高能天体物理研究组知识库

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 huminosity between these two states is different for different sources, ranging from 1% to 4% of the Eddington luminosity L(Edd). Even for the same source, the transition luminosity at different outbursts call be different. The transition call occur at luminosities from 0.0069 to 0.15 L(Edd). In order 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 parameters such as 0.1 <= alpha <= 0.9. 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 ce, L/L(Edd) proportional to alpha(2,34). For 0.1 <= alpha <= 0.6, the transition luminosity varies by two orders of magnitude, from 0.001 to 0.2 L(Edd). Comparing with observations, we find that transition luminosity can be fitted by adjusting the value of alpha, 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.