A MODEL FOR THE CORRELATION OF HARD X-RAY INDEX WITH EDDINGTON RATIO IN BLACK HOLE X-RAY BINARIES

In this work, we theoretically investigate the correlation between gamma and [lambda] within the framework of a disk-corona model. We improve the model by taking into account all cooling processes, including synchrotron and self-Compton radiations in the corona, Comptonization of the soft photons from the underlying accretion disk, and the bremsstrahlung radiations. Presuming that the coronal flow above the disk can reach up to the 0.1 Eddington rate at the outer region, we calculate the structure of the two-phase accretion flows and the emergent spectra for accretion rates from 0.003 to 0.1. We find that at accretion rates larger than ~0.01 Eddington rate, a fraction of coronal gas condenses into the disk and an inner disk can be sustained by condensation. We show that our model can roughly explain the observed evolution of gamma sub(3-25 keV) with L sub(0.5-25keV)/L sub(Edd) for the black hole X-ray transient H1743 -322 in the decay of 2003 from the thermal-dominated state to low/hard state.