Electron and Proton Acceleration in Trans-relativistic Magnetic Reconnection: Dependence on Plasma Beta and Magnetization
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Final Published version
Affiliation
Univ Arizona, Dept AstronUniv Arizona, Steward Observ
Issue Date
2018-07-20Keywords
accretion, accretion disksgalaxies: jets
magnetic reconnection
radiation mechanisms: nonthermal
X-rays: binaries
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IOP PUBLISHING LTDCitation
David Ball et al 2018 ApJ 862 80Journal
ASTROPHYSICAL JOURNALRights
© 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 islandISSN
1538-4357Version
Final published versionSponsors
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.4d6cf38312083f563656e5d76948123aae974a485f413a2113503eed53cd6c53
10.3847/1538-4357/aac820