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Radiation-mediated Shocks in Gamma-Ray Bursts: Pair Creation
KTH, School of Biotechnology (BIO), Centres, Albanova VinnExcellence Center for Protein Technology, ProNova. KTH, School of Engineering Sciences (SCI), 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.;AlbaNova, Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden..
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..
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.;Tartu Observ, EE-61602 Toravere, Tartumaa, Estonia..
2018 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 858, no 1, article id 7Article in journal (Refereed) Published
Abstract [en]

Relativistic sub-photospheric shocks are a possible mechanism for producing prompt gamma-ray burst (GRB) emission. Such shocks are mediated by scattering of radiation. We introduce a time-dependent, special relativistic code which dynamically couples Monte Carlo radiative transfer to the flow hydrodynamics. The code also self-consistently follows electron-positron pair production in photon-photon collisions. We use the code to simulate shocks with properties relevant to GRBs. We focus on plane-parallel solutions, which are accurate deep below the photosphere. The shock generates a power-law photon spectrum through the first-order Fermi mechanism, extending upward from the typical upstream photon energy. Strong (high Mach number) shocks produce rising nu F-nu spectra. We observe that in non-relativistic shocks the spectrum extends to E-max similar to m(e)v(2), where v is the speed difference between the upstream and downstream. In relativistic shocks the spectrum extends to energies E > 0.1 m(e)c(2) where its slope softens due to Klein-Nishina effects. Shocks with Lorentz factors gamma > 1.5 are prolific producers of electron-positron pairs, yielding hundreds of pairs per proton. The main effect of pairs is to reduce the shock width by a factor of similar to Z(+/-)(-1). Most pairs annihilate far downstream of the shock, and the radiation spectrum relaxes to a Wien distribution, reaching equilibrium with the plasma at a temperature determined by the shock jump conditions and the photon number per proton. We discuss the implications of our results for observations of radiation generated by sub-photospheric shocks.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD , 2018. Vol. 858, no 1, article id 7
Keywords [en]
gamma-ray burst: general, plasmas, radiation mechanisms: non-thermal, radiative transfer, scattering
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-227749DOI: 10.3847/1538-4357/aab9b3ISI: 000431046300007OAI: oai:DiVA.org:kth-227749DiVA, id: diva2:1205727
Funder
Swedish Research Council
Note

QC 20180515

Available from: 2018-05-15 Created: 2018-05-15 Last updated: 2018-05-15Bibliographically approved

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