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Radiation-mediated Shocks in Gamma-Ray Bursts: Subshock Photon Production
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. Columbia Univ, Phys Dept, 538 West 120th St, New York, NY 10027 USA.;Columbia Univ, Columbia Astrophys Lab, 538 West 120th St, New York, NY 10027 USA..ORCID iD: 0000-0002-0642-1055
Columbia Univ, Phys Dept, 538 West 120th St, New York, NY 10027 USA.;Columbia Univ, Columbia Astrophys Lab, 538 West 120th St, New York, NY 10027 USA..
2019 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 879, no 2, article id 83Article in journal (Refereed) Published
Abstract [en]

Internal shocks provide a plausible heating mechanism in the jets of gamma-ray bursts (GRBs). Shocks occurring below the jet photosphere are mediated by radiation. It was previously found that radiation-mediated shocks (RMSs) inside GRB jets are inefficient photon producers, and the photons that mediate the RMS must originate from an earlier stage of the explosion. We show that this conclusion is valid only for nonmagnetized jets. RMSs that propagate in moderately magnetized plasma develop a collisionless subshock that locally heats the plasma to a relativistic temperature, and the hot electrons emit copious synchrotron photons inside the RMS. We find that this mechanism is effective for mildly relativistic shocks and may be the main source of photons observed in GRBs. We derive a simple analytical estimate for the generated photon number per proton, Z, which gives Z = 10(5)-10(6), consistent with observations. The number is controlled by two main factors: (1) the abundance of electron-positron pairs created in the shock, which is self-consistently calculated, and (2) the upper limit on the brightness temperature of soft radiation set by induced Compton scattering. The photons are initially injected with low energies that are well below the observed GRB peak. The injected soft photons that survive induced downscattering and free-free absorption gain energy in the RMS via bulk Comptonization and shape its nonthermal spectrum.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2019. Vol. 879, no 2, article id 83
Keywords [en]
gamma-ray burst: general, magnetohydrodynamics (MHD), radiation mechanisms: thermal, radiative transfer, scattering, shock waves
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:kth:diva-255375DOI: 10.3847/1538-4357/ab229fISI: 000474750300004Scopus ID: 2-s2.0-85071870760OAI: oai:DiVA.org:kth-255375DiVA, id: diva2:1341300
Note

QC 20190808

Available from: 2019-08-08 Created: 2019-08-08 Last updated: 2020-03-09Bibliographically approved

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Lundman, Christoffer

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