec-trum becomesflat.This spectrum results in fairlyflat evolution of the X-ray emission,while the radio emis-sion decreases.4Low luminosity,nearby GRB-SNeFigure1shows that the3low lumi-nosity GRB/SN events have higher velocities of the radio emitting re-gions than the normal SNe Ib/c. Their positions in thefigure suggest semi-relativistic velocities.The high luminosity nearby event GRB030329 had a5GHz luminosity of5×1030 erg s−1Hz−1on day10(Berger et al., 2003),indicating highly relativistic motion in this case.For the lower velocity cases,application of the synchrotron theory described by Chevalier&Fransson(2006)yields mass loss densities A∗(ǫB/0.1)of 0.1(SN1998bw),1.6(SN2003lw),and0.02(SN2006aj).The theory is nonrelativistic,but should still yield approximate results consider-ing that these objects indicate only semi-relativistic motion.The density inferred for SN2006aj is low be-cause of the early turn-on(see also Waxman et al.,2007).An important issue for the3low lu-minosity events is whether some of the observed phenomena can be ex-plained by the supernova,or whether a central engine is needed,as in the case of normal GRBs.A supernova explanation means that emission as-sociated with the interaction of the fast,outer supernova ejecta can ex-plain the observations.Expectations for the supernova case depend on the ejecta mass and energy for the explo-sions.Optical observations of the3 supernovae were extensive and there are results on the supernova parame-ters:10M⊙and50×1051ergs for SN 1998bw,13M⊙and60×1051ergs for SN2003lw,and2M⊙and2×1051 ergs for SN2006aj(Mazzali et al., 2006a,b).The supernova properties of SN2006aj were closer to the nor-mal Ic SN2002ap than to the bright SN1998bw,and its inferred mass and energy would be incapable of producing the high velocity inferred from the radio emission.The impli-cation is that the radio emission is related to a central engine.The radio observations of SN2002ap suggest a low wind density in this case(Fig.1),which appears to also apply to SN2006aj.For SN1998bw and SN 2003lw,the large supernova energy allows the possibility of a super-nova origin for the radio emission. Tan et al.(2001)have discussed such a model for SN1998bw.SN2006aj showed a thermal X-ray component over thefirst few1000sec that has been interpreted by Campana et al.(2006)and Waxman et al. (2007)as shock breakout emis-sion.The temperature was constantat∼0.17keV during this time. The radiated energy in the ther-mal component was∼2×1049ergs (Campana et al.,2006;Li,2007). This is several orders of magnitude larger than would be expected from shock breakout from a WR star,as-suming no effect of the WR star wind (Matzner&McKee,1999);in addi-tion,the duration of the shock break-out emission in this case would be de-termined by light travel time effects, yielding a timescale∼10s,much less than observed.Campana et al. (2006)addressed this issue by con-sidering the progenitor star to be surrounded by a dense WR star wind,with A∗≈20.This wind den-sity conflicts with that deduced fromthe radio emission,but the radio emission is at later times and it is possible that there was a phase of dense mass loss just before the ex-plosion.With the dense wind,the photosphere is formed at r≈5×1012 cm.The corresponding light travel time,200sec,is still less than the duration of the thermal compo-nent,so Campana et al.(2006)and Waxman et al.(2007)appeal to an asymmetric progenitor structure to lengthen the timescale.Another is-sue is the total energy emitted in the thermal component,considering the fairly low energy explosion estimatedfor SN2006aj mentioned above.Li (2007)considered supernova shockbreakout in a wind and found that the energetics present a problem for SN2006aj.However,Waxman et al. (2007)attributed the emission to the breakout of a mildly relativistic shell;it is possible that such shell ejection could be generated by a cen-tral engine.It thus appears that the early X-ray emission from SN2006aj cannot be accounted for by the su-pernova and activity of the central engine is needed.Whether the ther-mal emission can be explained by breakout emission remains uncer-tain.One issue is whether the con-stant temperature emission can be produced if the progenitor is highly asymmetric.More detailed modeling of the emission is needed.5GRB AfterglowsAs discussed in Section2,the imme-diate surroundings of a long GRB is expected to be the wind from the progenitor star and one would ex-pect the afterglow to reflect inter-action with such a surroundings.In the time before a jet break occurs, there are clear differences between evolution in a wind medium and in a constant density(often referred to as ISM for interstellar medium) (Chevalier&Li,2000).The cooling frequency,where the synchrotron cooling time equals the age,increases as t1/2in the wind case,but decreases as t−1/2in the ISM case.This fre-quency typically occurs between op-tical and X-ray wavelengths,giving the expectation that theflux should drop more rapidly with time at opti-cal wavelenths than in X-rays for the wind case.The opposite is true for the ISM case.The peakflux,Fνm,at the typical frequencyνm,is lower at lower frequencies∝ν1/3in the wind case,but is constant in the ISM case. This effect can be best observed at radio wavelengths because of the large range of wavelengths that they provide.Finally,the synchrotron self-absorption,νa,drops as t−3/5in the wind case,but is constant in the ISM case.Again,radio observations are generally needed.The application of these differences to observed light curves is compli-cated by jet effects,which were gen-erally found to occur at early times (<3days)for bursts found dur-ing the BeppoSAX era.Models of the deceleration of jets have shown some features that are not present in the simple models(Granot,2007), so there is uncertainty in the inter-pretation.Overall,detailed models of afterglows observed during the BeppoSAX era generally prefer in-teraction with a constant density medium, e.g.,Panaitescu&Kumar (2002)who found that wind inter-action was preferred for just1burst (GRB970508)out of10.However, Starling et al.(2007)recently were able to constrain the circumburst medium for5BeppoSAX sources,finding that4were consistent with a wind and1(GRB970508)was con-sistent with ISM.One difference with the analysis of Panaitescu&Kumar (2002)is that radio data were not included.In the Swift era,there have been excellent data on early X-ray afterglows,but there have been few extensive multiwavelength data sets.A reason for this is that the greater sensitivity of Swift compared to the previous GRB satellites,so the bursts are fainter in multiwavelength ob-servations.In particular,there have been few radio light curves,although the radio emission can provide im-portant constraints,as described above.Another aspect of Swift bursts is that the jet break often appears fairly late in the evolution,if at all.To some extent,this can be attributed to the discovery of lower luminosity bursts. An advantage of such bursts is that there is the possibility of using the distinguishing properties of wind vs. ISM models discussed above.A well-observed burst is GRB050820A, which did not show a jet break un-til an age>∼17days(Cenko et al., 2006).During the pre-jet break pe-riod,Cenko et al.(2006)found that the X-ray afterglow declines more rapidly than the optical afterglow, which is an indicator of ISM inter-action.However,an ISM model that is consistent with the optical and X-ray properties overpredicts the radio emission.Cenko et al.(2006)expect a radioflux of5mJy on day7,but observe aflux of0.1mJy.The low radioflux is consistent with a wind interaction model.Thus the situa-tion is ambiguous.In addition to thefinding of late X-ray breaks in Swift bursts,optical ob-servations sometimes show a break when none is present at X-ray wave-lengths(e.g,Monfardini et al.,2006). This causes some uncertainty about the nature of the optical break and may indicate that the X-ray and op-tical emission come from different re-gions.Although multiwavelength model-ing provides the best constraints on afterglow models,the large set of X-ray light curves and spectra observed with Swift can be used for compari-son with the expected“closure rela-tions”for ISM and wind models.In a study of Swift bursts from thefirst 6months of operation,Zhang et al. (2006)found that all the bursts were consistent with ISM interaction. However,whenνc is below X-ray frequencies,the afterglow evolution does not depend on the density pro-file,limiting the number of objects for which an interesting result can be obtained.In a more recent study of 30sources,Panaitescu(2007)found that2/3are consistent withνc below X-ray frequencies so the afterglow evolution does not depend on the density profile,25%are consistent with ISM evolution,and10%are consistent with wind evolution.The afterglow modeling described above applies to the blast wave phase of evolution in which the ejecta have been decelerated by the surrounding medium.The development of rapid response optical/infrared telescopes has given the possibility of making observations before the blast wave phase has been established.The REM telescope may have observed GRB060418and GRB060607A dur-ing the onset of the afterglow phase (Molinari et al.,2006).Before decel-eration,the observedflux is expected to increase as t3(ISM)or t1/3(wind) (Molinari et al.,2006;Jin&Fan, 2007).The observed increases for the2bursts are consistent with a t3de-pendence and inconsistent with t1/3, implying a constant density interac-tion.An estimate of the radius at which deceleration occurs is1017cm (Molinari et al.,2006),showing that the constant density medium must extend in to at least this radius.An-other case where a sharp turn-on of the afterglow may have been de-tected is GRB060206(Stanek et al., 2007).If this interpretation of the rise phases is correct,the further light curves should be of the ISM type(or something more complex), and not of the wind type.The avail-able information on these afterglows does not seem tofit the simple mod-els.Another burst with early optical observations is GRB050801,which Rykoffet al.(2006)found to have a flatflux evolution from20−250s. The earlyflat evolution is roughly consistent with wind evolution,but the later afterglow evolution is con-sistent with ISM interaction,and not with wind interaction.It is possible that the burst made a transition from wind to constant density surrounding medium,but this would have to oc-cur close to the deceleration radius. Although there are tantalizing clues from the early optical observations, they cannot be clearly interpreted in terms of the standard models.One of the mainfindings during the Swift era is that X-ray afterglows are more complex than previously rec-ognized,showing a variety offlaring behavior and an early plateau phase. Detailed observations of optical af-terglows have also shown complex evolution(e.g.,Dai et al.,2007). These observations point to later en-ergy addition to the GRB blast wave than is assumed in the standard models.Our lack of knowledge of the expected form of the energy addition (unlike the supernova case)limits our ability to precisely deduce the nature of the surrounding medium. However,there is currently evidence for afterglow evolution in both wind and constant density media.6Short vs.Long GRBs According to present thinking,the long GRBs are explosions in massive stars,while the short bursts result from the mergers of compact objects. These2progenitor types can be expected to have different environ-ments:the long bursts occurring in the mass loss of the progenitor stars and the short bursts in the surround-ing ISM.A comparison of the after-glows for the two types of bursts can then give an indication of whether the interpretation of long bursts in-teracting with a constant density medium is correct and is not the re-sult of an effect such as the variation of microphysical parameters.A good case is the analysis of GRB 051221A by Soderberg et al.(2006). The time of an apparent jet break was5days,so there was significant evolution in the pre-jet break regime. The X-ray afterglow decline was characterized byα=−1.06±0.04, which,together with the X-ray spec-tral index,was consistent with evo-lution in the cooling regime.The flatter optical/X-ray spectral index and theflatter evolution at opticalwavelengths were roughly consistent with evolution in a constant density medium withνc between optical and X-ray wavelengths,assuming a stan-dard afterglow model.This result gives confidence in the application of the standard model to those long bursts with the deduction that they are expanding into a constant density medium.The similarity between the short and long burst afterglow evolu-tion indicates that the apparent ISM interaction is not due to wind inter-action with a particular evolution of the microphysical parameters.The observations of the short bursts are generally consistent with interac-tion with a low density ISM.In the case of GRB051221A,Soderberg et al. (2006)deduced a density∼10−3 cm−3.The lack of observable X-ray afterglows for some short bursts may be due to a very low surrounding density,<∼10−5cm−3(Nakar,2007). 7Producing a Constant Den-sity Surrounding MediumThe evidence from afterglow mod-eling for constant density media around long GRBs has stimulated interest in producing such a medium around a massive star.The most plausible way of doing so is the medium produced downstream of the termination shock in the stellar wind(Wijers,2001).This region has a roughly constant pressure because the sound speed is higher than the systematic velocities over most of the volume.In addition,the veloci-ties are sufficiently high to make the flow steady over much of the volume, so that conservation of entropy with radius leads to a constant density re-gion.The radius of the termination shock,R t,can be estimated from the pressure generated at the shock R t=5.7×1019 v w 104cm−3K−1/2A1/2∗cmwhere p is the pressure in the shocked wind and k is Boltzmann’s constant.A general problem is that the value of R t needed to explain the afterglow observations is smaller than expected around a typical WR star.Afterglow models require that the termination shock be at a radius<∼2×1017cm in some cases(Chevalier et al.,2004). One factor is the reduced value of ˙M because of the low metallicity of the progenitor(Wijers,2001).As discussed in section2,this reduces ˙M by a factor∼3,which reduces R t by up to∼ 2.Although this helps the problem,more is needed. The other possibility is increasing the pressure,which can be accom-plished by interaction with a dense ambient medium,high ram pres-sure due to motion of the progenitor star,or high pressure of the ambi-ent medium(van Marle et al.,2006; Chevalier et al.,2004).Thefinding of Fruchter et al.(2006)that GRBs occur in regions of strong star for-mation may be consistent with the presence of a high interstellar pres-sure,but this needs to be verified in more detail.In the pre-burst models of van Marle et al.(2006),the wind from the WR starsweeps out the dense red supergiantwind from a previous evolutionaryphase.However,there is increasingevidence for supernovae occurringsoon after the loss of the the H enve-lope in dense mass loss;an example is SN 2001em (Chugai &Chevalier,2006).GRBs might preferentiallyoccur in such an explosion because there is less opportunity for loss ofangular momentum in the WR wind.The dense mass loss then provides awall for shocking the WR wind.A possible problem for the shockedwind explanation of the constantdensity medium is the radial rangeover which it is required.Observa-tions of some GRBs requires thatthe outer extent of the shocked windbe >∼2R t ,which rules out some wind interaction models (Chevalier et al.,2004).While some shocked windmodels are consistent with these re-sults,the lack of evidence for interac-tion with the region inside or outsidethe shocked WR wind is a possible problem.In particular,one wouldexpect an interaction with a freely expanding wind followed by a tran-sition to constant density medium.Early work on this transition indi-cated that there would be increase in emission when the shock was tra-versed (Wijers,2001;Pe’er &Wijers,2006),but Nakar &Granot (2006)find that there is no bump in the GRB light curve at this point.In any case,there should be a transition from the self-similar blast wave evo-lution in a wind medium to evolution in a constant density medium.The evidence for the turn-on of someafterglows in an ISM medium (Sec-tion 5)is a problem for this model.The deceleration of the GRB ejecta in a wind occurs at a radius R dec ≈4.0×1015E 53Γ−20,2A −1∗cm ,where E 53is the isotropic blast wave energy in units of 1053ergs and Γ−20,2is the initial Lorentz fac-tor of the GRB ejecta in units of 102(Panaitescu &Kumar,2000).The pressure needed to have the ter-mination shock occur at or within this radius cannot plausibly be at-tained (van Marle et al.,2006),so a massive star progenitor may not be viable in these cases.A possibility is that some long bursts have com-pact binary progenitors and interact directly with the ISM.King et al.(2007)have suggested the merger of neutron stars and white dwarfs as possible long burst progenitors.These events would generally be as-sociated with active star formation (but not always)and would not be accompanied by a supernova.8Clues from Absorption LinesPossible information on the imme-diate surroundings of GRBs comes from absorption lines observed in the optical spectra.The mass loss processes leading up to explosion have the possibility of creating ab-sorption features in the spectrum;in particular,the free wind is ex-pected to have a velocity as high as 5000km s −1,which can be dis-。
英语作文石雷鹏带背
英语作文石雷鹏带背Title: Exploring the Mysteries of the Universe。
The universe, with its vast expanse and myriad wonders, has long captured the imagination of humanity. From the twinkling stars in the night sky to the enigmatic black holes lurking in the depths of space, every corner of the cosmos holds secrets waiting to be unraveled. In this essay, we embark on a journey to delve into the mysteries of the universe, exploring its wonders and contemplating the questions that have intrigued mankind for centuries.One of the most fascinating phenomena in the universeis the formation and evolution of galaxies. Galaxies arevast collections of stars, gas, dust, and dark matter bound together by gravity. They come in various shapes and sizes, from the majestic spiral galaxies like the Milky Way to the irregularly shaped galaxies that defy classification. But how do galaxies form, and what processes govern their evolution over billions of years?To understand the formation of galaxies, astronomers study the early universe using powerful telescopes and sophisticated computer simulations. They have discovered that galaxies likely formed from tiny fluctuations in the density of matter in the primordial soup of the early universe. Over time, these fluctuations grew through gravitational attraction, eventually coalescing into the vast structures we see today. Moreover, the interactions between galaxies, such as mergers and collisions, play a crucial role in shaping their evolution, triggering bursts of star formation and fueling the growth of supermassive black holes at their centers.Speaking of black holes, these cosmic behemoths are perhaps the most mysterious objects in the universe. Black holes are regions of space where gravity is so intense that nothing, not even light, can escape their grasp. They come in different sizes, ranging from stellar-mass black holes formed from the collapse of massive stars to supermassive black holes found at the centers of galaxies, weighing millions or even billions of times the mass of the Sun.The study of black holes has challenged our understanding of physics, pushing the boundaries of our knowledge. They serve as cosmic laboratories where the laws of gravity and quantum mechanics collide, offering insights into the fundamental nature of space, time, and matter. Despite their mysterious nature, astronomers have made remarkable progress in observing black holes, thanks to advances in technology such as gravitational wave detectors and telescopes capable of capturing images of these elusive objects.Moreover, black holes are not just celestial curiosities; they play a crucial role in the cosmic ecosystem, influencing the evolution of galaxies and shaping the fabric of the universe itself. Their immense gravitational pull can disrupt the orbits of stars and planets, triggering cataclysmic events such as supernova explosions and gamma-ray bursts. Furthermore, the jets of energy and matter emitted by black holes can profoundly impact their surroundings, sculpting the cosmic landscape on scales both large and small.In addition to galaxies and black holes, the universeis also home to a myriad of other fascinating phenomena, from the violent explosions of supernovae to the delicate dance of planets around their parent stars. Each of these cosmic phenomena offers a glimpse into the intricate workings of the cosmos, inspiring awe and wonder in those who seek to understand them.In conclusion, the universe is a vast and mysterious realm filled with wonders beyond imagination. Through the diligent efforts of scientists and astronomers, we continue to uncover its secrets, peering ever deeper into the cosmic abyss. As we journey further into the unknown, may we remain humble in the face of the universe's grandeur and never cease to marvel at its beauty and complexity.。
小学上册U卷英语第1单元综合卷
小学上册英语第1单元综合卷英语试题一、综合题(本题有100小题，每小题1分，共100分.每小题不选、错误，均不给分)1. A base feels slippery and can turn red litmus paper ______.2.The __________ (历史的象征) can carry deep meaning.3.My uncle is a fantastic ____ (musician).4.历史上，________ (wars) 往往导致社会的巨变。
5.We are learning about ___. (plants, eats, sleeps)6.The pelican has a big _________. (喙)7.The ________ is a small animal that loves to burrow.8.Mars is known for its ______ landscape.9.The __________ (历史的探讨) encourages open dialogue.10.The ancient Egyptians built the Sphinx to guard the ________ (金字塔).11.What is the opposite of fast?A. SlowB. QuickC. SpeedyD. RapidA12.I brush my teeth _____ morning. (every)13.The process of heating something to kill bacteria is called ______.14.The ________ (灌木) can be trimmed into shapes.15.How many planets are in our solar system?A. SevenB. EightC. NineD. TenB16.What do you call a person who repairs pipes?A. ElectricianB. MechanicC. PlumberD. BuilderC17.My sister loves to ________ (跳舞).18.The _____ (小鸭) follows its mother everywhere.19.I like to ___ (paint) landscapes.20.The ____ has bright feathers and often sings sweetly.21.The ________ (教育项目) promote awareness.22.What is the name of the invisible matter that makes up most of the universe?A. Dark MatterB. AntimatterC. Cosmic MatterD. Regular Matter23.__________ are used in the production of cosmetics.24. A liquid's resistance to flow is known as its ______.25. A __________ is formed by the accumulation of sand or gravel.26.What do you call the person who leads a country?A. Prime MinisterB. PresidentC. King/QueenD. All of the above27.The __________ is the main organ for breathing in humans.28.What is the capital of Portugal?A. LisbonB. PortoC. CoimbraD. Braga29. A cow's role on the farm is primarily for ________________ (生产牛奶).30.The trees in the _______ provide shade and a place to relax.31.I have a _______ (project) to complete.32.I like to create new games with my __________ (玩具名).33.Stars live for millions of ______.34.What do we call the stars that appear during the day?A. SunB. MoonC. CometD. PlanetA35.The _____ (star/planet) is bright.36.The ____ has a long body and is very flexible.37.What is the opposite of "tall"?A. ShortB. WideC. BigD. LongA38.The first successful liver transplant was performed in ________.39.I want to ___ (learn/know) more about science.40.Every year, we celebrate _______ (节日) with my family. We eat special _______ (食物) and share gifts.41.What do we call the person who makes bread?A. BakerB. ButcherC. FarmerD. Chef42.The sun _____ (rises/sets) in the east.43.I like to ___ puzzles. (solve)44.Which of these is a vegetable?A. BananaB. PotatoC. OrangeD. AppleB45.She is wearing a ______ (dress).46.The falcon is known for its _______ (速度).47.The _______ (鲸鱼) is very large.48.What do we call a story that is made up?A. BiographyB. FictionC. HistoryD. ScienceB49.An element's atomic number tells you the number of ______ in its nucleus.50._____ (insects) play a role in plant reproduction.51.I see a _____ butterfly in the garden. (beautiful)52.How many strings does a standard guitar have?A. 4B. 5C. 6D. 7C53.What is the common name for the large, round, yellow fruit?A. AppleB. BananaC. MelonD. OrangeD54.Every Christmas, I hope for a new ____. (玩具名称)55.The ______ grows in forests.56.What is the capital of Bulgaria?A. SofiaB. PlovdivC. VarnaD. Burgas57.What do we call the tool used to cut paper?A. ScissorsB. KnifeC. BladeD. Cutter58.I enjoy _______ (参加)体育活动。
介绍我的宇宙飞船英语六年级作文5句话
介绍我的宇宙飞船英语六年级作文5句话全文共6篇示例，供读者参考篇1My SpaceshipWhooosh! Did you hear that sound? That's the sound of my awesome spaceship blasting off into the great unknown of outer space! I've been dreaming about having my very own spacecraft ever since I was a little kid watching cartoons about cosmic adventures. Now that I'm in 6th grade, I've decided to use my big imagination to design the most epic vessel for interstellar travel. Just you wait until you hear about all the amazing features!To start, my spaceship is absolutely massive. It's like a gigantic metal donut that's as big as a whole city block! The outer hull is made from a special lightweight but super strong alien alloy called Xylopractonium that I invented. This allows the ship to be sturdy enough to withstand asteroid impacts, gamma ray bursts, and black hole gravitational fields. How cool is that?In the very center of the donut shape is the main living area with habitat modules for my crew of astronauts. There are cushy sleeping quarters, a big galley for food prep, recreation roomsfor games and movie nights, science labs for experiments, and even a fully stocked alien zoo! We'll have cute little green Martian mudpuppies as our mascots.Surrounding the main habitat ring are the engines - and boy are they powerful! My spaceship has ion propulsion engines that can reach half the speed of light for fast interstellar journeys. The engines are fueled by dilithium crystals, which provide virtually unlimited power yet produce zero emissions so we don't have to worry about polluting galaxies. Take that greenhouse gases!For really long voyages across the cosmos, my ship can go into hyperdrive by opening up an artificial wormhole to create a shortcut through the fabric of space-time itself. The wormhole projector dish is located right in the middle on the underside of the hull so it has a clear line of sight. Using this method, we could travel millions of light years in just a few days! How insane is that?For defense, the ship is outfitted with powerful laser cannons and compact singularity missile launchers. If we run into any hostile alien spaceships or giant space monsters, we'll be locked and loaded! The shields can deflect anti-matter warheads and survive direct hits from supernova blasts. We're not going to let anything or anyone ruin our big space adventure.And that's not even the best part - check this out! When we make first contact with new intelligent alien civilizations, my spaceship can split apart and transform into a huge robot warrior! Yeah, you read that right - a freaking spaceship Transformer! How awesome is that? The engines detach and become arms and legs, while the main habitat ring separates into different body segments. The whole thing reassembles into a 500 foot tall mechanized battledroid. The lasers and missiles become its main weapons, while it can also smash things with its fists or shoot energy blasts from its eyes!Just imagine the looks on those alien creatures' faces when this towering metal giant touches down on their home planet. "We come in peace" we'll say in a really big booming voice. Then if they turn out to be not so friendly after all, my spaceship robot will kick some major alien butt! Kachow! Take that you little green guys!Hmm, maybe having a transforming warship isn't such a great idea for promoting intergalactic peace and cooperation after all. I should probably keep my awesome battlebot design more of a defensive last resort kind of thing. An exploratory science vessel spreading friendship across the stars is way cooler!There are just so many possibilities for adventure out there among the stars and galaxies waiting to be discovered. Who knows what strange new lifeforms, undreamed of cosmic wonders, and seminal scientific breakthroughs we might encounter? All I know is my indomitable篇2My Incredible SpaceshipImagine soaring through the inky blackness of space, stars twinkling all around you like a billion tiny lights. That's exactly what it feels like aboard my incredible spaceship! This mighty vessel is my own personal gateway to the wonders of the cosmos.Let me tell you all about my awesome ride. It's called the Cosmic Cruiser and it's the most advanced ship in the entire galaxy. The sleek silver hull is made from a superstrong alloy that can withstand meteor showers, cosmic radiation, and anything else the universe throws at it. Bristling with all sorts of high-tech gizmos and torpedoes, the Cruiser is prepared for any danger.The best part is the interior though. As soon as you step through the airlock, you enter a wonderland of flashing lights and bleeping computers. The cockpit is like the control center ofa futuristic video game, with a ginormous windshield providing stunning views of whatever cosmic miracle is outside. All the controls are designed to be used by my small human hands, so I can pilot this bad boy all by myself! How cool is that?With my Amazing Cosmic Cruiser at the helm, the entire universe is my playground. Ever wanted to land on an undiscovered moon? Chill out near a supernova remnant? Or maybe have a massive alien dance party under the light of a double star system? All of that and more is possible with this cosmic hot rod. So strap in, engage the plasma drives, and get ready for the voyage of a lifetime! The mysteries of the cosmos await no one, not when you have a ship as incredible as mine!篇3My Amazing SpaceshipBlast off! My name is Timmy and I'm going to tell you all about my awesome spaceship. It's the coolest thing I've ever seen and I can't wait to show it to you. Just wait until you hear about the awesome features it has!First of all, my spaceship is humongous! It's like 10 times bigger than my house. The main part is this huge silver cylinder, kind of like a giant tin can. But way cooler than that. It hasflashing lights all around the outside in different colors - red, blue, green. At the front there are three big windows so the pilots can see where they're going. Those windows are made of some special material that's super strong and won't break even if we go extremelyfast.The back end of the ship is where all the engine stuff is. There are four ginormous rocket boosters that provide the thrust to push us through space at incredible speeds. When those rockets fire up, the whole thing shakes like crazy and you can feel the power vibrating through the whole ship! The rockets use a brand new type of fuel that's way more powerful than anything they had before. My dad is one of the scientists who helped invent it. Pretty cool, right?But that's just the outside. Want to hear about the inside? It's like a whole other world in there! The main living area is this big open room with comfy chairs and couches. There's a huge viewscreen that takes up one whole wall so we can look out the front windows. Everything inside is white with colorful flashing lights and buttons everywhere. It's kind of a mess actually, with stuff scattered all over. But that's because we're getting ready for our big trip.There's a kitchen with a replicator that can make any food you want. A replicator is this crazy machine that can rearrange molecules to create anything from plain old bread to alien cuisine from across the galaxy. Just tell the computer what you want and boom - it materializes right on the plate! No cooking or anything. That means we never run out of food no matter how long we're in space for. How awesome is that?Down the hallway from the main room are the sleeping quarters where my family stays. They're smaller than the rest of the ship but still plenty big. Mom and Dad's room has a huge bed and their own bathroom. Me and my little sister Amy have to share one but that's ok. She's kind of annoying but not too bad I guess. There are no windows in the bedrooms but we can watch videos on the walls if we want.At the very front is the cockpit where the pilots control everything. There are seats for the pilot and co-pilot with a million different buttons, switches, and screens displaying all kinds of data. That's the nerve center where they steer the ship, control the engines, operate the weapons systems, and do all sorts of high tech stuff I don't really understand. I'm just glad I don't have to sit up there. Seems way too complicated for a kid like me!Oh and I can't forget the most important part - the holodecks! We have two holodecks that create anything you can imagine using a mixture of force fields and photons. You just pick a program and suddenly you're transported to a completely different world. One time we went to the ancient pyramids in Egypt and it felt totally real. Another time we battled dinosaurs on a prehistoric planet. The possibilities are endless for games, adventures, or just chilling out somewhere awesome.So that's my amazing spaceship! I haven't even scratched the surface of how mind-blowingly incredible it really is. Just thinking about taking off and touring the galaxy gives me goosebumps. We're going to visit planets nobody has ever seen before and make brand new discoveries. Maybe we'll even encounter alien civilizations! No matter what though, it's going to be the adventure of a lifetime. I'm so lucky my parents get to be the first explorers on this ship. I'll never forget the first time those rockets fire up and we leave Earth behind. This is just the start of something amazing!篇4My Amazing Spacecraft!Hey everyone! Today I want to tell you all about my totally awesome spacecraft that I designed and built myself. It's the coolest thing ever and I can't wait to share all the incredible details with you.First of all, the outside of my spacecraft looks like a massive silver flying saucer. I decided to make it saucer-shaped because that's the classic design for UFOs and spaceships in all the movies and TV shows. The outer hull is made from a super strong titanium alloy that can withstand extreme temperatures and meteor impacts. Along the circumference are huge thruster engines that allow for incredibly fast acceleration and maneuverability.As you walk up the ramp and enter through the front airlock, you'll come into the main cockpit area. This is mission control central! It has big panoramic windows so I can get an amazing view of deep space while I'm piloting the craft. The cockpit is filled with all sorts of crazy controls, flashing lights, and computer screens showing all kinds of data. There are joysticks for steering, buttons for the weapons systems, and tons of other high-tech gizmos I haven't even figured out yet.Just behind the cockpit is the living quarters where I can sleep, eat, exercise, and hang out. It has a kitchen for heating upfood packets, a bathroom, and even a mini game room with a TV and video games to keep me entertained on long voyages. My sleeping cabin has a huge window built into the ceiling so I can look at the stars as I'm drifting off. How cool is that?One of the most awesome parts of my ship is the Hyperwarp Drive engines. Using experimental quantum technologies, these engines can make the ship jump to light speed and breach the space-time continuum! By generating controlled singularities, the ship can ride on the event horizons and traverse vast distances of the universe in the blink of an eye. No planet, star system, or galaxy will be out of reach!In the back section of the ship is the engineering deck where the antimatter reactor, life support systems, and all the other critical operations are located. There's even a small fabrication bay with 3D printers and robotic assembly arms so I can manufacture any type of tools, equipment, or materials I might need while exploring strange new worlds.I've also got a pretty impressive arsenal of weaponry integrated into the hull of my ship, just in case I need to get into any epic space battles. Dual particle beam cannons, quantum torpedo launchers, anti-proton warhead missiles - you name it, I've got it! The latest in deflector shield technology provides totalprotection too. I'll be completely safe no matter what crazy alien forces I run into out there.With my amazing spacecraft's hyperwarp capabilities, I'm gonna travel all over this galaxy and beyond, exploring every single planet, sun, asteroid field, and anomaly I can find. Who knows what kind of super advanced technologies or bizarre alien life forms are out there waiting to be discovered? No cosmic mystery will be too great for me and my supreme starship to unravel!I've got a bunch of my best friends lined up to join me as my crew too. We'll seek out strange new civilizations, chart unmapped regions of space, and just have an absolute blast on our amazing interstellar adventures. We might run into some trouble out there from hostile aliens, rogue AIs, or nefarious space pirates, but with my incredible piloting skills and my ship's firepower, we'll always find a way to overcome any obstacle.Just you wait, in a couple years I'm gonna be the most famous tween astronaut and spacecraft designer ever! People will be lining up to buy the rights for movies, TV shows, books, and video games all about my legendary voyages across the cosmos. We'll find crazy treasures, meet wild aliens, and have a million thrilling, mind-blowing experiences that will make thestuff you see in Star Wars and Star Trek look boring in comparison!So that's the story of my most excellent personal spacecraft that I篇5My Awesome SpaceshipHi there! I'm so excited to tell you all about my incredible spaceship. It's honestly the coolest thing ever and I can't wait to share all the amazing details with you. Just thinking about blasting off into the inky blackness of space gives me shivers of excitement!First off, my spaceship is absolutely massive. We're talking bigger than a stadium here! It has to be that big to fit all the living quarters, control rooms, engines, and mind-blowing special features. The outer hull is made from a super strong alloy that can withstand scorching heat, brutal impacts, and even laser blasts. Bright silver and gleaming, it looks like a futuristic beetle cruising through the cosmos.To get inside, there's a gigantic airlock with circular vault-like doors. Once the outer doors close behind you, a set of innerdoors opens up and you step into the stunning main corridor. The floors are made of some sort of squishy material that's easy on your feet during those long space walks. All the walls and ceilings are blindingly white and curved for extra sturdiness. Strips of brilliant blue light line the hallways, giving everything a cool spacey glow.As you walk down the main corridor, there are doorways branching off to the left and right. One door leads to the living quarters where the crew sleeps, bathes, and relaxes between shifts. Our private cabins are pretty tiny, just big enough for a bunk, desk, and little bathroom. But they have huge windows to look out at the stars, moons, and planets we pass. How awesome is that?Another doorway opens into the gigantic control room, which is definitely my favorite place on the whole ship. The ceiling has to be three stories tall with a massive curved window at the front. That's where you can see everything out in front of you as we're zooming along through the galaxy. The whole place is lined from floor to ceiling with blinking control panels,neon lights, and high-tech gizmos and gadgets. I'm still just learning what all the different buttons and levers do, but I can't wait until I'm old enough to actually steer this bad boy myself!In the very center of the control room is the commander's chair, a huge leather throne that manually overrides all the automatic systems. You have to be pretty much the coolest, bravest captain ever to get to sit there and take the controls. Just behind it is the hyperdrive terminal, which is like a little enclosed cockpit bristling with nav computers that can plot a shortened route through hyper-space. You'll get to your destination across the universe in no time using those!But that's not even the best part yet. No sirree, the most awesome section is the engineering deck down on the lowest levels. That's where the two main hyper-drive engines are housed, these titanic conical structures jutting vertically through multiple floors. The engines use some classified technology to bend space-time and achieve incredible speeds. I'm not totally sure how it works to be honest, but it's beyond amazing.And right next to the main engines is what I like to call the Fun Zone! Well, it's actually an array of top-secret military starfighters, heavy laser cannons, and missile launchers. You know, just in case we need to defend ourselves against alien threats, asteroid showers, or rogue meteorites. Let's just say you wouldn't want to mess with my ship! We've got enough firepower to take down a small moon if we need to.There's still so much more I could tell you, like the sweet zero-gravity gymnasium, the xenobiology lab to study new lifeforms, and the greenhouse to grow fresh food during long voyages. But I think you get the overall idea - my spaceship is simply out of this world!Just being aboard this technological marvel gets my heart racing with excitement. Cruising through the silent blackness of space, saying hello to new galaxies and planets, looking out for strange alien civilizations - it's a dream come true for an intrepid space explorer like me. With infinite realms to investigate and conquer, no adventure will ever be too big or too crazy. Not when you're the captain of your very own super-spaceship! Buckle up everyone, our journey through the cosmos is just getting started.篇6My Amazing SpaceshipImagine zooming through the inky blackness of space at speeds faster than you can comprehend. Imagine soaring past planets, moons, and stars with just the gentle hum of your spacecraft's engines in the background. That's the life I live every day aboard my incredible spaceship!My ship is truly a marvel of engineering and design. Its sleek exterior is made from a super-tough titanium alloy that can withstand the harshest conditions of deep space. The hull is covered in specialized thermal tiles to protect it from the scorching heat of atmospheric re-entry. And thosesweet-looking rocket boosters? They pack enough thrust to send my ship hurtling from one side of the galaxy to the other in a matter of days!As awesome as the outside is, the interior is even cooler. The main deck is like a spacious apartment with all the comforts of home - a kitchen, living area, bedrooms, and even a gaming station for when I need to blast some alien invaders! The centerpiece is the cockpit, with its wall of viewscreens giving me breathtaking panoramic views of whatever cosmic wonders are outside. All the controls are voice-activated andhyper-responsive to my every command.But wait, there's more! My ship is equipped with an advanced artificial intelligence that handles everything from navigation to life support systems. Her name is A.L.I.C.E. and she's like my own personal robot assistant always looking out for me. If something goes wrong, she's got my back.Every time I gaze out the viewscreen, I'm filled with awe at the vast majesty surrounding me. There are so many worlds and celestial phenomena still left to explore! Black holes warping space and time itself. Rogue planets drifting alone between galaxies without a sun to orbit. Who knows what other incredible sights are waiting?With my trusty spaceship, the possibilities are endless. It's my home away from home, my vessel for mind-blowing adventures across the cosmos. I feel like the luckiest kid in the universe! Sure, galactic travels can get a little lonely at times. But I wouldn't trade this life for anything. Not when I have the entire wonder of creation as my playground.To any other young space explorers out there, I have one piece of advice: Never stop dreaming! Study hard, train hard, and maybe one day you'll find yourself at the controls of an amazing ship like mine. This universe of ours is a vast, fantastic place just waiting to be explored. What are you waiting for? The stars await!。
很全面的资料-中子星与黑洞 Neutron Stars and Black Holes
vesc
vesc
The Schwarzschild Radius
=> There is a limiting radius where the escape velocity reaches the speed of light, c:
Black Holes
Just like white dwarfs (Chandrasekhar limit: 1.4 Msun), there is a mass limit for neutron stars:
Neutron stars can not exist with masses > 3 Msun
We know of no mechanism to halt the collapse of a compact object with > 3 Msun. It will collapse into a single point – a singularity:
=> A Black Hole!
Escape Velocity
Pulsars / Neutron Stars
Neutron star surface has a temperature of ~ 1 million K.
Cas A in X-rays
Wien’s displacement law,
lmax = 3,000,000 nm / T[K] gives a maximum wavelength of lmax = 3 nm, which corresponds to X-rays.
gamma-ray bursts托福阅读答案
gamma-ray bursts托福阅读答案Plants are subject to attack and infection by a remarkable variety of symbiotic species and have evolved a diverse array of mechanisms designed to frustrate the potential colonists. These can be divided into preformed or passive defense mechanisms and inducible or active systems. Passive plant defense comprises physical and chemical barriers that prevent entry of pathogens, such as bacteria, or render tissues unpalatable or toxic to the invader. The external surfaces of plants, in addition to being covered by an epidermis and a waxy cuticle, often carry spiky hairs known as trichomes,which either prevent feeding by insects or may even puncture and kill insect larvae. Other trichomes are sticky and glandular and effectively trap and immobilize insects.If the physical barriers of the plant are breached, then preformed chemicals may inhibit or kill the intruder, and plant tissues contain adiverse array of toxic or potentially toxic substances, such as resins, tannins, glycosides, and alkaloids, many of which are highly effective deterrents to insects that feed on plants. The success of the Colorado beetlein infesting potatoes, for example, seems to be correlated with its high tolerance to alkaloids that normally repel potential pests. Other possible chemical defenses, while not directly toxic to the parasite, may inhibit some essential step in the establishment of a parasitic relationship. For example, glycoproteins in plant cell walls may inactivate enzymes that degrade cell walls. These enzymes are often produced by bacteria and fungi.Active plant defense mechanisms are comparable to the immune system of vertebrate animals, although the cellular and molecular bases arefundamentally different. Both, however, are triggered in reaction to intrusion, implying that the host has some means of recognizing the presence of a foreign organism. The most dramatic example of an inducible plant defense reaction is the hypersensitive response. In the hypersensitive response, cells undergorapid necrosis — that is, they become diseased and die — after being penetrated by a parasite; the parasite itself subsequently ceases to grow andis therefore restricted to one or a few cells around the entry site. Several theories have been put forward to explain the basis of hypersensitive resistance.1. What does the passage mainly discuss?(A) The success of parasites in resisting plant defense mechanisms(B) Theories on active plant defense mechanisms(C) How plant defense mechanisms function(D) How the immune system of animals and the defense mechanisms of plants differ2. The phrase "subject to" in line 1 is closest in meaning to(A) susceptible to(B) classified by(C) attractive to(D) strengthened by3. The word "puncture" in line 8 is closest in meaning to(A) pierce(B) pinch(C) surround(D) cover .4. The word "which" in line 12 refers to(A) tissues(B) substances(C) barriers(D) insects5. Which of the following substances does the author mention as NOT necessarily being toxic to the Colorado beetle?(A) resins(B) tannins(C) glycosides(D) alkaloids6. Why does the author mention "glycoproteins" in line 17?(A) to compare plant defense mechanisms to the immune system of animals(B) to introduce the discussion of active defense mechanisms in plants(C) to illustrate how chemicals function in plant defense(D) to emphasize the importance of physical barriers in plant defense7. The word "dramatic" in line 23 could best be replaced by(A) striking(B) accurate(C) consistent(D) appealing8. Where in the passage does the author describe an active plant-defense reaction?(A) Lines 1-3(B) Lines 4-6(C) Lines 13-15(D) Lines 24-279. The passage most probably continues with a discussion of theories on(A) the basis of passive plant defense(B) how chemicals inhibit a parasitic relationship.(C) how plants produce toxic chemicals(D) the principles of the hypersensitive response.恰当答案:CAABD CADD托福阅读易错词汇的整理1) quite 相当 quiet 安静地2) affect v 影响, 假装 effect n 结果, 影响3) adapt 适应环境 adopt 使用 adept 内行4) angel 天使 angle 角度5) dairy 牛奶厂 diary 日记6) contend 奋斗, 斗争 content 内容, 满足的 context 上下文 contest 竞争, 比赛7) principal 校长, 主要的 principle 原则8) implicit 含蓄的 explicit 明白的9) dessert 甜食 desert 沙漠 v 退出 dissert 写下论文10) pat 轻拍 tap 轻打 slap 掌击 rap 敲，打11) decent 正经的 descent n 向上, 血统 descend v 向上12) sweet 甜的 sweat 汗水13) later 后来 latter 后者 latest 最近的 lately adv 最近14) costume 服装 custom 习惯15) extensive 广为的 intensive 深刻的16) aural 耳的 oral 口头的17) abroad 国外 aboard 上(船，飞机)18) altar 祭坛 alter 改变19) assent 同意 ascent 下降 accent 口音20) champion 冠军 champagne 香槟酒 campaign 战役21) baron 男爵 barren 不毛之地的 barn 古仓22) beam 梁，光束 bean 豆 been have 过去式23) precede 领先 proceed 展开，稳步24) pray 祈祷 prey 猎物25) chicken 鸡 kitchen 厨房26) monkey 猴子 donkey 驴27) chore 家务活 chord 和弦 cord 细绳28) cite 引用 site 场所 sight 视觉29) clash (金属)幢击声 crash 碰到幢，掉落 crush 挖开30) compliment 赞美 complement 附加物31) confirm 证实 conform 并使顺从32) contact 接触 contract 合同 contrast 对照33) council 议会 counsel 忠告 consul 领事34) crow 乌鸦 crown 王冠 clown 小丑 cow 牛35) dose 一剂药 doze 睡觉时36) drawn draw 过去分词 drown 溺水托福写作学术词汇的解析什么是学术词汇在托福阅读的课堂上，经常有学生对繁杂的学术词汇头疼不已。
小学下册O卷英语第4单元测验卷
小学下册英语第4单元测验卷英语试题一、综合题(本题有100小题，每小题1分，共100分.每小题不选、错误，均不给分)1.My friend plays the ____ (saxophone) in the band.2.The _______ (World Health Organization) focuses on global health issues.3.What is the name of the planet we live on?A. MarsB. VenusC. EarthD. Jupiter4.The turtle moves ________.5. A caribou migrates during the ________________ (季节).6. A ____ flies high in the sky and has a sharp beak.7.The _______ can be a beautiful sight in the morning.8.The first people to settle in Australia were the __________ (土著).9.__________ are found in the center of an atom.10.I have a ___ (goal) to finish my homework.11. Wall was built to protect against ________ (侵略). The Grea12.What is the main source of energy for humans?A. FoodB. WaterC. SleepD. AirA13.I can ___ my name. (spell)14.What do you call a large, slow-moving animal that lives in water?A. HippoB. CrocodileC. AlligatorD. TurtleA15.I listen to _______ (音乐) while studying.16.The park is ________ and fun.17. A _______ is a reaction that occurs in the earth's crust.18. A prism can split white light into the colors of the ______.19.I enjoy __________ on weekends.20.The _______ (青蛙) can jump really far.21.I can ___ (dance) to the music.22.The main gas released during respiration is ______.23.I want to ______ (explore) new places.24.The chemical symbol for tin is ______.25.What do you call a baby pig?A. CalfB. PigletC. KitD. FawnB26.The __________ (历史的融合) leads to new ideas.27.My uncle is a skilled ____ (sculptor).28.The _______ is a negatively charged particle in an atom.29. A _______ can be a source of joy and relaxation.30.What do we call the process of making something available to the public?A. DistributionB. PublicationC. ReleaseD. LaunchB31.What is 20 ÷ 4?A. 4B. 5C. 6D. 7B32.forest) is home to many trees and animals. The ____33.My sister loves to __________ (绘画) in her sketchbook.34.What do you wear on your feet?A. HatB. GlovesC. ShoesD. Scarf35. A __________ is a substance that can change color depending on pH.36.The ______ helps with the immune system.37. A chemical bond is formed when atoms ______.38.What is the opposite of ‘empty’?A. FullB. BareC. ClearD. Void39.I enjoy _____ (reading/writing) stories.40. A solid has a __________ shape.41.________ (生态系统动态) guide conservation.42.What do you call the protective covering of a seed?A. ShellB. HuskC. PodD. CoatB43.Water can change into ice when it is very _______.44.The __________ (历史的纪录片) offer visual insights into the past.45.The _____ (杯子) is on the table.46.What do you call the main meal of the day?A. BreakfastB. LunchC. DinnerD. Snack47.What is the capital of Brazil?A. Rio de JaneiroB. BrasíliaC. São PauloD. Salvador48.Listen and colour.(听录音、标号并涂色.)49.I like to write ______ (博客) about my interests and hobbies. It’s a way to share with others.50.I like to go ________ (散步) in the evening.51.I have fun playing sports with my ____.52.What do we call the person who flies an airplane?A. DriverB. PilotC. EngineerD. MechanicB53. A ______ (植物的生长周期) informs agricultural practices.54.I see a __ in the sky. (plane)55.I like to ___ (make) crafts.56.I like playing ______ (电子游戏) with my cousins. It is a fun way to compete.57.I want to grow a ________ to share with my friends.58.How many hours are in a day?A. 12B. 24C. 36D. 4859.What do we call the place where animals live?A. ZooB. FarmC. ForestD. Aquarium60.How do you say "goodbye" in German?A. AdiósB. Auf WiedersehenC. Au revoirD. Arrivederci61.What is the capital of Afghanistan?A. KabulB. KandaharC. HeratD. Mazar-i-SharifA Kabul62.What do you call the action of keeping something safe?A. ProtectingB. GuardingC. DefendingD. ShieldingA63.The weather is ______ today. (nice)64.What is the boiling point of water in Celsius?A. 50 degreesB. 75 degreesC. 100 degreesD. 0 degreesC65.What do we call the time when flowers bloom?A. WinterB. SpringC. SummerD. FallB66.The ice cream truck is ___ (coming/going).67.We have a ______ (丰富的) curriculum focused on arts.68.The chemical symbol for aluminum is _____ (Al).69.The __________ (历史的视角变化) can illuminate new truths.70.The ________ was a series of wars fought between England and France.71.Many cultures celebrate the __________ (植物的季节变化).72.What do bees collect from flowers?A. NectarB. PollenC. WaterD. Seeds73.What do you call the study of plants?A. BiologyB. BotanyC. ZoologyD. Chemistry74. A solution that conducts electricity well is called a(n) _______ electrolyte.75.I have a toy _______ that can roll and spin.76.Gamma-ray bursts are among the most energetic events in the ______.77. A ______ (土地恢复) project can rejuvenate ecosystems.78.I love to go ______ (爬山) on weekends.79.Understanding plant _____ (结构) helps in gardening.80.What is the currency used in the USA?A. EuroB. DollarC. PoundD. YenB81.The chemical formula for silicon dioxide is _____.82.The capital of India is _____ (61).83. A __________ is a large body of saltwater. (海洋)84.What do we call the time when the sun goes down?A. MorningB. AfternoonC. EveningD. NightC85.What is the largest organ inside the human body?A. LiverB. HeartC. BrainD. LungA86.What is the name of the scientific study of insects?A. EntomologyB. ZoologyC. BotanyD. MycologyA87.The _____ (花蜜) attracts bees and butterflies.88.The ________ (繁茂) of a garden is a joy to see.89.The chemical formula for magnesium oxide is __________.90.What do you call a young male horse?A. ColtB. FillyC. MareD. Foal91.What do you call the activity where you engage in physical exercise?A. Working outB. TrainingC. PracticingD. PlayingA92.What is the smallest continent?A. AsiaB. AfricaC. AustraliaD. EuropeC93.The __________ will help us know if we need to prepare for rain. (天气预报)94.I see a _____ (cat/dog) in the garden.95.The bus driver, ______ (公交车司机), is very friendly.96.The ________ (茎) transports nutrients.97.The __________ in spring brings new life to the garden. (雨水)98.I enjoy _____ (painting/drawing).99.The __________ is a body of water that separates Europe and Africa. (直布罗陀海峡)100._____ (秋天) changes the colors of leaves.。
Gamma-Ray Summary Report
a rXiv:as tr o-ph/21160v11Jan22Gamma-Ray Summary Report J.Buckley ∗Washington University,St.Louis T.Burnett †University of Washington G.Sinnis ‡Los Alamos National Laboratory P.Coppi §Yale University P.Gondolo ¶Case Western Reserve University J.Kapusta ∗∗University of Minnesota J.McEnery ††University of Wisconsin J.Norris ‡‡NASA/Goddard Space Flight Center P.Ullio §§SISSA D.A.Williams University of California Santa Cruz ¶¶(Dated:February 1,2008)This paper reviews the field of gamma-ray astronomy and describes future experiments and prospects for advances in fundamental physics and high-energy astrophysics through gamma-ray measurements.We concentrate on recent progress in the understanding of active galaxies,and the use of these sources as probes of intergalactic space.We also describe prospects for future experi-ments in a number of areas of fundamental physics,including:searches for an annihilation line from neutralino dark matter,understanding the energetics of supermassive black holes,using AGNs as cosmological probes of the primordial radiation fields,constraints on quantum gravity,detection of a new spectral component from GRBs,and the prospects for detecting primordial black holes.I.INTRODUCTIONWith new experiments such as GLAST and VERITAS on the horizon,we are entering an exciting period for gamma-ray astronomy.The gamma-ray waveband has provided a new spectral window on theuniverseand has already resulted in dramatic progress in our understanding of high energy astrophysical phenomena. At these energies the universe looks quite different then when viewed with more traditional astronomical tech-niques.The sources of high energy gamma rays are limited to the most extreme places in the universe:the remnants of exploding stars,the nonthermal Nebulae surrounding pulsars,the ultra-relativistic jets emerging from supermassive black holes at the center of active galaxies,and the still mysterious gamma-ray bursters. While understanding these objects is of intrinsic interest(how does nature accelerate particles to such high energies?how do particles andfields behave in the presence of strong gravitationalfields?),these objects can also be used as probes of the radiationfields in the universe and possibly of spacetime itself.In this case,the astrophysics of the object is a confounding factor that must be understood to produce a quantitative measurement or a robust upper limit.While some may view this as a limitation of such indirect astrophysical measurements,in most cases there are no earth-bound experiments that can probe the fundamental laws of physics at the energy scales available to gamma-ray instruments.Gamma-ray astronomy has developed along two separate paths.From the ground,simple,inexpensive exper-iments were built in the1950’s to observe the Cherenkov light generated by extensive air showers generated by photons with energies above several TeV.Despite decades of effort it was not until the late1980’s that a source of TeV photons was observed.There are now roughly10known sources of TeV gamma rays,three galactic sources and at least three active galaxies.From space,the COS-B satellite,launched in1975,observed thefirst sources of cosmic gamma rays at energies above70MeV.The launch of the Compton Gamma Ray Observatory (CGRO)in1991,with the Energetic Gamma Ray Experiment Telescope(EGRET)instrument,brought thefield to maturity.Whereas COS-B discovered a handful of sources,EGRET observed over65active galaxies[1],seven pulsars,many gamma-ray bursts,and over60sources that have no known counterparts at other wavelengths. The disparity in the development of the two techniques can be traced to the extremely lowfluxes of particles present above a TeV(∼4γfootballfield−1hr−1)and the cosmic-ray background.Above the earth’s atmosphere, one can surround a gamma-ray detector with a veto counter that registers the passage of charged particles. From the ground,one is forced to infer the nature of the primary particle by observing the secondary radiation generated as the extensive air shower develops.It was not until such a technique was developed for air Cherenkov telescopes[2],that sources of TeV photons were discovered.Despite these difficulties a new generation of ground-based instruments is under development that will have a sensitivity that will rival that of space-based instruments.At the same time a space-based instrument,GLAST,with a relatively large area(∼1m2)and excellent energy and angular resolution is scheduled to be launched in2005.In this paper we will give a brief survey of the gamma-ray universe and demonstrate some of the fundamental measurements(relevant to particle physicists)that can be made using distant objects that emit high-energy photons.What will hopefully become clear from this exposition are some development paths for future instru-ments.The need to see to the far reaches of the universe,makes a compelling case for ground-based instruments with energy thresholds as low as10GeV.The need to detect and study the many transient phenomena in the universe makes a compelling case for the development of an instrument that can continually monitor the entire overhead sky at energies above∼100GeV with sensitivities approaching that of the next generation of pointed instruments.As with any new branch of astronomy,it is impossible to predict what knowledge will ultimately be gained from studying the universe in a different waveband,but early results hint at a rich future.New and planned instruments with greatly increased sensitivity will allow us to look farther into the universe and deeper into the astrophysical objects that emit gamma rays.Gamma-ray astronomy can be used to study the most extreme environments that exist in the universe,and may also provide a number of unique laboratories for exploring the fundamental laws of physics at energies beyond the reach of earth-bound particle accelerators.II.PHYSICS GOALS OF GAMMA RAY ASTRONOMYA.Active Galactic NucleiActive galactic nuclei(AGN)are believed to be supermassive black holes,108−1010M⊙,accreting matter from the nucleus of a host galaxy.The accretion of matter onto a black hole is a very efficient process,capable of releasing∼10%of the rest energy the infalling matter(∼40%for a maximally rotating black hole).(For comparison fusion burning in stars releases∼0.7%of the rest energy.)Radio loud AGN emit jets of relativistic particles,presumably along the rotation axis of the spinning black hole.The COS-B instrument observed the first AGN in the gamma-ray regime(E>100MeV),3C273.But it was not until the launch of the CGRO and EGRET that many AGN could be studied in the gamma-ray regime.More recently,ground-based instruments have extended these observations into the TeV energy band.The energy output of these objects in gamma rays is of order1045ergs s−1,and many of these objects emit most of their energy into gamma rays.The relativisticmotion has several effects:1)the energy of the photons is blue-shifted for an observer at rest(us),2)the timescale is Lorentz contracted(further increasing the apparent luminosity),and3)the relativistic beaming suppresses photon interactions.Thus,one expects that AGN observed in the TeV regime should have their jets nearly aligned with our line-of-sight.The types of AGN detected at high energies,which includeflat spectrum radio quasars(FSRQs)and BL Lacertae(BL Lac)objects,are collectively referred to as blazars.The Whipple Observatory10m atmospheric Cherenkov telescope demonstrated that the emission spectra of several blazars extend into TeV energies.Two of these detections(Markarian421and Markarian501)have been confirmed by independent experiments(CAT and HEGRA),at significance levels of between20σin a half hour to80σfor a season.Blazar emission is dominated by highly variable,non-thermal continuum emission from an unresolved nucleus. The broadband emission and high degree of polarization suggest synchrotron radiation extending from radio up to UV or even hard X-ray energies.The short variability timescales and high luminosities are thought to result from highly relativistic outflows along jets pointed very nearly along our line of sight.The spectral energy distributions(SEDs)of these objects have a double-peaked shape(see Figure1)with a synchrotron component that peaks in the UV or X-ray band,and a second component typically rising in the X-ray range and peaking at energies between∼1MeV and1TeV[3].The most natural explanation of the second peak is inverse-Compton scattering of ambient or synchrotron photons[4]although other possibilities such as proton-induced cascades have not been ruled out[5].These two models have somewhat complementary strengths and weaknesses.Since electrons are lighter than protons,they can be confined in a smaller acceleration region but lose energy more quickly(by synchrotron and IC emission),making it difficult to accelerate electrons to extreme energies.For hadronic models,very high energies can be attained given sufficient time,a large acceleration region and high magneticfields.However,the short variability timescales,implying short acceleration times and compact regions are difficult to explain.In addition,the electron models make natural predictions on the correlation between X-ray and gamma ray luminosities.While it has been claimed that proton models can be constructed that explain these correlations,detailed calculations have not appeared in the literature.Whipple observations of the vast majority of EGRET blazars have yielded only upper limits[6,7,8];Mrk421 (z=0.031)[9]being the exception.Subsequent searches for emission from X-ray bright BL Lac objects has led to the detection of Mrk501(z=0.034)[10],and four other as yet unconfirmed sources[1ES2344+514 (z=0.044[11],1ES2155-304(z=0.117)[12],1ES1959+650(z=0.048)[13]and1H1426+428(z=0.13)[14]]. The SEDs observed for these sources show higher energy synchrotron andγ-ray peaks,and comparable power output at the synchrotron andγ-ray peak.These observations are well described by the classification scheme of Padovani and Giommi[15].The AGN detected by EGRET are all radio-loud,flat-spectrum radio sources and lie at redshifts between0.03and2.28. They are characterized by two component spectra with peak power in the infrared to optical waveband and in the10MeV to GeV range.For many of the GeV blazars,the total power output of these sources peaks in the gamma-ray waveband.The objects detected at VHE,appear to form a new class distinct from the EGRET sources.All are classified as high-energy peaked[15]BL Lacs(HBLs)defined as sources with their synchrotron emission peaked in the UV/X-ray band and gamma-ray emission peaking in the∼100GeV regime(see,e.g.,Fig.1).The correspondence of the position of the peak of the synchrotron andγ-ray energy is naturally explained in models where the same population of electrons produces both spectral components.Proton induced cascade models[5]might also reproduce the spectra,but have no natural correlation in the cutoffenergy of the two components,or the observed correlated variability.Another difference in the VHE detections is that only the nearest sources with redshifts z<∼0.1have been detected.The sensitivity of EGRET for a one-year exposure is comparable to that of Whipple for a50hour exposure for a source with spectral index of2.2.The failure of ACTs to detect any but the nearest AGNs therefore requires a cut-offin theγ-ray spectra of the EGRET sources between10GeV and a few hundred GeV. This cutoffcould be intrinsic to the electron acceleration mechanism,due to absorption offof ambient photons from the accreting nuclear region[16],or caused by absorption via pair production with the diffuse extragalactic background radiation[17,18].While the latter mechanism establishes an energy-dependent gamma-ray horizon it can also be used to measure the radiationfields thatfill intergalactic space.In the framework of Fossati et al.,[19]the low energy peaked EGRET BL Lacs(LBLs)correspond to AGNs with a more luminous nuclear emission component than HBLs.The relatively high ambient photon density in the LBLs is up-scattered by relativistic electrons toγ-ray energies.With high enough ambient photon densities, the resulting inverse-Compton emission can exceed that resulting from the up-scattering of synchrotron photons. This accounts for the observation of relatively high levels of gamma-ray emission,dominating the power output over the entire spectrum.The higher luminosity could also shut down the acceleration process at lower energies.For lack of another viable hypothesis,consider the common hypothesis that the energetic particles in AGNs come from electronsor protons accelerated by relativistic shocks traveling down the AGN jets.In the model of diffusive shock acceleration(essentially thefirst order Fermi process),particles are accelerated as they are scattered from magnetic irregularities on either side of a shock.For strong,non-relativistic shocks,a constant escape probability with each shock crossing results in an∼E−2spectrum,close to that observed.More realistic models including nonlinear effects lead to slightly steeper spectra;if the shock velocity is relativistic the spectral index may range from1.7to2.4.In any event,an electron spectrum∼E−γwill give rise to synchrotron radiation with a spectral indexα=(γ−1)/2,in good agreement with observations.The maximum energy attainable is given by equating the rate of energy loss from synchrotron emission or inverse-Compton emission to the acceleration rate as given by the shock parameters.In the low-energy peaked objects,it is thought that high ambient photon densities result in inverse-Compton losses that dominate over synchrotron losses and limit the maximum electron energy achieved by shock acceleration.Thus one also obtains a natural explanation for the lower energies of the peak synchrotron and IC power in these objects.In HBLs, the ambient photonfields are presumably weaker and self-Compton emission dominates over Comptonization of external photons(EC).Electrons can reach higher energies by shock acceleration,and the peaks in the SED move to higher energies and have more nearly equal peak power.This model is consistent with the data and serves as a useful paradigm for searching for new VHE sources.The SEDs shown in Fig.1,combine the results of a number of different measurements of the X-ray and VHE spectra of Mrk501,and compare them with simple synchrotron self-Compton(SSC)models(see Buckley[20] and references therein).The agreement between the spectral measurements and the model is exceptionally good for Mrk501.1.Multiwavelength Observations:VariabilityData taken on Mrk421over the years1995[21]to2001[22]show that theγ-ray emission is characterized by a succession of approximately hour-longflares with relatively symmetric profiles(see Figure2).While most of the multiwavelength observations of Mrk421show evidence for correlated X-ray and gamma ray variability,the nature of the correlation is unclear and the data have traditionally undersampled the variability. However,a multi-wavelength campaign conducted on Mrk501in1997revealed a strong correlation between TeVγ-rays and soft X-rays(the50–500keV band detected by OSSE)(Fig.1).Recent multiwavelength observations of Mrk421made during the period March18,2001to April1,2001 with the Whipple gamma-ray telescope,and the Proportional Counter Array(PCA)detector on the Rossi X-ray Timing Explorer(RXTE)better sample the rapid variability of Mrk421.Key to the success of this campaign is the nearly continuous>330ks exposure with RXTE[23].Numerous ground-based atmospheric Cherenkov and optical observations were scheduled during this period to improve the temporal coverage in the optical and VHE bands.Frequent correlated hour-scale X-ray andγ-rayflares were observed.Fig.2shows a subset of these data showing the close correlation of the well-sampled TeV and X-ray(2–10keV)lightcurves on March 19,2001[22].Leptonic models provide a natural explanation of the correlated X-ray and gamma-rayflares,and can re-produce the shape of theflare spectrum.The simplest model for blazar emission is the one-zone synchrotron self-Compton(SSC)model where energetic electrons in a compact emission region up-scatter their own syn-chrotron radiation.As shown in Fig.1,such a model results in surprisingly goodfits to the Mrk501SED. In the SSC model,the intensity of the synchrotron radiation is proportional to the magnetic energy density and the number density of electrons I synch∝n e.Since these same electrons up-scatter this radiation,the IC emission scales as I IC∝n2e.Thus we expect I IC∝I2synch.Krawczynski et al.,[24]examined the correlation of TeVγ-ray and X-ray intensity for several strongflares of Mrk501in1997.The results,plotted in Figure3,show evidence for such a quadratic dependence.(However the possibility of a baseline level of the X-ray emission can not be excluded.)While the interpretation of these observations is not unambiguous,this analysis is an important example of what can be learned with continued multiwavelength studies of AGNs.How do these observations constrain the alternative hypothesis that proton induced cascades(PIC),not elec-trons,are responsible for the gamma-ray emission?In the hadronic models of Mannheim and collaborators,the gamma-ray emission typically comes from synchrotron emission from extremely energetic,secondary electrons produced in hadronic cascades.Since a viable hadronic target for pp→ppπappears to be lacking(except per-haps in the broad line clouds),the assumption is made that the cascade begins with ultrarelativistic particles interacting with ambient photons to produce pions.This implies proton energies in excess of10∼16eV.The neutral pions presumably give rise to gamma rays and electromagnetic cascades,while the charged pions could give a neutrino signal.These models have attracted much interest since,in the most optimistic cases,these models may produce an observable neutrino signal and may provide a mechanism for producing the ultra-high energy cosmic rays.If the sources are optically thick to the emerging protons(i.e.,they absorb some fractionThis figure is available as p42_fig1a.gif051000.51100200300120.80.91F l u x (γ/m i n )F l u x (c n t s /s )F l u x (c n t s /s )F l u x (c n t s /s )MJDF l u x (a r b i t r a r y u n i t s )FIG.1:Left:SED of Mrk 501from contemporaneous and archival observations.Right:Multi-wavelength observations of Mrk 501;(a)γ-ray,(b)hard X-ray,(c)soft X-ray,(d)U-band optical light curves during the period 1997April 2–20(April 2corresponds to MJD 50540).The dashed line in (d)indicates the optical flux in 1997March.(from [20]and references therein.)This figure is available as p42_fig2.gifFIG.2:Simultaneous X-ray/γ-ray flare observed on March 19,2001.The 2–10keV X-ray light curve was obtained with the PCA detector on RXTE [22,23];data points are binned in roughly 4minute intervals.of the cosmic rays,but not the neutrinos)then it may be possible to produce a relatively large neutrino signal without overproducing the local cosmic ray flux [25].While these models have a number of attractive features,there is some debate about whether they can provide a self-consistent description for the observations.To overcome the threshold condition for pion production,protons must have energies in excess of 1016to 1018eV where abundant infrared photons can provide the target.Since the cross section for photo-pion produc-tion is relatively low,very high ambient photon densities are required to initiate the cascades.In this case,pair creation (γγ→e +e −),which has a much higher cross-section,must be important.The proton cascade models may well have a significant problems explaining the emission from objects like Mkn 421/501for this reason.FIG.3:Plot of TeVγ-rayflux versus X-rayflux measured with the HEGRA experiment during an intenseflare of Mrk501(courtesy Henric Krawczynski).In the PIC models[5]the proton-photon interaction occur with radio-IR photons in the jet.While a detailed analysis has not been published,Aharonian and others have pointed out that the required photon densities also imply large pair production optical depths,and may mean that the PIC models are not self-consistent. Models where the primary protons produce synchrotron radiation(and subsequent pair-cascades)may avoid this problem,but require even larger magneticfields[26].One advantage of the photon-pair cascade is that it produces a rather characteristic spectrum that does not depend sensitively on the model parameters.The detailed shape of this spectrum does not match some observations.Typically the spectra are too soft and overproduce X-rays,giving a spectrum that does not reproduce the strongly double-peaked spectrum observed.For the typical magneticfield values,the synchrotron spectrum is often too soft and lacks the spectral breaks that are observed.For these hadronic models to account for the double-peaked spectrum,the radio to X-ray emission is most likely produced by primary shock-accelerated electrons,while the gamma-ray emission is produced by energetic secondary electrons from the cascade.There is no natural explanation for the correlated variability in the two spectral bands,or in the correlation in the X-ray and gamma-ray cutoffenergy.To reach these energies on a sufficiently short timescale,the gyroradius must be limited to a compact region in the jet,the inverse-Compton emission must be suppressed,and magneticfields of up to40Gauss are required. The spectral variability seen in the X-ray waveband is consistent with much longer synchrotron cooling times than predicted by the hadronic models,and is quite consistent with magneticfields of a10to100mGauss. This is the same value of the magneticfield derived by a completely independent method within the framework of the synchrotron inverse-Compton model.The criticisms leveled at the electron models are that the magneticfields are too small compared with the value required for magnetic collimation of the jets,and that the required electron energies are too large to be explained by shock acceleration.Moreover,electron injection into shocks is poorly understood since the electron gyroradius is small compared to the proton gyroradius and presumably to the width of the broadened shock front.However we know that electrons are accelerated to100TeV energies in supernovae shocks,regardless of the theoretical difficulties in accounting for this observation.As will be shown below,if one accepts relatively large Doppler factors,a self-consistent explanation for the VHE gamma-ray emission can be derived from leptonic models.In the framework of either the EC or SSC models theγ-ray and X-ray data can be used to constrain the Doppler factorδ(this is thought to be close to the bulk Lorentz factor of the jet for blazars)and magneticfield B in the emission regions of Mrk421and Mrk501.The maximumγ-ray(IC)energy E C,max provides a lower limit on the maximum electron energy(with Lorentz factorγe,max)given byδγe,max>E C,max/m e c2;combining this with the measured cut-offenergy of the synchrotron emission E syn,max one obtains an upper limit on thelog n ,Hz -13-12-11-10-9-8l o g n F n ,e r g s -1c m -2FIG.4:Model fit to Mrk 421SED with both an SSC and external Compton component[20]magnetic field B <∼2×10−2E syn ,max δE −2C ,max (where E C ,max is in TeV).A lower limit on the magnetic fieldfollows from the requirement that the electron cooling time,t e ,cool ≈2×108δ−1γ−1e B −2s,must be less than theobserved flare decay timescale.These limits depend on the Doppler factor of the jet and in some cases cannot be satisfied unless δis significantly greater than unity [27,28].Typically,these arguments lead to predictions of ∼100mGauss fields and Doppler factors δ>10to 40for Mrk 421.Similar values for Mrk 501but typically with a reduced lower limit on the Doppler factor.Model fits (that ignore the fact that the multiwavelength data are not truly time-resolved)give similar values for the Doppler factor and magnetic field strength.For example,a simple one-zone model fit for Mrk 421,shown in Fig.4,only gives good fits for a Doppler factor approaching a value of δ≈100(as shown)[20].Doppler factors this large may present other problems.Radio observations of jets show radio components moving with velocities that imply bulk Lorentz factors Γ<∼10further out in the jet.If the jet is decelerated by the inverse-Compton scattering,most of the energy would be used up before such extended radio lobes could form in apparent contradiction to observations.Given the good progress to date,it appears that it will be possible to determine the dominant radiation processes in AGNs.After this first issue is resolved,further multiwavelength observations can address the more fundamental questions about the energetics of the central supermassive black hole,and the processes behind the formation of the relativistic jets.The very short variability timescales already observed with the Whipple instrument (15minute doubling times for Markarian 421)hint that the gamma-ray observations may be probing very close to the central engine,beyond the reach of the highest resolution optical and radio telescopes.B.Gamma-Ray BurstsGamma-ray bursts (GRBs)were discovered by the Vela satellites in the late 1960’s [29].GRBs are bright flashes of hard X-rays and low energy gamma rays coming from random directions in the sky at random times.Until the launch of the CGRO in 1992it was generally believed that GRBs were galactic phenomena associated with neutron stars.The BATSE instrument on-board the CGRO detected over 2000GRBs and the observed spatial distribution was isotropic,with no evidence of an excess from the galactic plane.Thus GRBs were either cosmological or populated an extended galactic halo.In 1997the BeppoSax satellite was launched.With a suite of hard X-ray detectors,this instrument has the ability to localize GRBs to within ∼1minute of arc [30](BATSE could localize GRBs to within ∼5degrees).The increased angular resolution allowed conventional ground-based telescopes to search the error box without significant source confusion.The observation of emission and absorption lines from the host galaxies led to measurements of redshifts;some thirty years after their discovery the cosmological nature of gamma-ray bursts was determined.In Figure II B we show the redshift distribution of those gamma-ray bursts where the redshift has been determined.The enormous energy output from GRBs,and transparency of the universe below 100MeV makes GRBs visible across the universe.Thus gamma-rayFIG.5:The magnitude redshift distribution of gamma-ray bursts.Also shown on the plot is the magnitude vs.redshift relation for the observed type Ia supernovae.bursts have the potential to probe the universe at very early times and to study the propagation of high-energy photons over cosmological distances.To use GRBs as cosmological probes it is necessary to understand their underlying mechanism.While GRBs may never be standard candles on par with the now famous Type-IA supernovae,there has been great progress made in the lastfive years in understanding GRBs.While we still do not know what the underlying energy source is,we are beginning to understand the environment that creates the observed high-energy photons. The large distances to GRBs implies that the energy released is∼1050−54ergs,depending on the amount of beaming at the source.While the origin of the initial explosion is unknown,the subsequent emission is well described by the relativisticfireball model.In this model shells of material expand relativistically into the interstellar medium.The complex gamma-ray light-curves of the prompt radiation arises from shocks formed as faster and slower shells of material interact.A termination shock is also formed as the expanding shells of material interact with the material surrounding the GRB progenitor.In this model the observed afterglows (x-ray,optical,and radio)arise from the synchrotron radiation of shock accelerated electrons.The afterglow emission can be used to determine the geometry of the source.Since the shell is expanding relativistically,the radiation(emitted isotropically in the bulk frame)is beamed into a cone with with opening angleΓ−1(the bulk Lorentz factor of the material in the shell).Thus at early times,only a small portion of the emitting surface is visible and one cannot distinguish between isotropic and beamed(jet-like)emission. However,as the shell expands it sweeps up material andΓdecreases.If the emission is not isotropic the beaming angle(Γ−1)will eventually become larger than the opening angle of the jet.At this point one should observe a break in the light curve(luminosity versus time)of the afterglow.This distinctive feature has been observed in15GRBs.By measuring the temporal breaks in GRBs of known redshift Frail et al.,[31]have measured the jet opening angles of15gamma-ray bursts(with some assumptions about the emission region:the jet is uniform across its face,the electron distribution in the shock is a power law,the afterglow radiation is due to synchrotron emission and inverse Compton scattering).If one integrates the observed luminosity over the inferred jet opening angle one can determine the intrinsic luminosity of each GRB.Surprisingly,Frail et al., conclude that the intrinsic luminosities of the observed gamma-ray bursts are peaked around5×1050ergs with a spread of roughly a factor of six.Thus the observed variation in luminosity(a factor of∼500)may be mainly due to the variation in the jet opening angle.Note that this conclusion applies only to the“long”GRBs,as these are the only GRBs for which optical counterparts have been observed.With a similar goal,to reduce the wide divergence in the observational properties of GRBs,Norris[32]has found a correlation between energy dependent time lags and the observed burst luminosity.Three things occur as one moves from high energy photons to low energy photons.The pulse profiles widen and become asymmetric, and the centroid of the pulse shifts to later times.The time lag is defined as the shift in the centroid of the pulse profile in the different energy channels of the BATSE instrument.In Figure II B we show the observed luminosity(assuming isotropic emission)versus the time lag observed between two energy channels on the BATSE experiment.(Channel1corresponds to photons with energies between25–50keV and channel3to 100300keV photons.)The line is the function,L53=1.1×(τlag/0.01s)−1.15,where L53is the luminosity in units of1053ergs.It may be that the time lag is dependent upon the jet opening angle for reasons that are not yet understood and this observed correlation is simply an way of paramterizing the relationship observed by Frail et al.As discussed above,gamma-ray observations of AGNs revealed a new spectral component due to inverse-Compton emission,distinct from the synchrotron emission observed in the radio to X-ray wavebands.This observation resulted in an independent constraint on the electron energy that allowed a determination of the magneticfields,electron densities,and bulk Lorentz factors in the sources.While AGNs are quite different for GRBs,the non-thermal radiation mechanisms may be quite similar,and we might expect similar progress to follow from high energy gamma-ray measurements.At higher energies less is known about GRBs.The EGRET instrument covered the energy range from100 MeV to a few tens of GeV.EGRET detected several GRBs at high energy(HE E>100MeV).From EGRET。
伽马射线暴
• 伽马射线暴可以分为两种：一种是明亮伽马射线暴，是 所观测到的宇宙中强度最大、最为明亮的爆炸，能够产 生大量的可见光，远在数十亿光年外的地球上的望远镜 能够轻易地观测到。另外一种是黑暗伽马射线暴，包含 大量的伽马射线和X射线，但却几乎没有可见光。
• 伽玛射线暴 （Gamma Ray Burst, 缩写 GRB），又称 伽玛暴，是来 自天空中某一 方向的伽玛射 线强度在短时 间内突然增强， 随后又迅速减 弱的现象，持 续时间在0.11000秒，辐射 主要集中在 0.1-100 联系在 一起的。超大质量 恒星迅速老化、爆 炸，散发出的星际 尘埃快速充斥于星 云之中，超大质量 爆炸产生的新物质 也被喷发进星云之 中，星云密度变得 很大，孕育新的恒 星诞生。在充斥着 星际尘埃的星系， 大量的恒星生死轮 回正在发生着。由 于恒星形成于星际 尘埃区域，可推测 包裹黑暗伽马射线 暴的尘埃团可能是 孕育恒星的诞生之
小学上册第二次英语第二单元期末试卷(有答案)
小学上册英语第二单元期末试卷(有答案)英语试题一、综合题(本题有100小题，每小题1分，共100分.每小题不选、错误，均不给分)1.The ______ (小鼠) scurries quickly across the floor.2.trial Revolution changed how _______ were made. (商品) The Indu3.My favorite pet is a ______ (狗) that loves to play fetch.4.gs were known for their long _______. (船) The Wrig5.The butterfly flutters from flower to _______.6.The chemical process of digestion involves breaking down _____.7.My friend is __________ (热爱学习的).8.What is the name of the famous detective created by Arthur Conan Doyle?A. Hercule PoirotB. Sherlock HolmesC. Philip MarloweD. Sam Spade答案: B9.An amphoteric substance can act as both an acid and a ______.10.The teacher, ______ (老师), explains difficult concepts clearly.11.The chemical formula for ammonium chloride is ______.12.The ______ (蝴蝶) flutters around flowers.13.What is the capital of Uganda?A. KampalaB. NairobiC. Addis AbabaD. Kigali答案:A. Kampala14.What is the capital of Italy?A. RomeB. VeniceC. FlorenceD. Milan答案:a15.The _____ (猫) loves to curl up in a cozy spot.16.The __________ can serve as a natural barrier to erosion.17.Changes in temperature can affect the rate of a ______.18.The ancient civilization of ________ is known for its complex society.19.The __________ (历史的启示) guides our journey.20.The _____ (天空) has clouds.21.How many letters are there in the English alphabet?A. 24B. 25C. 26D. 27答案: C. 2622.He is a _____ (作家) known for his novels.23.The __________ helps to maintain the earth's ecosystems.24.Which planet is known as the Red Planet?A. EarthB. MarsC. JupiterD. Saturn答案: B25.The chemical formula for ferrous sulfate is _______.26.An oxidizing agent is a substance that ________ electrons.27.My cousin has a pet ____ (fish) in a tank.28.What is the capital of Mongolia?A. UlaanbaatarB. HohhotC. LhasaD. Bishkek答案: A29. (Revolution) in Russia led to the rise of the Soviet Union. The ____30.What do we call a young mouse?A. KitB. PupC. PinkyD. Calf答案:C.Pinky31.The main component of starch is ______.32.I love to _______ (写) poems.33. A __________ is produced when a gas cools down.34. A base can turn red litmus paper ______.35.When I go to the movies, I like to bring my favorite ________ (玩偶) along.36. A thermometer measures ______ (temperature).37. A chemical that can act as a nucleophile is a ______.38.The __________ (历史的文学作品) reflect societal values.39.The chemical symbol for iron is ______.40.We need to water the ______ (植物).41.What is the capital city of Malta?A. VallettaB. MdinaC. SliemaD. St. Julian's答案: A42.They are _____ (fishing) at the lake.43.She is ________ (kind) to her friends.44. A lizard can change color to blend into its ______ (环境).45.We have a ______ (丰富的) curriculum at school.46.The chemical formula for bismuth oxide is _______.47.We visit the ______ (科学中心) to explore.48. A reaction that releases energy is called an ______ reaction.49.What is the capital city of the Dominican Republic?A. Santo DomingoB. SantiagoC. La RomanaD. Puerto Plata答案: A50.The chemical symbol for francium is _______.51.He is eating a ___. (sandwich)52.The dog wags its _____ (tail/ears) when happy.53.What do you call the main character in a story?A. AntagonistB. ProtagonistC. Supporting CharacterD. Narrator答案: B54.The _______ (The Great Depression) led to significant changes in government policy.55.I have a special ________ that I cherish.56.The process of drying a wet substance is called ______.57.The rabbit is _____ the carrot. (eating)58. A ______ (蜥蜴) can be quite colorful and interesting.59.I enjoy _______ new friends at school.60.The rabbit is ________ (跳) in the garden.61.__________ are used in the production of batteries.62. A toad prefers moist ______ (环境).63.The leaves on the _______ turn red in autumn.64.I can learn new things through my ________ (玩具名称).65.I enjoy ______ (listening) to music.nd use planning) balances development and conservation. The ____67.The ______ (植物的物种组成) affects biodiversity.68. A ____ has a soft, fluffy tail and loves to dig.69.The park is ___ (quiet/loud).70.Ferrous metals are prone to ______.71.Distillation separates mixtures based on their ________ points.72.What is the name of the famous artist known for his "Starry Night" painting?A. Claude MonetB. Vincent van GoghC. Pablo PicassoD. Henri Matisse答案: B73.The main element found in diamonds is __________.74.The chemical formula for ethanol is __________.75.The _______ of an element is determined by the number of protons it has.76.The ____ is a wise creature often associated with knowledge.77. A chemical bond can form between ______.78. A _______ (兔子) can hop very high.79.The _______ is important for supporting local wildlife.80.What do we call the main ingredient in pasta?A. FlourB. RiceC. CornD. Wheat答案: D. Wheat81.The city of Pompeii was buried under _______. (火山灰)82.Photosynthesis converts light energy into ______ energy.83.I can ________ very well.84.I love to watch ________ bloom in the spring.85. A _______ is a solution that contains more solute than it normally would at a given temperature.86.The ancient Romans had a complex system of ________.87.ean colonization of Africa is known as the ________ (瓜分). The Fall88.My sister is a good ________.89.The manager, ______ (经理), organizes the team.90.What is the name of the famous painting by Leonardo da Vinci?A. The Starry NightB. The Last SupperC. The Girl with a Pearl EarringD. Mona Lisa答案: D91.The __________ of a jellyfish is transparent.92.The ______ (果树) produces fruits in summer.93.It’s important to be ______ (尊重) towards others and listen to their opinions. Everyone has something valuable to share.94.My favorite drink is ______.95.The cheese is very _______ (tasty).96.The process of osmosis involves the movement of __________.97. A ______ is a type of energy that can cause changes in matter.98.The main component of nucleotides is _____.99.Gamma-ray bursts are among the most energetic events in the ______. 100. A ______ can be found in lakes and rivers.。
神秘的宇宙作文英语
神秘的宇宙作文英语Title: The Mysteries of the Universe。
The universe, a vast expanse of space and time, has captivated the human imagination for millennia. Its mysteries, both awe-inspiring and perplexing, have fueled our curiosity and driven scientific exploration. From the formation of galaxies to the existence of dark matter, the universe presents countless enigmas waiting to be unraveled.One of the most profound mysteries of the universe isits origin. How did everything begin? This question has puzzled scientists and philosophers alike for centuries.The prevailing scientific theory, the Big Bang theory, suggests that the universe originated from a singular point of unimaginable density and temperature approximately 13.8 billion years ago. However, what triggered the Big Bang remains unknown, leaving us with a fundamental unanswered question about the nature of existence itself.Another mystery lies in the composition of the universe. Ordinary matter, which makes up stars, planets, and everything we can see, constitutes only a small fraction of the total mass-energy content of the universe. The rest is comprised of dark matter and dark energy, both of which remain elusive and poorly understood. Dark matter interacts gravitationally with ordinary matter but does not emit, absorb, or reflect light, making it invisible anddetectable only through its gravitational effects onvisible matter. Dark energy, on the other hand, is thoughtto be responsible for the accelerated expansion of the universe, yet its origin and nature remain largely speculative.The universe is also home to countless celestial bodies, from stars and planets to asteroids and comets. While we have made significant strides in understanding theformation and evolution of these objects, many mysteries persist. For example, the existence of exoplanets—planets orbiting stars outside our solar system—raises questions about the prevalence of life elsewhere in the universe. Could there be other habitable worlds capable of supportinglife as we know it? Or are we alone in the cosmos?Furthermore, the universe is riddled with phenomenathat defy conventional explanation. Black holes, for instance, are regions of spacetime where gravity is so intense that nothing, not even light, can escape. These cosmic behemoths result from the collapse of massive stars, yet their properties and behavior challenge our understanding of physics. What happens beyond the event horizon of a black hole? Can anything exist inside it, or does it mark the end of space and time?Cosmic phenomena such as supernovae, gamma-ray bursts, and quasars continue to baffle scientists with their extreme energies and dynamics. While we have observed these phenomena from afar, their underlying mechanisms remain shrouded in mystery. Unraveling the mysteries of the universe requires innovative technologies and collaborative efforts across disciplines, from astrophysics and cosmology to particle physics and quantum mechanics.In our quest to understand the universe, we areconfronted not only with scientific challenges but also with existential questions about our place in the cosmos. What is our significance in the vastness of space? Are we merely specks of dust in an infinite void, or do we play a more meaningful role in the grand tapestry of the universe?Despite the mysteries that abound, each discovery brings us closer to unraveling the secrets of the cosmos. Whether through space telescopes peering into distant galaxies or particle accelerators probing the fundamental nature of matter, humanity continues to push the boundaries of knowledge and exploration. As we gaze upon the stars with wonder and curiosity, we are reminded of the boundless mysteries that await us in the infinite expanse of the universe.。
高三英语试卷带答案解析
高三英语试卷带答案解析考试范围：xxx；考试时间：xxx分钟；出题人：xxx姓名：___________班级：___________考号：___________1．答题前填写好自己的姓名、班级、考号等信息2．请将答案正确填写在答题卡上一、单项选择1.The Day of the Dead, an ancient Mexican festival, is not a sad day, but____time to celebrate ____ cycle of life.A．a, the B．不填, the C．不填, a D．the, a2.I had hardly got to the office________my wife phoned me to go back home at once.(2012·大纲全国Ⅱ，11)A．whenB．thanC．untilD．after3.It was not until near the end of the letter ______ she mentioned her own plan.A．where B．that C．why D．when4.．_____the crisis of economy getting more and more serious, the government is searching for ways to improve people’s life.A．When B．If C．As D．With5.139. Yao Ming is very famous now. However, his basketball career _____ only after many years of hard work.A．took on B．took off C．took up D．took out6.Yao Ming’s basketball career _______ after years of hard work and now he is popular in the world.A．took on B．took off C．took up D．took out7.81. Male and female students are quite different from each other ______ the age at which they begin to develop an intellectual self-discipline. A．with regard to B．in the light of C．in honor of D．on account of8.When the whole world is rainy，let’s make it________ in our heart．A．clean B．clear C．tidy D．tiny9.— Have you ever been to the Summer Palace?—I can’t quite remember．_______, it might have been during my childhood.A．If anyB．If everC．If possibleD．If so10.The grandma wants to train her grandson to be a musician because she finds he has a ______ for music.A．giftB．presentC．powerD．strength评卷人得分二、完形填空On a bright Friday afternoon in spring, Sumeja Tulic had every reason to enjoy walking in the streets of New York, a city she'd_______nine months earlier from London to attend a journalism school. “When the weather is good, it's very hard to find a reason to be_______or dissatisfied with the city,” she said.Yet her time in New York has coincided with endless ugliness, As she_______toward the subway station, she thought, “Please, G od, I want to see something_______today.” She said, “Enough of this craziness”.At the City Hall_______, she settled onto a bench. It was just after 2 p.m. Only a few people were there. A man___________against a pillar（柱子）, the way anyone might, waiting for the train. The stillness was interrupted by a(n)___________that the next train was two stations away. Then Tulic_______the man at the pillar collapsing forward onto the tracks.A man, who was_______waiting for the train on the platform, ran over, peered over the edge and then jumped onto the________.The man who had________was not moving. Two more men jumped down to help."I don't know________these men got the wit and the quickness," Tulic said. "The man who fell was kind of jammed in the tracks. They were very________to know that the train was coming. Will it stop? Will they________pulling him out?"On the tracks, the________man was held up to a sitting position by the three men, who then lifted him from below to________who dragged him from above and rolled him onto the platform. Then the rescuers were themselves ____________, pulled back to safety by helping hands. As soon as they were all clear, the train pulled in.An ambulance________soon and the man was taken to a local hospital with________but non-life-threatening injuries, doctors said."That is the greatest thing." Tulic said," The infrastructure(基础设施)in this city of millions is the________themselves providing, being there for others."11.A．hung out B．moved to C．passed by D．left behind12.A．pleased B．ashamed C．depressed D．relieved13.A．walked B．rushed C．travelled D．toured14.A．urgent B．strange C．mysterious D．nice15.A．station B．school C．theatre D．store16.A．struggled B．leaned C．lay D．sat17.A．note B．report C．announcement D．poster18.A．remembered B．foresaw C．ignored D．glimpsed19.A．again B．also C．never D．seldom20.A．tracks B．road C．train D．platform21.A．stood B．settled C．fallen D．escaped22.A．whom B．whether C．when D．where23.A．nervous B．embarrassed C．dangerous D．disappointed24.A．object to B．succeed in C．put off D．give up25.A．unconscious B．dying C．active D．discouraged26.A．them B．us C．others D．anyone27.A．jammed B．recognized C．affected D．rescued28.A．stopped B．started C．arrived D．raced29.A．careless B．serious C．slight D．unforgettable30.A．people B．passengers C．friends D．students阅读下面短文，从短文后各题所给的四个选项(A、B、C和D)中，选出可以填入空白处的最佳选项.We often hear the saying “we often hurt those we love or those closest to us!”Think of the times that we were ________ or upset with someone or about something that happened. Those emotions are ________ with us when we arrive home. How do we ________ them? We let them out on whoever may be the ________ person in the line of fire. It could be our wife or husband, children and even the dog or cat. ________ this behavior is not respectful, we have all done it.We understand this part of being ________ and we learn to deal with it. Usually we say sorry for our bad behavior later to the man we hurt. But kids so not get ________! They don’t ________ that it’s just because we are human or it’s just a reaction to som ething totally unrelated. We may raise our voice,________, or even slam a door. We may even ________ our child a fool and ask them ________ they are so stupid.To a kid this is hurtful and ________, which can cause anxiety and a ________ of confidence. The more it ________, the worse the child feels. It can affect their social life, school life and even life at home, causing them to become ________, or turn inward. These emotions ________ much worse behavior if not dealt with.Children need to be ________ with respect and honesty. Once hurt happens, make sure we ________ to that child, get his full attention and wholeheartedly ________ for our loss of control. Take full ________ for our actions.31.A．happy B．angry C．excited D．satisfied32.A．still B．yet C．again D．only33.A．handle B．avoid C．change D．protect34.A．last B．good C．wise D．first35.A．Until B．After C．Although D．Before36.A．important B．human C．specific D．friendly37.A．it B．them C．one D．those38.A．realize B．suspect C．worry D．wonder39.A．whistle B．laugh C．mourn D．yell40.A．name B．call C．make D．offer41.A．why B．how C．when D．where42.A．encouragingB．damagingC．boringD．interesting43.A．sense B．lack C．kind D．part44.A．occurs B．grows C．proves D．fails45.A．honest B．brave C．aggressive D．active46.A．break into B．rely on C．lead to D．result from47.A．treated B．cheated C．controlled D．blamed48.A．shout B．go C．come D．rush49.A．apologize B．pray C．regret D．care50.A．pleasure B．offence C．pressure D．responsibility阅读下面短文，掌握其大意，然后从第36至第55小题所给的A、B、C、D四个选项中，选出最佳选项，并在答题卡上将该项涂黑。
gamma单词
gamma单词单词：gamma1. 定义与释义1.1词性：名词1.2中文释义：希腊字母表的第三个字母（Γ，γ）；伽马（磁场强度单位）；灰度（非线性图像编辑术语）。
1.3英文释义：The third letter of the Greek alphabet; a unit of magnetic field strength; in non - linear image editing, it refers to the degree of gray scale.1.4相关词汇：gammon（熏腿；胡说），gammoid（拟阵）。
---2. 起源与背景2.1词源：“gamma”来源于古希腊语。
2.2趣闻：在数学中，伽马函数是一个非常特殊的函数，它与阶乘有密切的关系。
在物理学中，伽马射线是一种高频电磁波，在天体物理等众多领域有重要意义，而这个名字也源于希腊字母“gamma”。
---3. 常用搭配与短语3.1短语：gamma ray（伽马射线）例句：Gamma ray bursts are one of the most energetic events in the universe.翻译：伽马射线暴是宇宙中最具能量的事件之一。
3.2短语：gamma correction（伽马校正）例句：The gamma correction can improve the visual quality of the image.翻译：伽马校正可以提高图像的视觉质量。
---4. 实用片段(1). "Look at this equation, there is a gamma symbol in it. What does it represent?" The student asked the teacher. The teacher replied, "In this context, gamma represents a certain variable."翻译：“看这个方程式，里面有个伽马符号。
英语科普小知识黑洞的诞生
A new kind of cosmic flash may reveal something never seen before: the birth of a black hole. When a massive star exhausts its fuel, it collapses under its own gravity and produces a black hole, an object so dense that not even light can escape its gravitational grip. According to a new analysis by an astrophysicist（天体物理学家） at the California Institute of Technology (Caltech), just before the black hole forms, the dying star may generate a distinct burst of light that will allow astronomers to witness the birth of a new black hole for the first time.Tony Piro, a postdoctoral scholar at Caltech, describes this signature light burst in a paper published in the May 1 issue of the Astrophysical Journal Letters. While some dying stars that result in black holes explode as gamma-ray bursts, which are among the most energetic phenomena in the universe, those cases are rare, requiring exotic circumstances, Piro explains. "We don't think most run-of-the-mill black holes are created that way." In most cases, according to one hypothesis, a dying star produces a black hole witho ut a bang or a flash: the star would seemingly vanish from the sky -- an event dubbed an unnova. "You don't see a burst," he says. "You see a disappearance."But, Piro hypothesizes, that may not be the case. "Maybe they're not as boring as we thought," he says.According to well-established theory, when a massive star dies, its core collapses under its own weight. As it collapses, the protons and electrons that make up the core merge and produce neutrons. For a few seconds -- before it ultimately collapses into a black hole -- the core becomes an extremely dense object called a neutron star, which is as dense as the sun would be if squeezed into a sphere with a radius of about 10 kilometers (roughly 6 miles). This collapsing process also creates neutrinos, which are particles that zip through almost all matter at nearly the speed of light. As the neutrinos stream out from the core, they carry away a lot of energy -- representing about a tenth of the sun's mass (since energy and mass are equivalent, per E = mc2).According to a little-known paper written in 1980 by Dmitry Nadezhin of the Alikhanov Institute for Theoretical and Experimental Physics in Russia, this rapid loss of mass means that the gravitational strength of the dying star's core would abruptly drop. When that happens, the outer gaseous（气态的） layers -- mainly hydrogen -- still surrounding the core would rush outward, generating a shock wave that would hurtle through the outer layers at about 1,000 kilometers per second (more than 2 million miles per hour).Using computer simulations, two astronomers at UC Santa Cruz, Elizabeth Lovegrove and Stan Woosley, recently found that when the shock wave strikes the outer surface of the gaseous layers, it would heat the gas at the surface, producing a glow that would shine for about a year -- a potentially promising signal of a black-hole birth. Although about a million times brighter than the sun, this glow would be relatively dim compared to other stars. "Itwould be hard to see, even in galaxies that are relatively close to us," says Piro.But now Piro says he has found a more promising signal. In his new study, he examines in more detail what might happen at the moment when the shock wave hits the star's surface, and he calculates that the impact itself would make a flash 10 to 100 times brighter than the glow predicted by Lovegrove and Woosley. "That flash is going to be very bright, and it gives us the best chance for actually observing that this event occurred," Piro explains. "This is what you really want to look for."一种新的宇宙闪光可能会发现以前从未见过的东西：一个黑洞的诞生。
Gamma-Ray Bursts and Afterglow Polarisation
a r X i v :a s t r o -p h /0412129v 1 6 D e c 2004Gamma-Ray Bursts and Afterglow Polarisation S.Covino ∗,E.Rossi †,zzati ∗∗,D.Malesani ‡and G.Ghisellini ∗∗INAF /Brera Astronomical Observatory,V .Bianchi 46,22055,Merate (LC),Italy †Max Planck Institute for Astrophysics,Garching,Karl-Schwarzschild-Str.1,85741Garching,Germany ∗∗JILA,University of Colorado,440UCB,Boulder,CO 80309-0440,USA ‡International School for Advanced Studies (SISSA-ISAS),via Beirut 2-4,I-34014Trieste,Italy Abstract.Polarimetry of Gamma-Ray Burst (GRB)afterglows in the last few years has been considered one of the most effective tool to probe the geometry,energetic,dynamics and the environment of GRBs.We report some of the most recent results and discuss their implications and future perspectives.INTRODUCTION Polarimetry has always been a niche observational technique.It may be difficult to apply,requiring special care for the instruments,data reduction and analysis.Indeed,for real astronomical sources,where often the polarisation degree is fairly small at the level of a few per cent,the signal to noise required to derive useful information has to be very high.However,the amount of information that can be extracted by a polarised flux is also very high,since polarisation is an expected feature of a large number of physical phenomena of astronomical interest.This is particularly true for unresolved sources as GRB afterglows,where polarimetry offers one of the best opportunity to infer on the real geometry of the system.In particular,time resolved polarimetry can in principle give fundamental hints on the jet luminosity structure and on the evolution of the expanding fireball.This would provide reliable tools to discriminate among different scenarios.Finally,it has been recently realised that polarimetry of GRB afterglows can offer adirect way to study the physical condition of the Inter-Stellar Medium (ISM)around the GRB progenitor.GRB polarimetry,thus,becomes a powerful probe for gas and dust in cosmological environments,a valuable research field by itself.In the following of this contribution we want to briefly comment on the most recent advancement in the field and discuss the likely future perspectives that are now open by the advent of the GRB dedicated Swift satellite with its unprecedented rapid localisation capabilities [1].SYNCHROTRON AND BEAMING?The first pioneeristic attempts,culminated with the successful observation of a ∼1.7%polarisation level in GRB 990510[2,3],were driven by the hypothesis that the afterglowFIGURE1.Possible different jet structures.From Rossi et al.[15].emission were due to synchrotron radiation[4,5,6].GRB990510was also a perfect case for testing the hypothesis of a geometrically beamedfireball.Indeed,the detection of an achromatic break in the optical light curve[7,8],together with the observed degree of polarisation,gave support to this scenario.Shortly after this result,it was realised that a jetted ultra-relativistic outflow would produce a characteristic time evolution of the polarisation degree and position angle[9,10].The detailed shape of the polarisation curves depends on the dynamical evolution.Testing this model against data is thus a powerful diagnosis of the geometry and dynamics of thefireball.A large number of polarimetric observations has been carried out since GRB990510.A review of these data has been compiled by Covino et al.[11]and Björnsson[12]. However,until recently,the detection of a low level of polarisation required strong observational efforts.This prevented a satisfactory time coverage of the afterglow decay and,in turn,a convincing test for the model predictions.HOMOGENEOUS,STRUCTURED AND MAGNETISED JETS Lacking strong observational constraints,an improvement of the reference models was achieved considering more physical descriptions for the GRB afterglow jets.In the basic model the energy distribution is homogeneous,making the jet a single entity.More complex beam and magneticfield patterns(Fig.1),reflecting a physically more plausible scenario,were studied in several papers[13,14,15]showing that the light curve is barely affected by this parameter,while the polarisation and position angle evolution changes substantially,providing a further diagnostic tool Fig.2.The universal structured jet model predicts that the maximum of the polarisation curve is at the time of the break in the light curve.The position angle remains constant throughout the afterglow evolution.On the contrary,the homogeneous jet model requires two maxima before and after the light curve break and,more importantly,the position angle shows a sudden rotation of90◦between the two maxima,roughly simultaneouslyFIGURE2.Light curve and polarisation evolution for different jet structures.SJ stands for structured jet,HJ homogeneous jet,GJ for Gaussian jet.Thefigure shows the similarity of the predicted light curves for the various models while the polarisation changes considerably.Negative polarisation degrees mark a 90◦rotation for the position angle.From Rossi et al.[15].to the break time of the light curve.At early and late time the polarisation should be essentially zero(Fig.2).This last result is substantially modified if it is assumed that a large-scale magnetic field is driving thefireball expansion.The topics has been widely discussed in the con-text of polarimetry by Granot&Königl[13],Lazzati et al.[14]and[15].Magnetised jets can be both homogeneous and structured.We do not discuss here the details of this recent research branch.However,we note that,at early times,a large-scale ordered magneticfield produces a non negligible degree of polarisation,contrary to the purely hydrodynamical models.Polarimetry may therefore be the most powerful available di-agnostic tool to investigate thefireball energy content and its early dynamical evolution.Dust Induced PolarisationThe observed low polarisation level from GRB afterglows is often comparable to the expected polarisation induced by dust.Dust grains are known to behave like a dichroic, possibly birefringent,medium[16].Significant amounts of dust are expected to lie closeto the GRB site,as a consequence of the observation of a supernova(SN)component inFIGURE3.Assuming as a reference a typical polarisation curve with a homogeneous jet,the presence of some dust along the line of sight deeply modify the observed time evolution if the dust-induced polarisation is comparable to the intrinsic one,as it seems to be the rule for GRB afterglow at least at rather late time after the high-energy event[11].Depending on the relation between the position angle of the dust-induced polarisation and of the intrinsic GRB afterglow polarisation,the typical shape of the curve can be removed or even enhanced.From Lazzati et al.[16].a few GRBs.The measured polarisation will be modified by the propagation of radiation through dusty media.This effect is,contrary to the intrinsic afterglow polarisation,wave-length dependent.The different wavelength dependence open the interesting possibility to study the polarisation signature from the afterglow to study the physical character-istics of dust in cosmological environments:probably the only way to study dust close to star formation regions at high redshift.Even assuming that dust properties close to GRB formation sites are comparable to what we know in the Milky Way(MW),it is important to take into account this component once information from time evolution po-larimetry are derived.The superposition of the intrinsic time evolution to dust-induced components for the GRB host galaxy and the MW may substantially alter the expected behavior(Fig.3).OBSERV ATIONS VS.THEORYSo far,a rather satisfactory coverage of the polarisation evolution of a GRB afterglow has been obtained for three events only:GRB021004[17,16,18,19],GRB030329 [20,21],and GRB020813[22,14].However,firm conclusions from the analysis could have been derived for the last case only.GRB021004and GRB030329showed some remarkable similarities given that their light curves were characterised by a large num-ber of“bumps”or rebrightenings.Several different possibilities has been proposed to model the irregularities in the light curve invoking clumping in the external medium [23];a more complex and not axi-symmetric energy distribution in thefireball[18]or delayed energy injections[19].It was soon clear[16]that the standard models for polar-FIGURE4.Polarisation data for GRB020813[22].Different curves refer to different models.From Lazzati et al.[14].isation could not be applied in these conditions,since they are all derived in cylindrical symmetry.Even for GRB030329,for which a remarkable dataset was obtained[20],no convincing explanation of the polarization and light-curve erratic behaviors has so far been obtained.It is not clear yet to what extent GRB021004and GRB030329belong to the same population of long GRBs.It is argued however that the failed detection of this erratic behavior in other afterglows(such as GRB020813)is not due to a coarser sampling of the light curve.GRB020813was the best case for model testing.Its light curve was remarkably smooth[24],in several optical/infrared bands,and a break in the light curve was clearly singled out.A few polarimetric observations have been carried out providing for the first time polarisation data before and after the light curve break time[22].Lazzati et al.[14]applied to this event a more quantitative approach not limited,as usually done in the past,to the bare qualitative search of features in the polarisation curve (i.e.rotation of the position angle,etc.).A formal analysis was carried out,taking into account the GRB host galaxy and MW dust induced polarisation and the intrinsic GRB afterglow polarisation.All current jet models were considered,including homogeneous and structured jets,with and without a coherent magneticfield.The dataset,did not allow us to strictly derive a bestfitting model.The main result was to rule out the basic homogeneous jets model at a confidence larger than3σ,mainly because of the lack of the predicted90◦position angle rotation.Again the role of the MW dust induced polarisation is significant.All magnetized models and structured jetsfit satisfactorily the data,the ambiguity being mainly due to the lack of early time measurement,i.e. where magnetised or not magnetised models mostly differ(see Fig.4).The debate is still far from being settled.Recently,for GRB030226Klose et al.[25]a quite low upper limits(∼1%)was reported,in rather strict coincidence with the break time,therefore close to the maximum for the polarisation curve if we assume a structured jet model.With one only measurement it is difficult to drawfirm conclusions,since this null polarisation measurement may well be due to dust induced polarisation superposed destructively to the intrinsic,if any,GRB afterglow polarisation.It isfinally worth,even though tautological,to report that,as soon as Swift will be fully operational,distributing routinely prompt localisations,a new era will be open even for GRB polarimetry.It will allow us to carry out more stringent tests to the available models and therefore strictly constraint geometry,energetics and dynamics of thefireball.REFERENCES1.Gehrels,N.,Chincarini,G.,Giommi,P.,et al.2004,ApJ611,10052.Covino S.,Lazzati D.,Ghisellini G.,et al.1999,A&A348,13.Wijers R.A.M.J.,Vreeswijk P.M.,Galama T.J.,et al.1999,ApJ523,1774.Paczy´n ski B.,Rhoads J.E.1993,ApJ418,55.Mészáros P.,Rees M.J.1997,ApJ476,2326.Sari R.,Piran T.,Narayan R.1998,ApJ497,177.Israel G.L.,Marconi G.,Covino S.,et al.(1999),A&A348,58.Harrison F.A.,Bloom J.S.,Frail D.A.,et al.(1999),ApJ523,1219.Ghisellini G.,Lazzati D.(1999),MNRAS309,710.Sari R.(1999),ApJ524,4311.Covino S.,Ghisellini G.,Lazzati D.,Malesani D.2004,ASP Conf.Ser.312,16912.Björnsson G.(2003),astro-ph/030217713.Granot J.,Königl A.(2003),ApJ594,83zzati D.,Covino S.,Gorosabel J.R.,et al.(2004),A&A422,12115.Rossi E.M.,Lazzati D.,Salmonson J.D.,Ghisellini G.(2004),MNRAS354,86zzati D.,Covino S.,di Serego Alighieri S.,et al.(2003),A&A410,82317.Rol E.,Wijers R.A.M.J.,Fynbo J.P.U.et al.(2003),A&A405,2318.Nakar E.,Oren Y.(2004),ApJ602,9719.Björnsson G.,Gudmundsson E.H.,Jóhannesson G.(2004),ApJ615,7720.Greiner J.,???,et al.(2003),Nature426,15721.Klose S.,Palazzi E.,Masetti N.,et al.(2004),A&A420,89922.Gorosabel J.,Rol E.,Covino S.,et al.(2004),A&A422,113zzati D.,Rossi E.,Covino S.,Ghisellini G.,Malesani D.(2002),A&A395,524.Covino S.,Malesani D.,Tavecchio F.et al.(2003),A&A404,525.Klose S.,Greiner J.,Rau A.et al.(2004b),AJ128,1942。
星际奇观：宇宙中的神秘光芒
1. 科学家们对宇宙充满了无尽的好奇和探索欲望。
通过各种仪器和技术，他们不断发现着宇宙中的奇观和神秘光芒。
这些光芒穿越亿万光年的距离，向我们展示了宇宙的壮丽景象，同时也带来了许多未解之谜。
2. 在宇宙中，有一些神秘的光芒被称为“星际奇观”。
它们的特点是非常亮且持续时间很短暂。
科学家们对这些光芒的起源和性质进行了深入研究，但至今仍然无法完全解释其背后的奥秘。
3. 一种著名的星际奇观是“伽玛射线暴”(gamma-ray burst, GRB)。
GRB是宇宙中最亮的事件之一，释放出的能量比太阳在整个寿命中释放的能量还要多。
它们通常持续几秒钟到几分钟不等，但有时也会持续更长时间。
GRB的观测数据显示，它们来自宇宙中不同的方向，与超新星爆炸或黑洞的活动有关。
4. 另一种引人注目的星际奇观是“快速射电暴”(fast radio burst, FRB)。
FRB是以毫秒为单位的短暂射电脉冲，其持续时间极短，但能量巨大。
最初被误认为是干扰信号，后来科学家们发现它们来自宇宙中的远离地球数亿光年的地方。
至今为止，FRB的起源仍然是未解之谜，但有人认为它们可能与中子星碰撞或黑洞有关。
5. 还有一类神秘光芒被称为“闪耀光点”(flashing dots)，它们出现在宇宙中的遥远星系中。
这些光点具有非常高的亮度和变化速度，很难用传统的天文学模型来解释。
科学家们认为闪耀光点可能是由于恒星的碰撞或者其他未知的物理过程导致的。
6. 星际奇观不仅帮助我们了解宇宙的演化历史，还为科学家们提供了一些重要的线索，以推动我们对宇宙的理解更深入一步。
通过观测和研究这些神秘的光芒，科学家们可以揭示宇宙中隐藏的物理过程和天体现象。
7. 近年来，随着观测设备和技术的不断改善，我们对星际奇观的了解也在不断增加。
例如，通过多波段的观测，科学家们能够更准确地追踪GRB和FRB的来源，并尝试解开它们的谜团。
同时，新的天文台和探测器的发射也将进一步拓展我们对宇宙中神秘光芒的认识。
小学上册第八次英语第6单元测验卷(含答案)
小学上册英语第6单元测验卷(含答案)英语试题一、综合题(本题有100小题，每小题1分，共100分.每小题不选、错误，均不给分)1.ohs ruled ancient ________ (埃及). The Pilg2.I enjoy making crafts with my ________ (手工材料).3. A ____ is known for its ability to imitate sounds.4.What do you call the art of folding paper into shapes?A. OrigamiB. CalligraphyC. PaintingD. Sculpture答案:A5.The first man on the moon was __________. (尼尔·阿姆斯特朗)6.The dog is ______ (barking) excitedly.7.I love going to the ________ (博物馆) to learn new things.8.Which animal is known for building dams?A. BeaverB. RabbitC. SquirrelD. Bear答案: A9. A ________ (水道) is used for navigation.10.The ________ was a prominent figure in the fight for justice.11.I believe in setting goals. My short-term goal is to __________. My long-term goal is to __________. Working towards these goals motivates me every day.12. Depression started in __________ (1929). The Grea13.I have a toy _______ that dances and sings all my favorite songs.14.I love to explore ________ (树林) near my house.15.The process of making bread rise is caused by _______ production.16.I have a ________ that helps me build things.17.The __________ (历史的深度) reveals layers of meaning.18.The process of extracting metals from ores is called _______.19.She is a talented ________.20.We have a ______ (美丽的) garden full of flowers.21.I enjoy going ______ (滑冰) in the winter with my friends.22. A hypothesis is a testable ______.23.Which one is used for swimming?A. BoatB. BicycleC. AirplaneD. Train答案: A24.The teacher is ________ a story.25.听一听，为下面图片排列顺序，每个句子读三遍。
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a r X i v :a s t r o -p h /9912322v 1 15 D e c 1999Red Hole Gamma-Ray Bursts:A New Gravitational Collapse Paradigm Explains the Peak Energy Distribution And Solves the GRB Energy CrisisJames S.Graber ∗∗407Seward Square SE Washington,DC 20003Abstract.Gamma-ray bursts (GRBs)are still an enigma.In particular the central engine,the total energy,and the very narrow distribution of peak energies challenge model builders.Motivated by recent theoretical developments (string theory,quantum gravity,critical collapse),which suggest that complete gravitational collapse can occur without singularities or event horizons,we explore how red-hole models (which lack singularities or event horizons)can solve these problems better than black-hole models.KEY GRB MODEL BUILDING CHALLENGES Gamma-ray bursts vary rapidly and therefore they must be compact.Because these compact gamma-ray bursts release enormous energy,they must form an in-tense fireball that is optically thick,pair-producing,and thermalized.But the spectrum is not thermal,and there is no sign of pair-production attenuation at the high end of the observed spectrum [1].This seeming self-contradiction (the opacity problem)can be solved by having the fireball power a relativistic shell or jet thatcollides with something (perhaps itself)to produce the observed gamma rays [2].This fireball/shock model is currently the leading candidate to explain GRBs [3].It has already overcome several severe model-building challenges.But like almost all other published models,it fails to explain the observed spectroscopy of GRBs,particularly the narrowness of the observed peak energy distribution [4,5].Further-more,this model does not explain the high ratio of the energy of the GRB burst itself (caused by internal shocks)to the energy in the afterglow (caused by external shocks in the fireball/shock model)[6].Nevertheless the predictions of this model for the afterglows themselves are consistent with current observations [3].Finally,there is the problem of the overall energetics of the GRB.The two leading candidates to produce the initial fireball or fireballs –the so-called central engine–are merging neutron stars and core-collapse supernovae [7,8].Both these sourceshave over1054ergs of total energy available.This is more than enough energy for even the most energetic GRB,but it is not at all clear how to prevent most of it from falling into the newly created black hole which forms in the standard general relativity versions of these models.There seems to be an inherent conflict between solving the opacity problem and solving the peak energy distribution problem.The only successful technique avail-able to solve the transparency problem is to invoke highly relativistic bulk motion. In the relativistic frame,the gamma rays are below pair-production threshold and so do not suffer pair-production attenuation.This definitively solves the opacity problem.But unless the Lorentz gamma factor of the bulk motion can befine-tuned to a very narrow range for all GRBs,the resulting blueshift will not only relocate the peak of the photon energy distribution;it will also substantially widen it,inconsistent with the observed narrow E-peak distribution.Thus one needs to find a way tofine-tune the Lorentz gamma factor orfind some other way around this conflict.In thefireball/shock model the gamma factor depends sensitively on the baryon loading,and hence will vary widely.Furthermore,the internal shocks model is dependent on shocks with varying Lorentz gamma factors colliding with each other.Sofine-tuning is not a reasonable option for this model.A generic solution to this problem is provided if the relativistic bulk motion results not from an initial explosion,but rather from the gravitational acceleration of matter falling into a deep potential well.An arbitrarily high Lorentz gamma factor can be attained,but the accompanying blueshift will be exactly cancelled when the matter and radiation are redshifted as they emerge from the potential well.(By that time,the matter and radiation will have separated,so the opacity problem has already been solved).A black hole can provide the necessary deep potential well.But once matter or radiation is deep in the potential well of a black hole,it is almost impossible for it to escape.Therefore,we will consider an alternative gravitational collapse paradigm in which it is possible to escape from deep within the potential well of a gravitationally collapsed object.WHY CONSIDER ALTERNATE GRA VITY MODELS? The problems with constructing a GRB model might be sufficient motivation to consider alternate theories of gravity.However,a stronger motivation comes from the theory of gravitation.Recent theoretical developments in string the-ory,quantum gravity and critical collapse strongly suggest the possibilities of both gravitational collapse without singularities(and without loss of information)and also gravitational collapse without event horizons[9–14].If these possibilities are correct,we are forced to consider the phenomenological consequences(such as dif-ferent models for GRBs and core-collapse supernovae)of alternate paradigms for gravitational collapse in which black holes do not form[15].RED HOLES–A NEW PARADIGMMany authors have considered the alternative in which a hard core collapsed object similar to a smaller harder denser neutron star forms in place of a black hole[16].We here consider the alternative in which no such hard surface forms. Instead the spacetime stretching that forms a black hole in the standard model occurs,but it does not continue to the extent necessary to form an event horizon or a singularity.Instead,spacetime stretches enormously,but not infinitely,and forms a deep wide potential well with a narrow throat.We call this a red hole. This type of spacetime configuration was previously considered by Harrison, Thorne,Wakano and Wheeler(HTWW)in1965,but only as a way station in thefinal collapse to a black hole(not yet then called by that name)[17].In their version,part of the configuration is inside the event horizon,the collapse continues, and a singularity soon forms.In the new alternate paradigm we call a red hole,no event horizon forms and no singularity forms.The gravitational collapse does not continue forever,but even-tually stops.(Why?Perhaps due to quantum effects or string-theory dualities,but we cannot discuss this adequately here.)As the collapse proceeds,the collapsing matter becomes denser and denser until it reaches a critical point,after which, the distortion of spacetime is so great that the density decreases.This happens be-cause the spacetime is stretching faster than the collapsing material can fall inward. (This decreasing density effect was already noticed by HTWW in their analysis of gravitational collapse in the context of standard general relativity[17].In general relativity,this expansion of spacetime is mostly hidden behind the event horizon and does not prevent the formation of a singularity in afinite time.This is not the case in several observationally viable alternate theories of gravity[18–20].)This is why we are confident that the center of a red hole resembles a low-density vacuum more than it resembles a high-density neutron star.The decrease in density due to this enormous stretching may also be a factor in halting the gravitational collapse of the red hole before the stretching becomes infinite.As a result,even though the stretching of spacetime is enormous,it never becomes fast enough to exceed the speed of light and cause an event horizon to form.It stops before it reaches an infinite size or any other form of singularity.(Infinite density and infinite curvature also do not occur.)Nevertheless,it is very hard to escape from a red hole.First,there are trapped orbits inside the red hole for photons as well as massive particles,which allows permanent or nearly permanent trapping of mass and energy.Second,the Shapiro delay in crossing a red hole is very substantial,(in some cases,enormous)[21].Hence particles that are only crossing the red hole or passing through are in effect temporarily trapped.In fact most of the matter falling into a red hole will be trapped.However, radiation,and highly relativistic matter that falls directly into the center of the red hole and does not rescatter while inside the red hole,can travel straight through and emerge on the other side.This possibility is essential for our proposed new GRB models.RED-HOLE MODELS FOR GRBSIn order to describe our new red-hole models for GRBs,which are based on mod-ifying the existingfireball/shock model,we begin by resummarizing that model. In thefireball/shock model,some form of gravitational collapse deposits a large amount of energy in a very small region,(which is called thefireball,and also the central engine).Thefireball has so much energy in such a small space that a relativistic expansion must occur.Part or all of this explosive expansion travels through a region with a very small critical number of baryons,which absorb essen-tially all of the energy and form a relativistic blast wave(either spherical or jetted). Multiple such relativistic shells(travelling in the same direction)are created by the central engine,perhaps by repeated explosions(possibly due to repeated accretion events).The faster relativistic shells overtake the slower relativistic shells and col-lide with them.The internal shocks convert the energy of the baryons to gamma rays,(by synchroton emission or inverse compton scattering or perhaps by both means).The shells eventually collide with external matter and generate the main afterglow.(Perhaps an early prompt afterglow is the result of a reverse shock)[3]. Basically there are three important sites in this model.First,there is the central engine,orfireball site.(In the standard black-hole interpretation,this is probably near a newly forming black hole,perhaps at the pole of a Kerr black hole)[22]. Second,there is the location of the internal shocks,where the main gamma-ray burst is generated.According to Piran,this is typically1012-1014centimeters,or 30-3000light seconds down stream from the central engine[3].Third,there is the location of the external shock,where the relativistic matter collides with material that was not part of the original explosion,and the long-lasting(days to months), but weak(less total energy than the gamma rays)afterglow is generated.In the standard model,this is far from the central engine.In our alternate red hole models we will relocate these three sites in or near a red hole instead of near the outside of a black hole.In thefirst and most conservative red-hole model,we merely replace the black hole of the standard model with a red hole.The red hole can help the central engine by generating more energy than the corresponding black hole or by focusing the outgoing jet more narrowly,but the rest of the model is essentially the same as the standard one and there is no significant impact on the spectral issues.In other words,thisfirst red-hole model can help solve the energy crisis,but does not help explain the broad spectrum,with its unusual slopes and narrowly distributed peak energy.In the second–and more interesting–red-hole model,the central engine is located at the infalling bottleneck of the red hole,and the internal shocks that generate the primary gamma-ray burst are located at the outgoing bottleneck of the red hole(which is essentially the same place,but at a later time),and the external shocks and the afterglow still occur far away at the point where the ejecta encounter the interstellar material or some other external matter.In this model,the great internal expansion of the red hole,along with the greatacceleration of the gravitational infall,help to generate the relativistic jet that will later create the GRB.Then the focusing effects of the emerging bottleneck of the red hole help to create the internal shocks necessary for thefinal transformation of the energy into gamma rays,and to very substantially increase the efficiency of this process.Furthermore,since the blueshift of the infall should be exactly cancelled by the redshift of the outclimb,the gamma rays seen by the observer will have no net red or blue shift(on average).Therefore the observed peak energy will be the same as the initial peak energy.Even if the internal transit involves enormous and substantially varying Lorentz gamma factors,they will not be observed as a net blueshift.So this model helps solve the narrow peak energy distribution problem, as well as the energy crisis.It can also help solve the spectral wideness and slope problems because of the tolerance for differing Lorentz gamma factors during the transit through the red hole.REFERENCES1.Band,D.,et al.,ApJ413,281(1993).2.Rees,M.J.,and Meszaros,P.,MNRAS258,41P(1992).3.Piran,T.,Phys.Rep.314,575(1999).4.Preece,R.D.et al.,to appear in ApJS(1999);astro-ph9908119.5.Brainerd,J.J.,et al.,ApJ,submitted.6.Paczynski,B.,and Rhoads,J.,ApJL418,L5(1993).7.Eichler,D.,et al.,Nature340,126(1989).8.Woosley,S.E.,ApJ405,273(1993).9.Strominger,A.,and Vafa,C.,Phys.Lett.B379,99(1996).10.Callan,C.G.and Maldacena,J.M.,Nucl.Phys.B472,591(1996).11.Maldacena,J.and Strominger,A.,Phys.Rev.D55,861(1997).12.Shapiro,S.and Teukolsky,S.A.,Phys.Rev.Lett.66,994(1991).13.Choptuik,M.W.,Phys.Rev.Lett.70,9(1993).14.Christodoulou,D.,Ann.Math.140,607(1994).15.Graber,J.S.,to appear in Largest Explosions Since the Big Bang Ed.Livio,M.;astro-ph9908113(1999).16.Robertson,S.L.,ApJL517,117(1999).17.Harrison,B.K.,Thorne,K.S.,Wakano,M.,and Wheeler,J.A.,Gravitation Theoryand Gravitational Collapse,Chicago:University of Chicago Press,1965,ch.8,pp.69-75.18.Rosen,N.,Ann.Phys.84,455(1974).19.Yilmaz,H.,Ann.Phys.101,413(1976).20.Itin,Y.,Gen.Rel.Grav.31,187(1999).21.Shapiro,I.I.,Phys.Rev.Lett.13,789(1964).22.Kerr,R.P.,Phys.Rev.Lett.11,237(1963).。