Journal Article

A faint optical flash in dust-obscured GRB 080603A: implications for GRB prompt emission mechanisms

C. Guidorzi, S. Kobayashi, D. A. Perley, G. Vianello, J. S. Bloom, P. Chandra, D. A. Kann, W. Li, C. G. Mundell, A. Pozanenko, J. X. Prochaska, K. Antoniuk, D. Bersier, A. V. Filippenko, D. A. Frail, A. Gomboc, E. Klunko, A. Melandri, S. Mereghetti, A. N. Morgan, P. T. O’Brien, V. Rumyantsev, R. J. Smith, I. A. Steele, N. R. Tanvir and A. Volnova

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 417, issue 3, pages 2124-2143
Published in print November 2011 | ISSN: 0035-8711
Published online October 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19394.x
A faint optical flash in dust-obscured GRB 080603A: implications for GRB prompt emission mechanisms

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We report the detection of a faint optical flash by the 2-m Faulkes Telescope North simultaneously with the second of two prompt γ-ray pulses in INTEGRAL gamma-ray burst (GRB) 080603A, beginning at trest= 37 s after the onset of the GRB. This optical flash appears to be distinct from the subsequent emerging afterglow emission, for which we present comprehensive broad-band radio to X-ray light curves to 13 d post-burst and rigorously test the standard fireball model. The intrinsic extinction towards GRB 080603A is high (AV, z= 0.8 mag), and the well-sampled X-ray-to-near-infrared spectral energy distribution is interesting in requiring an LMC2 extinction profile, in contrast to the majority of GRBs. Comparison of the γ-ray and extinction-corrected optical flux densities of the flash rules out an inverse-Compton origin for the prompt γ-rays; instead, we suggest that the optical flash could originate from the inhomogeneity of the relativistic flow. In this scenario, a large velocity irregularity in the flow produces the prompt γ-rays, followed by a milder internal shock at a larger radius that would cause the optical flash. Flat γ-ray spectra, roughly F∝ν−0.1, are observed in many GRBs. If the flat spectrum extends down to the optical band in GRB 080603A, the optical flare could be explained as the low-energy tail of the γ-ray emission. If this is indeed the case, it provides an important clue to understanding the nature of the emission process in the prompt phase of GRBs and highlights the importance of deep (R > 20 mag), rapid follow-up observations capable of detecting faint, prompt optical emission.

Keywords: radiation mechanisms: non-thermal; gamma-ray burst: individual: GRB 080603A

Journal Article.  16554 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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