Journal Article

Lorentz factor constraint from the very early external shock of the gamma-ray burst ejecta

Yuan-Chuan Zou and Tsvi Piran

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 402, issue 3, pages 1854-1862
Published in print March 2010 | ISSN: 0035-8711
Published online February 2010 | e-ISSN: 1365-2966 | DOI:
Lorentz factor constraint from the very early external shock of the gamma-ray burst ejecta

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While it is generally agreed that the emitting regions in gamma-ray bursts move ultra-relativistically towards the observer, different estimates of the initial Lorentz factors, Γ0, lead to different at times conflicting estimates. We show here that the quiet periods in which the signals goes down below the instrumental thresholds put strong upper limits on the values of Γ0. According to the standard internal–external shock model, an external shock should develop during the prompt stage. This external shock radiates in the hard X-rays to soft gamma-rays bands and this emission should be seen as a smooth background signal. The observed deep minima indicate that this contribution is negligible. This limits, in turn, Γ0. We obtain upper limits on Γ0 for several bursts with typical values around hundreds. We compare these values with those obtained by the other methods, which typically yield lower limits. The results are marginally consistent leaving only a narrow range of allowed values for Γ0.

Keywords: radiation mechanism: non-thermal; gamma rays: bursts

Journal Article.  7465 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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