X-ray spectral transitions 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 have found two distinct hard-to-soft state transitions in the initial part of the outburst. The distinction is made on the basis of the transition luminosity, its duration, the shape of the track in the hardness-luminosity diagram and evolution of the hardness ratio. The bright/slow transition occurs at ∼30 per cent of Eddington (estimated bolometric) luminosity and takes ≳30 d, during which the source quickly reaches the intermediate/very high state and then proceeds to the soft state at much slower pace. The dark/slow transition is less luminous (≲10 per cent of Eddington), shorter (≲15 d) and the source does not slow its transition rate before reaching the soft state. We speculate that the distinction is due to irradiation and evaporation of the disc, which sustains the Comptonizing corona in the bright intermediate/very high state.