The effect of advection at luminosities close to Eddington: The ULX in M 31

The transient, ultra-luminous X-ray source CXOM31 J004253.1+411422 in the Andromeda galaxy is most likely a 10 solar mass black hole, with super-Eddington luminosity at its peak. The XMM-Newton spectra taken during the decline then trace luminosities of 0.86-0.27 L_Edd. These spectra are all dominated by a hot disc component, which roughly follows a constant inner radius track in luminosity and temperature as the source declines. At the highest luminosity the disc structure should change due to advection of radiation through the disc. This advected flux can be partly released at lower radii thus modifying the spectral shape. To study the effect of advection at luminosities close to Eddington we employ a fully relativistic slim disc model, SLIMBH, that includes advective cooling and full radiative transfer through the photosphere based on TLUSTY. The model also incorporates relativistic photon ray-tracing from the proper location of the disc photosphere rather than the mid-plane as the slim disc is no longer geometrically thin. We find that these new models differ only slightly from the non-advective (standard) BHSPEC models even at the highest luminosities considered here. While both discs can fit the highest luminosity data, neither is a very good fit to the *lower* luminosities. This could indicate a missing physical process that acts in low luminosity discs and subsides as the disc luminosity approaches the Eddington limit.
Comment: 5 pages, 4 figures, improved figures