The very high state accretion disc structure from the Galactic black hole transient XTE J1550 - 564

The 1998 outburst of the bright Galactic black hole binary system XTE J1550 - 564 was used to constrain the accretion disc structure in the strongly Comptonized very high state spectra. These data show that the disc emission is not easily compatible with the constant area L ∝ T⁴ behaviour seen during the thermal-dominated high/soft state and weakly Comptonized very high state. Even after correcting for the effects of the scattering geometry, the disc temperature is always much lower than expected for its derived luminosity in the very high state. The simplest interpretation is that this indicates that the optically thick disc is truncated in the strongly Comptonized very high state, so trivially giving the observed continuity of properties between the low/hard and very high states of Galactic black holes.