Spectral state transitions in low-mass X-ray binaries - the effect of hard and soft irradiation

In neutron star and black hole X-ray binaries the transitions between the two spectral states, hard and soft, signals the change between accretion via a hot advection-dominated flow(ADAF) and disk accretion. In a few cases the hard/soft transition were observed during the rise to the nova outburst, mostly only the soft/hard transition during the luminosity decrease. Surprisingly the luminosities at the second transition is always lower by a factor of 3 to 5. A model for this hysteresis was presented in a preceding paper: It was shown that this switch in the accretion mode at different mass accretion rates and therefore different luminosities is caused by the different amount of Compton cooling or heating of the accretion disk corona as it is irradiated by hard or soft radiation from the central light source, respectively. We now give detailed results on the dependence on hardness of this radiation and on radiation efficiency. We further discuss the influence of the inclination and a possible warping of the disk on the observed hysteresis.