Ambient magnetic field amplification in shock fronts of relativistic jets : an application to GRB afterglows
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Strong downstream magnetic fields of the order of ∼1 G, with large correlation lengths, are believed to cause the large synchrotron emission at the afterglow phase of gamma-ray bursts (GRBs). Despite the recent theoretical efforts, models have failed to fully explain the amplification of the magnetic field, particularly in a matter-dominated scenario. We revisit the problem by considering the synchrotron emission to occur at the expanding shock front of a weakly magnetized relativistic jet over a magnetized surrounding medium. Analytical estimates and a number of high-resolution 2D relativistic magnetohydrodynamical (RMHD) simulations are provided. Jet opening angles of θ = 0°–20°, and ambient to jet density ratios of 10−4–102 were considered. We found that most of the amplification is due to compression of the ambient magnetic field at the contact discontinuity between the reverse and forward shocks at the jet head, with substantial pile-up of the magnetic field lines as the jet propagates sweeping the ambient field lines. The pile-up is maximum for θ → 0, decreasing with θ, but larger than in the spherical blast problem. Values obtained for certain models are able to explain the observed intensities. The maximum correlation lengths found for such strong fields is of lcorr ≤ 1014 cm, 2–6 orders of magnitude larger than the found in previous works.
Rocha da Silva , G , Falceta-Goncalves , D , Kowal , G & de Gouveia Dal Pino , E M 2015 , ' Ambient magnetic field amplification in shock fronts of relativistic jets : an application to GRB afterglows ' Monthly Notices of the Royal Astronomical Society , vol 446 , no. 1 , pp. 104-119 . DOI: 10.1093/mnras/stu2104
Monthly Notices of the Royal Astronomical Society
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
DescriptionGRS thanks CNPQ for financial support. DFG thanks the European Research Council (ADG-2011 ECOGAL) and the Brazilian agencies CNPq (No. 300382/2008-1), CAPES (3400-13-1) and FAPESP (No. 2011/12909-8) for financial support. GK thanks FAPESP (No. 2009/50053-8, 2011/51275-4, 2013/04073-2, 2013/18815-0) for financial support. EMGDP thanks FAPESP (No. 2006/50654-3) and CNPq (306598/2009-4) for financial support. Date of Acceptance: 06/10/2014
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