Formation of Overheated Regions and Truncated Disks around Black Holes; Three-dimensional General Relativistic Radiation-magnetohydrodynamics Simulations

TL;DR

This study uses 3D general relativistic radiation magnetohydrodynamics simulations to explore the formation of truncated, overheated accretion disks around black holes, revealing temperature structures and dependencies on accretion rate and black hole spin.

Contribution

First detailed 3D GRRMHD simulations showing the formation of overheated regions and disk truncation around black holes, linking simulation results with observed high/ultraluminous X-ray states.

Findings

Truncated cold disk at ~30 r_g for accretion rate ~L_Edd/c^2.

Overheated regions appear within the truncation radius, sandwiching the cold disk.

Truncation radius decreases to ~10 r_g with higher accretion rates, near the innermost stable orbit.

Abstract

Using three-dimensional general relativistic radiation magnetohydrodynamics simulations of accretion flows around stellar mass black holes, we report that the relatively cold disk ($\gtrsim 10^{7}$K) is truncated near the black hole. Hot and less-dense regions, of which the gas temperature is $ \gtrsim 10^9$K and more than ten times higher than the radiation temperature (overheated regions), appear within the truncation radius. The overheated regions also appear above as well as below the disk, and sandwich the cold disk, leading to the effective Compton upscattering. The truncation radius is $\sim 30 r_{\rm g}$ for $\dot{M} \sim L_{\rm Edd}/c^2$, where $r_{\rm g}, \dot M, L_\mathrm{Edd}, c$ are the gravitational radius, mass accretion rate, Eddington luminosity, and light speed. Our results are consistent with observations of very high state, whereby the truncated disk is thought to be embedded in the hot rarefied regions. The truncation radius shifts inward to $\sim 10 r_{\rm g}$ with increasing mass accretion rate $\dot{M} \sim 100 L_{\rm Edd}/c^2$, which is very close to an innermost stable circular orbit. This model corresponds to the slim disk state observed in ultra luminous X-ray sources. Although the overheated regions shrink if the Compton cooling effectively reduces the gas temperature, the sandwich-structure does not disappear at the range of $\dot{M} \lesssim 100L_{\rm Edd}/c^2$. Our simulations also reveal that the gas temperature in the overheated regions depends on black hole spin, which would be due to efficient energy transport from black hole to disks through the Poynting flux, resulting gas heating.