Magnetic flux stabilizing thin accretion disks

TL;DR

This paper determines the minimum large-scale magnetic flux needed to stabilize thin accretion disks around black holes, comparing it with stellar magnetic flux and discussing implications for system stability and variability.

Contribution

It introduces a calculation for the minimal magnetic flux required for thermal stability in thin disks, linking magnetic field strength to accretion rate and system stability.

Findings

Minimal magnetic flux depends on accretion rate as (dot M/dot M_Edd)^{20/21}

Companion star's magnetic flux may be insufficient at high accretion rates

High variability in GRS 1915+105 may be due to magnetic flux shortage

Abstract

We calculate the minimal amount of large-scale poloidal magnetic field that has to thread the inner, radiation-over-gas pressure dominated region of a thin disk for its thermal stability. Such a net field amplifies the magnetization of the saturated turbulent state and makes it locally stable. For a $10 M_\odot$ black hole the minimal magnetic flux is $10^{24}(\dot M/\dot M_{\rm Edd})^{20/21}\,\rm G\cdot cm^{2}$. This amount is compared with the amount of uniform magnetic flux that can be provided by the companion star -- estimated to be in the range $10^{22}-10^{24}\,\rm G\cdot cm^2$. If accretion rate is large enough, the companion is not able to provide the required amount and such a system, if still sub-Eddington, must be thermally unstable. The peculiar variability of GRS 1915+105, an X-ray binary with the exceptionally high BH mass and near-Eddington luminosity, may result from the shortage of large scale poloidal field of uniform polarity.