Hot accretion flow with radiative cooling: state transitions in black hole X-ray binaries

TL;DR

This study uses hydrodynamical simulations to explore how radiative cooling influences state transitions in black hole X-ray binaries, identifying a critical accretion rate that triggers structural changes in the accretion flow.

Contribution

It introduces a self-consistent simulation approach incorporating multiple radiative processes to explain state transitions and the formation of two-phase accretion structures.

Findings

State transitions occur at a critical accretion rate of about 3α times the Eddington rate.

Formation of cold, dense clumpy structures within hot gas at high accretion rates.

Transition to a disc-corona configuration as accretion rate increases.

Abstract

We investigate state transitions in black hole X-ray binaries through different parameters by using two-dimensional axisymmetric hydrodynamical simulation method. For radiative cooling in hot accretion flow, we take into account the bremsstrahlung, synchrotron and synchrotron-self Comptonization self-consistently in the dynamics. Our main result is that the state transitions occur when the accretion rate reaches a critical value $\dot M \sim 3\alpha\ \dot M_{\rm Edd}$, above which cold and dense clumpy/filamentary structures are formed, embedded within the hot gas. We argued this mode likely corresponds to the proposed two-phase accretion model, which may be responsible for the intermediate state of black hole X-ray binaries. When the accretion rate becomes sufficiently high, the clumpy/filamentary structures gradually merge and settle down onto the mid-plane. Eventually the accretion geometry transforms to a disc-corona configuration. In summary our results are consistent with the truncated accretion scenario for the state transition.