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    Electron and Proton Acceleration in Trans-relativistic Magnetic Reconnection: Dependence on Plasma Beta and Magnetization

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    Ball_2018_ApJ_862_80.pdf
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    Final Published version
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    Author
    Ball, David
    Sironi, Lorenzo
    Özel, Feryal
    Affiliation
    Univ Arizona, Dept Astron
    Univ Arizona, Steward Observ
    Issue Date
    2018-07-20
    Keywords
    accretion, accretion disks
    galaxies: jets
    magnetic reconnection
    radiation mechanisms: nonthermal
    X-rays: binaries
    
    Metadata
    Show full item record
    Publisher
    IOP PUBLISHING LTD
    Citation
    David Ball et al 2018 ApJ 862 80
    Journal
    ASTROPHYSICAL JOURNAL
    Rights
    © 2018. The American Astronomical Society. All rights reserved.
    Collection Information
    This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu.
    Abstract
    Nonthermal electron acceleration via magnetic reconnection is thought to play an important role in powering the variable X-ray emission from radiatively inefficient accretion flows around black holes. The trans-relativistic regime of magnetic reconnection-where the magnetization sigma, defined as the ratio of magnetic energy density to enthalpy density, is similar to 1-is frequently encountered in such flows. By means of a large suite of two-dimensional particle-in-cell simulations, we investigate electron and proton acceleration in the trans-relativistic regime. We focus on the dependence of the electron energy spectrum on sigma and the proton beta (the ratio of proton thermal pressure to mio agnetic pressure). We find that the electron spectrum in the reconnection region is nonthermal and can be modeled as a power law. At low beta, the slope, p, is independent of beta and hardens with increasing sigma as p similar or equal to 1.8 + 0.7/root sigma. Electrons are primarily accelerated by the nonideal electric field at X-points, either in the initial current layer or in current sheets generated between merging magnetic islands. At higher values of beta, the electron power law steepens, and the electron spectrum eventually approaches a Maxwellian distribution for all values of sigma. At values of beta near beta(max) approximate to 1/4 sigma, when both electrons and protons are relativistically hot prior to reconnection, the spectra of both species display an additional component at high energies, containing a few percent of particles. These particles are accelerated via a Fermi-like process by bouncing between the reconnection outflow and a stationary magnetic island
    ISSN
    1538-4357
    DOI
    10.3847/1538-4357/aac820
    Version
    Final published version
    Sponsors
    NSF [AST-1715061, ACI-1657507, AST1716567]; Chandra Award [TM6-17006X]; John Simon Guggenheim Memorial Foundation; DoE [DE-SC0016542]; NASA [Fermi NNX-16AR75G, ATP NNX-17AG21G]
    Additional Links
    http://stacks.iop.org/0004-637X/862/i=1/a=80?key=crossref.4d6cf38312083f563656e5d76948123a
    ae974a485f413a2113503eed53cd6c53
    10.3847/1538-4357/aac820
    Scopus Count
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    UA Faculty Publications

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