Dependence of spectral state transition and disk truncation on viscosity parameter a

Abstract

A wealth of Galactic accreting X-ray binaries has been observed in both the low/hard and high/soft states. The transition between these two states has often been detected. Observations show that the transition luminosity between these two states is different for different sources, ranging from 1 % to 4% of the Eddington luminosity ^Edd- Even for the same source, the transition luminosity at different outbursts can be different. The transition can occur at luminosities from 0.0069 to 0.15 Lgdd- In order to investigate the underlying physics, we study the influence of viscosity parameter a 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 parameters such as 0.1 < a < 0.9. Fitting the numerical results, we obtain fitting formulae for both the transition accretion rate and the corresponding radius as a function of a. We find that the transition luminosity is very sensitive to the value of a, L/L-Edd °c a234. For 0.1 < a < 0.6, the transition luminosity varies by two orders of magnitude, from 0.001 to0.2Z,Edd- Comparing with observations, we find that transition luminosity can be fitted by adjusting the value of a, and the model-determined values of a are mostly in the range of observationally inferred values. We also investigate the truncation of the disk in the low/hard state for some luminous sources. Our results agree roughly with the observations. © 2009 Astronomical Society of Japan.