A mechanism for hysteresis in black hole binary state transitions

Abstract

We suggest that the hysteretic cycle of black hole state transitions arises from two established properties of accretion disks: the increase in turbulent stress in disks threaded by a net magnetic field and the ability of thick (but not thin) disks to advect such a field radially. During quiescence, magnetic field loops are generated by the magnetorotational instability at the interface between the inner hot flow and outer thin disk. Vertical flux is advected into and accumulates stochastically within the inner flow, where it stimulates the turbulence so that $\alpha \sim 1$. The transition to a geometrically thin inner disk occurs when $L \sim \alpha^2 L_{\rm Edd} \sim L_{\rm Edd}$, and the first "thin" disk to form is itself moderately thick, strongly magnetized, and able to advect field inward. These properties favor episodic jet production. As the accretion rate declines magnetic flux escapes, $\alpha$ decreases to $\alpha \sim 0.01 - 0.1$, and a hot inner flow is not re-established until $L \ll L_{\rm Edd}$. We discuss possible observational consequences of our scenario.