Temperature properties in magnetised and radiatively cooled two-temperature accretion flows onto a black hole

TL;DR

This paper develops a self-consistent two-temperature GRMHD simulation method for accretion flows onto black holes, revealing significant impacts of Coulomb coupling and radiative cooling on thermodynamics and potential observational signatures.

Contribution

It introduces a novel approach to simulate magnetised, radiatively cooled two-temperature accretion flows, including new temperature ratio relations based on plasma-$\beta$.

Findings

Coulomb interaction and radiative cooling significantly alter electron and ion temperature distributions.

Accretion mode (SANE vs MAD) affects temperature properties despite similar accretion rates.

Proposed temperature ratio relations can effectively model two-temperature flows around supermassive black holes.

Abstract

Simplified assumptions about the thermodynamics of the electrons are normally employed in general-relativistic magnetohydrodynamic (GRMHD) simulations of accretion onto black holes. To counter this, we have developed a self-consistent approach to study magnetised and radiatively cooled two-temperature accretion flows around a Kerr black hole in two spatial dimensions. The approach includes several heating processes, radiative cooling, and a coupling between the electrons and the ions via Coulomb interaction. We test our approach by performing axisymmetric GRMHD simulations of magnetised tori accreting onto a Kerr black hole under various astrophysical scenarios. In this way, we find that the inclusion of the Coulomb interaction and the radiative cooling impacts the thermodynamical properties of both the ions and electrons, changing significantly the temperature distribution of the latter, and underlining the importance of a two-temperature approach when imaging these flows. In addition, we find that the accretion rate influences the bulk properties of the flow as well as the thermodynamics of the electrons and ions. Interestingly, we observe qualitatively distinct temperature properties for SANE and MAD accretion modes while maintaining the same accretion rates, which could help distinguishing MAD and SANE accretion flows via observations. Finally, we propose two new relations for the temperature ratios of the electrons, ions, and of the gas in terms of the plasma-$\beta$ parameter. The new relations represent a simple and effective approach to treat two-temperature accretion flows on supermassive black holes such as Sgr A* and M\,87*.