A new description of the X-ray spectra of ULXs

Details A new description of the X-ray spectra of ULXs A new description of the X-ray spectra of ULXs

Aug 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004head....8.1514r&link_type=abstract

American Astronomical Society, HEAD meeting #8, #15.14; Bulletin of the American Astronomical Society, Vol. 36, p.928

Astronomy and Astrophysics

Astronomy

Scientific paper

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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