# Evolving Nonthermal Electrons in Simulations of Black Hole Accretion

## TL;DR

This paper introduces a self-consistent method to evolve nonthermal electron populations in black hole accretion simulations, revealing their spectral features and minimal impact on overall dynamics.

## Contribution

It presents a novel approach to simulate nonthermal electrons in GRMHD models, capturing their evolution and spectral properties in accretion flows.

## Key findings

- Nonthermal electrons have negligible effect on system dynamics.
- The electron energy distribution shows a synchrotron cooling break.
- Radiative power is enhanced at large radii due to simple injection assumptions.

## Abstract

Current simulations of hot accretion flows around black holes assume either a single-temperature gas or, at best, a two-temperature gas with thermal ions and electrons. However, processes like magnetic reconnection and shocks can accelerate electrons into a nonthermal distribution, which will not quickly thermalise at the very low densities found in many systems. Such nonthermal electrons have been invoked to explain the infrared and X-ray spectra and strong variability of Sagittarius A* (Sgr A*), the black hole at the Galactic Center. We present a method for self-consistent evolution of a nonthermal electron population in the GRMHD code KORAL. The electron distribution is tracked across Lorentz factor space and is evolved in space and time, in parallel with thermal electrons, thermal ions, and radiation. In the present study, for simplicity, energy injection into the nonthermal distribution is taken as a fixed fraction of the local electron viscous heating rate. Numerical results are presented for a model with a low mass accretion rate similar to that of Sgr A*. We find that the presence of a nonthermal population of electrons has negligible effect on the overall dynamics of the system. Due to our simple uniform particle injection prescription, the radiative power in the nonthermal simulation is enhanced at large radii. The energy distribution of the nonthermal electrons shows a synchrotron cooling break, with the break Lorentz factor varying with location and time, reflecting the complex interplay between the local viscous heating rate, magnetic field strength, and fluid velocity.