Scale-invariance of black hole accretion: modeling emission from a black hole X-ray binary with relativistic accretion flow simulations

TL;DR

This study uses relativistic accretion flow simulations to model the emission from a black hole X-ray binary, testing the scale-invariance of accretion physics across different black hole systems.

Contribution

It demonstrates that GRMHD simulations can reproduce observed spectra and polarization in a quiescent stellar-mass black hole system, supporting scale-invariance of accretion processes.

Findings

Model with strong electron-ion coupling matches observed X-ray spectra.

Predicted polarization levels align with NIR/optical observations.

Winds carry away insufficient angular momentum to explain orbital decay.

Abstract

We model non-thermal emission spectrum of the extremely sub-Eddington X-ray binary system A0620-00. It is believed that this non-thermal emission is produced by a radiatively inefficient "quiescent" accretion onto a stellar-mass black hole present in the system. We post-process GRMHD simulations with multiwavelength, fully polarized, relativistic radiative transfer calculations to predict broadband spectra and emission polarization levels for a range of electron models and accretion rates. We find that a model with strong coupling of electrons and ions in the accretion disk and accretion rate of only $\dot{M}=3\times10^{-13} [M_\odot yr^{-1}]$ is able to recover the observed X-ray spectral slope as well as the excess of linear polarization detected in the source in NIR/optical bands. Our models constrain the spectral properties of a putative relativistic jet produced in this system. In addition we show that the magnetized winds from our hot accretion flow carry away a small fraction of the orbital angular momentum of the binary unable to explain the observed rapid orbital decay of the system. Similar to the present GRMHD simulations are often used to explain emission from sub-Eddington supermassvie black holes in Sgr A* or M87; the present simulations allow us to test whether some aspects of the quiescent black hole accretion are scale invariant.