Abstract
We report the detection of a faint optical flash by the 2-m Faulkes Telescope North simultaneously with the second of two prompt gamma-ray pulses in INTEGRAL gamma-ray burst (GRB) 080603A, beginning at t_rest = 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 broadband radio to X-ray light curves to 13 days post-burst and rigorously test the standard fireball model. The intrinsic extinction toward GRB 080603A is high (A_V,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 gamma-ray and extinction-corrected optical flux densities of the flash rules out an inverse-Compton origin for the prompt gamma-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 gamma-rays, followed by a milder internal shock at a larger radius that would cause the optical flash. Flat gamma-ray spectra, roughly F propto nu^-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 gamma-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.
Highlights
The exact mechanism that produces the prompt radiation of a gamma-ray burst (GRB) is still unknown
Given the apparent lack of chromatic changes, to better constrain the evolution we simultaneously fitted the light curves of the various bands with the same function, only allowing different normalizations and no colour change
We considered two different models, either a simple power law and a broken power law with a cooling break, β = 0.5, combined with three different extinction profiles according to the parametrization of Pei (1992): Small Magellanic Cloud (SMC), LMC2 [for the Large Magellanic Cloud (LMC)], and Milky Way (MW)
Summary
The exact mechanism that produces the prompt radiation of a gamma-ray burst (GRB) is still unknown. As a non-thermal process, synchrotron and inverse Compton (IC) are the main candidates. The former can successfully account for most of the afterglow emission evolution, and is naturally expected from shock-accelerated electrons. As such, this has been considered for explaining the γ -ray prompt emission itself. The value generally observed, α ≈ −1, differs from the value of −3/2 expected for rapidly cooling electrons (the so-called ‘fast-cooling death line’; Ghisellini, Celotti & Lazzati 2000). Under some assumptions most GRB spectra could be reconciled with a synchrotron origin (e.g. Lloyd & Petrosian 2000; Daigne, Bosnjak & Dubus 2011), the question of whether it is the dominant process in the GRB production remains unanswered
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