The Spectral Features of Disk and Corona with Mass Evaporation in the Low/Hard State

We investigate the spectral features of accretion flows composed of an outer cool, optically thick disk and inner hot, optically thin, advection dominated accretion flows (ADAF) within the framework of disk and corona with mass evaporation (Liu et al. 2002a). In this work, both the magnetic field and Compton scattering of soft photons from the disk by electrons in the corona are included to calculate the evaporation rates at different distances. The disk is truncated at the distance where the evaporation rate equals to the accretion rate ($\dot m_{\rm evap}(r_{\rm tr})=\dot m$). For a series of accretion rates, the corresponding truncation radii are calculated out, with which we are able to calculate the emergent spectra from the inner ADAF + outer disk + corona. At very low accretion rates, the spectra are similar to that of a pure ADAF because the disk is truncated at large distances. The disk component becomes important at high accretion rates since the truncation occurs at small distances. An anti-correlation between the Eddington ratio $\xi \equiv L_{\rm 0.5-25 \,keV}/L_{\rm Edd}$ and the hard X-ray photon index $\Gamma_{\rm 3-25 \,keV}$ at low/hard states is predicted by the model. Comparing the theoretical results with observations, we find that our model can reproduce the anti-correlation between the Eddington ratio $\xi$ and the hard X-ray photon index observed for the X-ray binary XTE J1118+480.Comment: 11 pages and 7 figures, published by PAS

The Dependence of Spectral State Transition and Disk Truncation on Viscosity Parameter $\alpha

A wealth of Galactic accreting X-ray binaries have been observed both in low/hard state and high/soft state. The transition between these two states was often detected. Observation shows that the transition luminosity between these two states is different for different sources, ranging from 1% to 4% of the Eddington luminosity. Even for the same source the transition luminosity at different outbursts is also different. The transition can occur from 0.0069 to 0.15 Eddington luminosity. To investigate the underlying physics, we study the influence of viscosity parameter $\alpha$ on the transition luminosity on the basis of the disk-corona model for black holes. We calculate the mass evaporation rate for a wide range of viscosity parameter, $0.1\le \alpha\le 0.9$. By fitting the numerical results, we obtain fitting formulae for both the transition accretion rate and the corresponding radius as a function of $\alpha$. We find that the transition luminosity is very sensitive to the value of $\alpha$, $L/L_{\rm Edd}\propto\alpha^{2.34}$. For $0.1\le\alpha\le 0.6$, the transition luminosity varies by two orders of magnitude, from 0.001 to 0.2 Eddington luminosity. Comparing with observations we find that the transition luminosity can be fitted by adjusting the value of $\alpha$, and the model determined values of $\alpha$ are mostly in the range of observationally inferred value. Meanwhile we investigate the truncation of the disk in the low/hard state for some luminous sources. Our results are roughly in agreement with the observations.Comment: 8 pages, 3 figures,accepted by PAS