An optically thick inner corona geometry for the Very High State Galactic Black Hole XTE J 1550 – 564

We investigate the accretion disc structure in the strongly Comptonised spectra seen at high luminosities from galactic black holes (variously termed the very high or steep power law dominant state). We use simultaneous ASCA-RXTE data on two such spectra from the black hole transient XTE J1550-564 to constrain the disc and Comptonized luminosity. The broad bandpass of the data gives robust estimates of the disc temperature and luminosity despite the strong Comptonisation distorting the spectrum. The disc temperature is much lower than that seen from the same source at the same luminosity in its disc dominated spectra (high/soft or thermal dominant state). This implies that the disc emissivity or/and the disc area has changed between the two states. We build a spectral model of a Comptonising corona over an inner disc to show explicitally that a standard disc emissivity requires a large increase in inner disc radius for the very high state spectra. While such models can describe the spectra, it seems more probable that the disc emissivity changes in the presence of the corona. We build a specific model of an inner corona which is coupled energetically to the disc, in which some fraction f of the accretion power is dissipated in the corona leaving only a fraction 1− f to be dissipated in the optically thick disc (Svensson & Zdziarski 1994). We show that this can explain the low temperature/high luminosity disc emission seen in the very high state with only a small increase in radius of the disc. While the inferred disc truncation is probably not significant given the model uncertainties, it is consistent with the low frequency QPO and gives continuity of properties with the low/hard state spectra. Irrespective of such details, there is clear evidence in the very high state spectra that the growth of the corona affects the disc, strongly favouring models in which the corona is part of the same accretion flow structure as gives rise to the disc.