We perform 2D particleincell simulations of magnetic reconnection in electronion plasmas subject to strong Compton cooling and calculate the Xray spectra produced by this process. The simulations are performed for transrelativistic reconnection with magnetization 1 ≤ σ ≤ 3 (defined as the ratio of magnetic tension to plasma restmass energy density), which is expected in the coronae of accretion discs around black holes. We find that magnetic dissipation proceeds with inefficient energy exchange between the heated ions and the Comptoncooled 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 Epk. We find Epk ∼ 40 keV for σ = 1 and Epk ∼ 100 keV for σ = 3. In addition to the Xray peak around 100 keV, the simulations show a nonthermal MeV tail emitted by a nonthermal electron population generated near Xpoints 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 X1 is well explained by electronion reconnection with σ ∼ 3.
Effective Resistivity in Relativistic Collisionless Reconnection
Abstract Magnetic reconnection can power spectacular highenergy astrophysical phenomena by producing nonthermal energy distributions in highly magnetized regions around compact objects. By means of twodimensional fully kinetic particleincell (PIC) simulations, we investigate relativistic collisionless plasmoidmediated reconnection in magnetically dominated pair plasmas with and without a guide field. In Xpoints, where diverging flows result in a nondiagonal thermal pressure tensor, a finite residence time for particles gives rise to a localized collisionless effective resistivity. Here, for the first time for relativistic reconnection in a fully developed plasmoid chain, we identify the mechanisms driving the nonideal electric field using a full Ohm law by means of a statistical analysis based on our PIC simulations. We show that the nonideal electric field is predominantly driven by gradients of nongyrotropic thermal pressures. We propose a kinetic physics motivated nonuniform effective resistivity model that is negligible on global scales and becomes significant only locally in Xpoints. It captures the properties of collisionless reconnection with the aim of mimicking its essentials in nonideal magnetohydrodynamic descriptions. This effective resistivity model provides a viable opportunity to design physically grounded global models for reconnectionpowered highenergy emission.
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 Award ID(s):
 2206609
 NSFPAR ID:
 10431429
 Date Published:
 Journal Name:
 The Astrophysical Journal
 Volume:
 950
 Issue:
 2
 ISSN:
 0004637X
 Page Range / eLocation ID:
 169
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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