Journal Article

Constraints on cold magnetized shocks in gamma-ray bursts

Ramesh Narayan, Pawan Kumar and Alexander Tchekhovskoy

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 416, issue 3, pages 2193-2201
Published in print September 2011 | ISSN: 0035-8711
Published online September 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19197.x
Constraints on cold magnetized shocks in gamma-ray bursts

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We consider a model in which the ultrarelativistic jet in a gamma-ray burst (GRB) is cold and magnetically accelerated. We assume that the energy flux in the outflowing material is partially thermalized via internal shocks or a reverse shock, and we estimate the maximum amount of radiation that could be produced in such magnetized shocks. We compare this estimate with the available observational data on prompt γ-ray emission in GRBs. We find that, even with highly optimistic assumptions, the magnetized jet model is radiatively too inefficient to be consistent with observations. One way out is to assume that much of the magnetic energy in the post-shock, or even pre-shock, jet material is converted to particle thermal energy by some unspecified process, and then radiated. This can increase the radiative efficiency sufficiently to fit observations. Alternatively, jet acceleration may be driven by thermal pressure rather than magnetic fields. In this case, which corresponds to the traditional fireball model, sufficient prompt GRB emission could be produced either from shocks at a large radius or from the jet photosphere closer to the centre.

Keywords: acceleration of particles; MHD; radiation mechanisms: non-thermal; relativistic processes; shock waves; gamma-ray burst: general

Journal Article.  7414 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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