X-ray spectral states of black holes from RXTE All-Sky Monitor

We have analysed X-ray outbursts from several Galactic black hole (GBH) transients, as seen by the All-Sky Monitor (ASM) on board Rossi X-ray Timing Explorer (RXTE). We have used the best estimates of distance and black hole mass to find their luminosity (scaled to the Eddington limit), which allowed for direct comparison of many sources. We found that the very high and intermediate X-ray spectral states, characterized by very similar energy and power spectra, have in fact different evolutionary history. The distinction is made on the basis of the transition luminosity, duration and the shape of the track in the hardness-luminosity diagram. The very high state is brighter (∼30 per cent of Eddington luminosity) and longer (&30 days), while the intermediate state is dimmer (.5 per cent of Eddington) and shorter (.16 days). We speculate that the distinction is due to irradiation and evaporation of the disc, which sustains the Comptonizing corona in the very high state.