Electron-positron Pair Production in Global GRMHD Simulations of Black Hole Accretion Flows

TL;DR

This paper uses 3D general relativistic MHD simulations to study electron-positron pair production in black hole accretion flows, revealing pair distributions, equilibrium states, and implications for jet physics.

Contribution

It introduces a novel simulation approach incorporating pair physics as a passive scalar, providing insights into pair dynamics and their role in black hole accretion and jet environments.

Findings

Maximum pair fraction of ~0.01 in high accretion models

Presence of a 'pair void' near the black hole

Pairs in the upper corona and jets can exceed equilibrium values

Abstract

We present global, three-dimensional general relativistic magnetohydrodynamic simulations of accreting black holes that incorporate pair physics. Pairs are modeled as a passive scalar that maintains a constant temperature. For high accretion rate models, we observe a maximum pair fraction of $\sim \mathcal{O}(0.01)$, consistent with those inferred from some X-ray binaries, and identify a `pair void' extending to a few gravitational radii from the black hole. Pair fractions peak in the midplane just outside the plunging region and within a thin strip at the base of the corona. For moderate to high accretion rate models, pairs are near equilibrium close to the disk midplane, where the scattering optical depth is high and pair equilibrium timescales are short, and could be comparable to the Coulomb collision timescale. This suggests the possibility of a pair-regulated coronal temperature. In contrast, the upper corona and jets, where the scattering optical depth is relatively low and pair equilibrium timescales are long, are populated with pairs that may exceed their equilibrium value by orders of magnitude. These pairs are transported by advection from the disk, which dominates over local pair processes. This result highlights advection as a significant source of pair injection, which may be relevant for certain X-ray binaries exhibiting $\gamma$-ray signatures. The pair density along the magnetically dominated poles exceeds the Goldreich-Julian density in some models.