Accretion disk dynamo as the trigger for X-ray binary state transitions

TL;DR

This paper proposes a magnetic dynamo-driven model for accretion disk state transitions in X-ray binaries, linking magnetic field dynamics to spectral states and variability phenomena.

Contribution

It introduces a new model connecting magnetic dynamo activity and MRI zones to spectral states and variability in X-ray binary accretion disks.

Findings

Magnetically supported disks can have two MRI-active regions separated by a dead zone.

The spectral states of X-ray binaries are linked to the MRI activity in different disk zones.

The model explains the absence of thermal/viscous instability in luminous soft state disks.

Abstract

Magnetohydrodynamic accretion disk simulations suggest that much of the energy liberated by the magnetorotational instability (MRI) can be channeled into large-scale toroidal magnetic fields through dynamo action. Under certain conditions, this field can dominate over gas and radiation pressure in providing vertical support against gravity, even close to the midplane. Using a simple model for the creation of this field, its buoyant rise, and its coupling to the gas, we show how disks could be driven into this magnetically dominated state and deduce the resulting vertical pressure and density profiles. Applying an established criterion for MRI to operate in the presence of a toroidal field, we show that magnetically supported disks can have two distinct MRI-active regions, separated by a "dead zone" where local MRI is suppressed, but where magnetic energy continues to flow upward from the dynamo region below. We suggest that the relative strengths of the MRI zones, and the local poloidal flux, determine the spectral states of X-ray binaries. Specifically, "intermediate" and "hard" accretion states occur when MRI is triggered in the hot, upper zone of the corona, while disks in "soft" states do not develop the upper MRI zone. We discuss the conditions under which various transitions should take place and speculate on the relationship of dynamo activity to the various types of quasi-periodic oscillations that sometimes appear in the hard spectral components. The model also explains why luminous accretion disks in the "soft" state show no signs of the thermal/viscous instability predicted by standard alpha models.