In X-ray transients the change over from the hard to the soft spectrum occurs during the luminosity rise in outburst when the ADAF region disappears due to the increased accretion rate. The change back is connected with the decreasing accretion rate in outburst decline. In connection with these spectral transitions a special feature is observed, a hysteresis in the luminosity at the transition: the hard-soft transition is found at a luminosity higher by a factor of 3 to 5 than that at the soft-hard transition.

The change over from disk accretion to a hot coronal flow can be investigated by studying the interaction of a hot corona with the cool disk underneath. The location of the transition follows from the balance between mass flow rate in the thin disk and the rate at which gas evaporates into the corona. We here explain the hysteresis as caused by a different amount of Compton cooling of the corona by radiation from the inner region, very little for the hard photons of the hot inner flow at the change from hard to soft state, and more for the soft photons from the inner disk, in case of the change from soft to hard state.