MODELING X-RAY DATA OF BLACK HOLE BINARIES

We test the truncated disc models using multiwavelength (optical/UV/X-ray) data from the 2005 hard state outburst of the black hole SWIFT J1753.5-0127. This system is both fairly bright and has fairly low interstellar absorption, so gives one of the best datasets to study the weak, cool disc emission in this state. We fit these data using models of an X-ray illuminated disc to constrain the inner disc radius throughout the outburst. Close to the peak, the observed soft X-ray component is consistent with being produced by the inner disc, with its intrinsic emission enhanced in temperature and luminosity by reprocessing of hard X-ray illumination in an overlap region between the disc and corona. This disc emission provides the seed photons for Compton scattering to produce the hard X-ray spectrum, and these hard X-rays also illuminate the outer disc, producing the optical emission by reprocessing. However, towards the end of the outburst, all these conclusions may change. The optical points clearly lie on an extrapolation of the hard X-ray flux, which may indicate that the seed photons for Compton scattering are now self-generated in the flow by Cyclo-Synchrotron radiation rather than being from the disc. The weak soft X-ray emission implies a small disc radius, unchanged from the outburst peak, in conflict with the expectations of the truncated disc model. However, this also requires that the energy to power the corona is advected vertically and radially in a dissipationless fashion from the disc. Thus it seems more likely that the soft X-ray component is not direct emission from the disc itself. We show that a similarly dim low/hard state spectrum from XTE J1118+480 puts similar constraints on the soft X-ray emission region, but here the very low interstellar absorption (an order of magnitude smaller than in SWIFT J1753.5-0127) allows detection of a much larger, cooler, UV component which is well fit by a truncated disc. Thus whatever the origin of the additional weak soft X-ray emission (irradiation of the inner face as opposed to the top surface of the inner edge of the disc, residual inner disc left from evaporation, ionised reflection, jet etc), its existence as a clearly separate component from the truncated disc in XTE J1118+480 shows that it does not trace the inner disc radius, so cannot be used to constrain the truncated disc models