Affiliation:
1. National Astronomical Observatories, Chinese Academy of Sciences , 20A Datun Road, Beijing 100101, China
2. School of Astronomy and Space Sciences, University of Chinese Academy of Sciences , 19A Yuquan Road, Beijing 100049, China
Abstract
ABSTRACTObservationally, two main spectral states, i.e. the low/hard state and the high/soft state, are identified in black hole X-ray binaries (BH-XRBs). Meanwhile, the transitions between the two states are often observed. In this paper, we re-investigate the transition luminosities in the framework of the self-similar solution of the advection-dominated accretion flow (ADAF). Specifically, we search for the critical mass accretion rate $\dot{m}_{\rm crit}$ of ADAF for different radii r, respectively. It is found that $\dot{m}_{\rm crit}$ decreases with decreasing r. By testing the effects of BH mass m, the magnetic parameter β and the viscosity parameter α, it is found that only α has significant effects on $\dot{m}_{\rm crit}\text{--}r$ relation. We define the minimum $\dot{m}_{\rm crit}$ (roughly at the innermost stable circular orbit) as the hard-to-soft transition rate $\dot{m}_{\rm tr:H\text{--} S}$, above which BH will gradually transit from the low/hard state to the high/soft state, and $\dot{m}_{\rm crit}$ at 30 Schwarzschild radii as the soft-to-hard transition rate $\dot{m}_{\rm tr:S\rightarrow H}$, below which BH will gradually transit from the high/soft state to the low/hard state. We derive fitting formulae of $\dot{m}_{\rm tr:H\rightarrow S}$ and $\dot{m}_{\rm tr:S\rightarrow H}$ as functions of α, respectively. By comparing with observations, it is found that the mean value of α are α ∼ 0.85 and α ∼ 0.33 for the hard-to-soft transition and the soft-to-hard transition, respectively, which indicates that two classes of α are needed for explaining the hysteresis effect during the state transition. Finally, we argue that such a constrained α may provide valuable clues for further exploring the accretion physics in BH-XRBs.
Funder
National Natural Science Foundation of China
National Astronomical Observatories, Chinese Academy of Sciences
Chinese Academy of Sciences
Publisher
Oxford University Press (OUP)
Subject
Space and Planetary Science,Astronomy and Astrophysics
Cited by
1 articles.
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