Catching NGC4051 in the low state with XMM-Newton

a r X i v :a s t r o -p h /0309179v 2 29 S e p 2003

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Catching NGC 4051in the low state with XMM-Newton

P.Uttley a R.D.Taylor a ,I.M.M c Hardy a ,M.J.Page b ,K.O.Mason b ,mer c and A.Fruscione d a

School of Physics and Astronomy,University of Southampton,Southampton SO171BJ,United Kingdom

b Mullard Space Science Laboratory,University College London,Holmbury St Mary,Dorking RH56NT,United Kingdom

c

Astrophysikalisches Institut Potsdam,An der Sternwarte 16,D-14482Potsdam,Germany

d

Harvard-Smithsonian Center for Astrophysics,60Garden Street,Cambridge,MA 02138,USA

The Narrow Line Seyfert 1(NLS 1)galaxy NGC 4051shows unusual low flux states,lasting several months,when the 2-10keV X-ray spectrum becomes unusually hard (photon index <1)while the spectrum at lower X-ray energies is dominated by a large soft excess.A Chandra TOO of the low state has shown that the soft excess and hard components are variable and well-correlated.The variability of the hard component rules out an origin in a distant reflector.Here we present results from a recent XMM-Newton TOO of NGC4051in the low state,which allows a much more detailed examination of the nature of the hard and soft spectral components in the low state.We demonstrate that the spectral shape in the low state is consistent with the extrapolation of the spectral pivoting observed at higher fluxes.The XMM-Newton data also reveals the warm absorbing gas in emission,as the drop in the primary continuum flux unmasks prominent emission lines from a range of ion species.

1.INTRODUCTION

The NLS 1galaxy NGC 4051is highly vari-able in X-rays on both long and short time-scales [1].In particular,on long time-scales,the X-ray light curve of NGC 4051shows unusual low-flux states lasting months,during which the source varies little (compared with the normal variabil-ity amplitude)and the 2-10keV X-ray spectrum becomes extremely hard (photon index Γ∼1)[2,3,4].The first broadband X-ray spectrum of a confirmed low state,obtained in 1998by Bep-poSAX [2],suggested that the hard spectral shape (above ∼3keV)could be explained as being due to pure reflection from distant cold material (possibly the putative molecular torus).At lower energies,a different,much softer,spectral com-ponent was revealed (Γ∼3)which,it was sug-gested,may be associated with extended emission from hot gas on scales of hundreds of parsecs [5].

Thus the lack of variability in the low state spec-trum,and its unusual shape,were explained en-tirely in terms of emission and reprocessing from large-scales,while the primary continuum was it-self ‘switched off’.

Subsequently however,a Chandra observation of NGC 4051in a normal state in April 2000[10](and a later low state observation in February 2001[4])showed that there is in fact no signifi-cant soft X-ray emission on scales of hundreds of parsecs.Furthermore,although the source spec-trum had the same unusual hybrid form of the 1998low state (albeit at a higher continuum flux level),Chandra showed that the continuum in the low state is significantly variable in both hard and soft bands,and both hard and soft compo-nents are well-correlated.Thus,Chandra showed that the unusual shape of the low state spectrum was in fact intrinsic to the primary continuum emission and not due entirely to reflection and