Journal Article

Is GeV emission from Gamma-Ray Bursts of external shock origin?

Amanda Maxham, Bin-Bin Zhang and Bing Zhang

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 415, issue 1, pages 77-82
Published in print July 2011 | ISSN: 0035-8711
Published online July 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.18648.x
Is GeV emission from Gamma-Ray Bursts of external shock origin?

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Recent observations of Gamma-Ray Bursts (GRBs) by the Fermi Large Area Telescope (LAT) revealed a power-law decay feature of the high-energy emission (above 100 MeV), which led to the suggestion that it originates from an external shock. We analyse four GRBs (080916C, 090510, 090902B and 090926A) jointly detected by Fermi LAT and Gamma-ray Burst Monitor (GBM), which have high-quality light curves in both instrument energy bands. Using the MeV prompt emission (GBM) data, we can record the energy output from the central engine as a function of time. Assuming a constant radiative efficiency, we are able to track energy accumulation in the external shock using our internal/external shell model code. By solving for the early evolution of both an adiabatic and a radiative blastwave, we calculate the high-energy emission light curve in the LAT band and compare it with the observed one for each burst. The late time LAT light curves after T90 can be well fitted by the model. However, due to continuous energy injection into the blastwave during the prompt emission phase, the early external shock emission cannot account for the observed GeV flux level. The high-energy emission during the prompt phase (before T90) is most likely a superposition of a gradually enhancing external shock component and a dominant emission component that is of an internal origin.

Keywords: shock waves; gamma-ray burst: general; gamma-ray burst: individual: 080916C; gamma-ray burst: individual: 090510; gamma-ray burst: individual: 090902B; gamma-ray burst: individual: 090926A

Journal Article.  3936 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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