Revisiting the Thermal Stability of Radiation-dominated Thin Disks

TL;DR

This paper revisits the thermal stability of radiation-dominated thin disks by incorporating magnetic field effects, explaining observed stability in high-luminosity black hole binaries and the unique instability of GRS 1915+105.

Contribution

It introduces a linear stability analysis including magnetic field responses, showing increased instability thresholds and aligning theory with observations.

Findings

Magnetic fields raise the accretion rate threshold for thermal instability.

Observed high-luminosity sources remain stable due to magnetic effects.

GRS 1915+105's instability is explained by its exceptionally high accretion rate.

Abstract

The standard thin disk model predicts that when the accretion rate is over a small fraction of the Eddington rate, which corresponds to $L \ga 0.06 L_{Edd}$, the inner region of the disk is radiation-pressure-dominated and thermally unstable. However, observations of the high/soft state of black hole X-ray binaries with luminosity well within this regime ($0.01L_{Edd} \la L \la 0.5L_{Edd}$) indicate that the disk has very little variability, i.e., quite stable. Recent radiation magnetohydrodynamic simulations of a vertically stratified shearing box have confirmed the absence of the thermal instability. In this paper, we revisit the thermal stability by linear analysis, taking into account the role of magnetic field in the accretion flow. By assuming that the field responses negatively to a positive temperature perturbation, we find that the threshold of accretion rate above which the disk becomes thermally unstable increases significantly compared with the case of not considering the role of magnetic field. This accounts for the stability of the observed sources with high luminosities. Our model also presents a possible explanation as to why only GRS 1915+105 seems to show thermally unstable behavior. This peculiar source holds the highest accretion rate (or luminosity) among the known high state sources, which is well above the accretion rate threshold of the instability.