The Radiative Efficiency and Spectra of Slowly Accreting Black Holes from Two-Temperature GRRMHD Simulations

TL;DR

This study uses advanced simulations to explore how radiative efficiency and emitted spectra of slowly accreting black holes depend on accretion rate, revealing a transition from synchrotron to inverse Compton dominance and a gradual increase in radiative efficiency.

Contribution

It provides the first self-consistent, frequency-dependent radiation transport simulations of low-rate accretion flows, revealing new insights into their thermodynamics and spectra.

Findings

Spectra vary from synchrotron to inverse Compton dominance with increasing accretion rate.

Flow approaches ~1% radiative efficiency at accretion rates of 10^{-5} M_Edd.

Most radiation at higher rates is produced far from the black hole.

Abstract

We present axisymmetric numerical simulations of radiatively inefficient accretion flows onto black holes combining general relativity, magnetohydrodynamics, self-consistent electron thermodynamics, and frequency-dependent radiation transport. We investigate a range of accretion rates up to $10^{-5} \dot{M}_{\mathrm{Edd}}$ onto a $10^8 M_{\odot}$ black hole with spin $a_{\star} = 0.5$. We report on averaged flow thermodynamics as a function of accretion rate. We present the spectra of outgoing radiation and find that it varies strongly with accretion rate, from synchrotron-dominated in the radio at low $\dot{M}$ to inverse Compton-dominated at our highest $\dot{M}$. In contrast to canonical analytic models, we find that by $\dot{M} \approx 10^{-5} \dot{M}_{\mathrm{Edd}}$, the flow approaches $\sim 1\%$ radiative efficiency, with much of the radiation due to inverse Compton scattering off Coulomb-heated electrons far from the black hole. These results have broad implications for modeling of accreting black holes across a large fraction of the accretion rates realized in observed systems.