Comptonization by reconnection plasmoids in black hole coronae II: Electron-ion plasma. Issue 1 (26th September 2022)

• Record Type: Journal Article
• Authors: Sridhar, Navin; Sironi, Lorenzo; Beloborodov, Andrei M
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 518:Issue 1(2023)
• Page Start: 1301
• Page End: 1315
• Publication Date: 2022-09-26
• Subjects: acceleration of particles -- black hole physics -- magnetic reconnection -- radiation mechanisms: non-thermal -- radiative transfer -- X-rays: binaries; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac2730
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We perform 2D 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 ≤ σ ≤ 3 (defined as the ratio of magnetic tension to plasma rest-mass 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 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 pk . We find E pk ∼ 40 keV for σ = 1 and E pk ∼ 100 keV for σ = 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 σ ∼ 3.