Comptonization by Reconnection Plasmoids in Black Hole Coronae III: Dependence on the Guide Field in Pair Plasma

TL;DR

This study uses particle-in-cell simulations to explore how the guide field strength influences magnetic reconnection and plasmoid dynamics in pair plasmas, relevant for understanding X-ray emissions in black hole coronae.

Contribution

It demonstrates that guide field strength significantly affects plasmoid bulk energies and motion stochasticity, providing new insights into reconnection processes in black hole environments.

Findings

Bulk energy depends weakly on magnetization but strongly on guide field.

Guide field reduces stochasticity of plasmoid motions.

Strong guide fields lead to slower, more ordered plasmoid flows.

Abstract

We perform two-dimensional particle-in-cell simulations of magnetic reconnection for various strengths of the guide field (perpendicular to the reversing field), in magnetically-dominated electron-positron plasmas. Magnetic reconnection under such conditions could operate in accretion disk coronae around black holes. There, it has been suggested that the trans-relativistic bulk motions of reconnection plasmoids containing inverse-Compton-cooled electrons could Compton-upscatter soft photons to produce the observed non-thermal hard X-rays. Our simulations are performed for magnetizations $3 \leq \sigma \leq 40$ (defined as the ratio of enthalpy density of the reversing field to plasma enthalpy density) and guide field strengths $0 \leq B_{\rm g}/B_0 \leq 1$ (normalized to the reversing field strength $B_0$). We find that the mean bulk energy of the reconnected plasma depends only weakly on the flow magnetization but strongly on the guide field strength -- with $B_{\rm g}/B_0 = 1$ yielding a mean bulk energy twice smaller than $B_{\rm g}/B_0 = 0$. Similarly, the dispersion of bulk motions around the mean -- a signature of stochasticity in the plasmoid chain's motions -- is weakly dependent on magnetization (for $\sigma \gtrsim 10$) but strongly dependent on the guide field strength -- dropping by more than a factor of two from $B_{\rm g}/B_0 = 0$ to $B_{\rm g}/B_0 = 1$. In short, reconnection in strong guide fields ($B_{\rm g}/B_0 \sim 1$) leads to slower and more ordered plasmoid bulk motions than its weak guide field ($B_{\rm g}/B_0 \sim 0$) counterpart.