Bulk Comptonization by Reconnection Plasmoids in Black Hole Coronae
Abstract
The typical photon spectrum of accreting X-ray binaries in the hard state is modelled with a non-thermal power-law component. This component is usually interpreted as thermal Comptonization of disk photons by a cloud of trans-relativistic electrons in the disk ``corona''. However, the electron energization mechanism needed to balance the inverse Compton (IC) cooling remains uncertain. We perform first-principle 2D particle-in-cell simulations of magnetic reconnection--with a wide range of magnetizations (0 . 3 <= σ <= 40)--in electron-positron and electron-proton plasma, subject to different levels of IC cooling. We find that, for all the magnetizations we explored, the electrons' energy spectra are comprised of a high-energy peak dominated by particles with Lorentz factors of γ ∼ σ / 4 , and a low-energy component populated by cold particles residing inside plasmoids, which move as a bulk at trans-relativistic speeds. For σ >= 1 , the latter can be fit with a Maxwellian distribution with an effective temperature of Teff ∼ 100 keV, and so it could play the role of the electron distribution used in thermal Comptonization models. In summary, bulk Comptonization in reconnection may explain the hard state spectrum of accreting X-ray binaries.
- Publication:
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APS Division of Plasma Physics Meeting Abstracts
- Pub Date:
- 2020
- Bibcode:
- 2020APS..DPPJ03007S