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T.P. Roberts, R.S. Warwick, A.-M. Stobbart (University of Leicester), M.R. Goad (University of Southampton), M.J. Ward (University of Leicester), R.E. Kilgard (SAO), L.P. Jenkins (University of Leicester)
We present the results of XMM-Newton and Chandra X-ray spectroscopy of two ultraluminous X-ray sources (ULXs) that point to a new empirical description of the X-ray spectra of this class. This description does not require the presence of an intermediate-mass black hole (IMBH) in these systems. Detailed XMM-Newton data for an ULX in NGC 55 shows its spectral form to evolve over the course of ~60 ks of contiguous observations, that include dipping episodes. This culminates in a spectrum dominated above 2 keV by a ~ 0.9 keV disc-blackbody component, and below 2 keV by a steep (Gamma ~4) power-law continuum. The X-ray spectrum of NGC 5204 X-1, derived from two short XMM-Newton observations, is equally well fit by both the new empirical model derived from the NGC 55 ULX data, and the standard IMBH spectrum including a cool (~0.2 keV) disc-blackbody component. This spectral ambiguity is in part resolved by a further deep Chandra observation of NGC 5204 X-1, that shows the hard spectral component to be preferentially modelled by a disc-blackbody spectrum (~ 1.3 keV) at that epoch, though in this observation the soft component shows some evidence of weak emission lines. In all cases where a disc-blackbody models the hard spectral component its parameterization is consistent with an accretion disc around a stellar-mass black hole. We will discuss the challenges this new model poses for the standard disc-corona model of the X-ray emission from accreting black holes, and what it implies for the nature of the accretors in ULXs.
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.