An accretion disk-outflow model for hysteretic state transition in X-ray binaries

TL;DR

This paper proposes a magnetically driven outflow model for ADAFs to explain the hysteretic state transitions in X-ray binaries, highlighting how magnetic fields influence accretion dynamics and state changes.

Contribution

It introduces a new model incorporating magnetic outflows in ADAFs to explain hysteresis in XRB state transitions, emphasizing the role of magnetic fields in accretion processes.

Findings

Critical luminosity of ADAF with magnetic outflows is an order of magnitude higher than conventional ADAF.

Magnetic outflows can carry away significant angular momentum, increasing ADAF radial velocity.

The model naturally explains the observed hysteretic state transitions in X-ray binaries.

Abstract

We suggest a model of the advection dominated accretion flow (ADAF) with magnetically driven outflows to explain hysteretic state transition observed in X-ray binaries (XRBs). The transition from a thin disk to an ADAF occurs when the mass accretion rate is below a critical value. The critical mass accretion rate for the ADAF can be estimated by equating the equilibration timescale to accretion timescale of the ADAF, which is sensitive to its radial velocity. The radial velocity of thin disks is very small, which leads to the advection of the external field in thin disks very inefficient. ADAFs are present in the low/hard states of XRBs, and their radial velocity is large compared with the thin disk. The external field can be dragged inward efficiently by the ADAF, so a strong large-scale magnetic field threading the ADAF can be formed, which may accelerate a fraction of gas in the ADAF into the outflows. Such outflows may carry away a large amount of angular momentum from the ADAF, which increases the radial velocity of the ADAF significantly. This leads to a high critical mass accretion rate, below which an ADAF with magnetic outflows can survive. Our calculations show that the critical luminosity of the ADAF with magnetic outflows can be one order of magnitude higher than that for a conventional ADAF, if the ratio of gas to magnetic pressure $\beta\sim 4$ in the disk. This can naturally explain the hysteretic state transition observed in XRBs.