Radiation GRMHD Simulations of the Hard State of Black Hole X-ray Binaries and the Collapse of a Hot Accretion Flow

TL;DR

This paper uses advanced radiation GRMHD simulations to study hot accretion flows onto black holes, revealing their spectral properties, efficiency, and collapse behavior at different luminosities, relevant for understanding black hole X-ray binary states.

Contribution

It presents the first self-consistent radiation GRMHD simulations of hot accretion flows, showing their spectral features, efficiency, and collapse at specific luminosities.

Findings

Emergent spectra resemble observed hard states with power-law shape up to 100 keV.

Radiative efficiency is approximately 24% at certain luminosities.

Hot accretion flows can undergo thermal collapse at higher accretion rates.

Abstract

We present global radiation GRMHD simulations of strongly magnetized accretion onto a spinning, stellar mass black hole at sub-Eddington rates. Using a frequency-dependent Monte Carlo procedure for Compton scattering, we self-consistently evolve a two-temperature description of the ion-electron fluid and its radiation field. For an Eddington ratio $L/L_{\rm Edd} \gtrsim 10^{-3}$, the emergent spectrum forms an apparent power law shape from thermal Comptonization up to a cutoff at $\simeq 100$ keV, characteristic of that seen in the hard spectral states of black hole X-ray binary systems. At these luminosities, the radiative efficiency is high ($\approx 24\%$) and results in a denser midplane region where magnetic fields are dynamically important. For $L/L_{\rm Edd} \sim 10^{-2}$, our hot accretion flow appears to undergo thermal runaway and collapse. Our simulations demonstrate that hot accretion flows can be radiatively efficient and provide an estimate of their maximum luminosity.