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

¹ National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, PO Box 110, Kunming 650011, PR China
² Max-Planck-Institut für Astrophysik, Karl-Schwarzschildstr. 1, 85740 Garching, Germany e-mail: emm@mpa-garching.mpg.de

Received: 7 April 2005
Accepted: 24 May 2005

Abstract

In neutron star and black hole X-ray binaries the transitions between the two spectral states, hard and soft, signal the change between accretion via a hot advection-dominated flow(ADAF) and disk accretion. In a few cases the hard/soft transition was observed during the rise to the nova outburst, but in most cases only the soft/hard transition during the luminosity decrease. Surprisingly the luminosities at the second transition are always lower by a factor of 3 to 5. A model for this hysteresis was presented in a preceding paper where 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 for the dependence on the hardness of this radiation and the radiation efficiency. We further discuss the influence of the inclination and a possible warping of the disk on the observed hysteresis.