Comptonization by Reconnection Plasmoids in Black Hole Coronae II: Electron-Ion Plasma

TL;DR

This study uses particle-in-cell simulations to explore how magnetic reconnection in electron-ion plasmas near black holes produces X-ray spectra, emphasizing the role of plasmoid motions in generating observed photon energies.

Contribution

It demonstrates that magnetic reconnection with mildly relativistic plasmoid motions can produce the observed X-ray spectra in black hole coronae, highlighting the importance of bulk motions over thermal electron heating.

Findings

Reconnection generates ~100 keV photons via plasmoid motions.

Spectrum peak E_pk depends on magnetization sigma, reaching ~40 keV for sigma=1 and ~100 keV for sigma=3.

Non-thermal MeV tail arises from electron acceleration near X-points.

Abstract

We perform two-dimensional particle-in-cell simulations of magnetic reconnection in electron-ion plasmas subject to strong Compton cooling and calculate the X-ray spectra produced by this process. The simulations are performed for trans-relativistic reconnection with magnetization $1\leq \sigma\leq 3$ (defined as the ratio of magnetic tension to plasma rest-mass density), which is expected in the coronae of accretion disks around black holes. We find that magnetic dissipation proceeds with inefficient energy exchange between the heated ions and the Compton-cooled electrons. As a result, most electrons are kept at a low temperature in Compton equilibrium with radiation, and so thermal Comptonization cannot reach photon energies $\sim 100$ keV observed from accreting black holes. Nevertheless, magnetic reconnection efficiently generates $\sim 100$ keV photons because of mildly relativistic bulk motions of the plasmoid chain formed in the reconnection layer. Comptonization by the plasmoid motions dominates the radiative output and controls the peak of the radiation spectrum $E_{\rm pk}$. We find $E_{\rm pk}\sim 40$ keV for $\sigma=1$ and $E_{\rm pk}\sim100$ keV for $\sigma=3$. In addition to the X-ray peak around 100 keV, the simulations show a non-thermal MeV tail emitted by a non-thermal electron population generated near X-points of the reconnection layer. The results are consistent with the typical hard state of accreting black holes. In particular, we find that the spectrum of Cygnus~X-1 is well explained by electron-ion reconnection with $\sigma\sim 3$.