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

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 $3\le \sigma \le 40$ (defined as the ratio of enthalpy density of the reversing field to plasma enthalpy density) and guide field strengths $0\le B_{\mathrm{g}}/B_0\le 1$ (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 $B_{\mathrm{g}}/B_0=1$ yielding a mean bulk energy twice smaller than $B_{\mathrm{g}}/B_0=0$. 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 $B_{\mathrm{g}}/B_0=0$ to $B_{\mathrm{g}}/B_0=1$. In short, reconnection in strong guide fields ($B_{\mathrm{g}}/B_0\sim 1$) leads to slower and more ordered plasmoid bulk motions than its weak guide field ($B_{\mathrm{g}}/B_0\sim 0$) counterpart.

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