On the observed disc temperature of accreting intermediate mass black holes

Some ultraluminous X-ray sources in nearby galaxies show soft components resembling thermal disc emission. Calculations based on blackbody emission then indicate that the accreting black holes at the centres of these discs have masses of hundreds to thousands solar mass. Establishing the existence of such intermediate mass black holes is of considerable importance, so the assumptions and approximations lying behind blackbody spectral fits must be examined. We study here the basic assumption that the thermal emission is well-characterized by a multi-temperature blackbody spectrum. Because the opacity in the surface layers of a disc decreases at high energies, the emergent spectrum is hardened. We compute the observed spectra from discs around a non-spinning 1000-M-circle dot black hole and fit them over the XMM-Newton pn band with multicolour disc models. The typical overall spectra hardening factor usually adopted for discs around a stellar mass black hole (assuming an inclination of 60 and including relativistic effects for a non-spinning black hole) is found to be appropriate.