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

Abstract

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.

GRB 091024a and the Nature of Ultra-Long Gamma-Ray Bursts

“…GRBs 080407A, 091024A, and 110709B have long interpulse regions lasting ∼ 1400, 500, and 600 s, respectively. The BATSE bursts discussed previously have interpulse episodes of a similar timescale, with a few bursts reaching 600-1400 s. All bursts tend to have emission episode durations of 50-200 s. An intermediate case is found in bursts like GRB 010619A, discovered by BeppoSAX (Frontera et al 2009;Guidorzi et al 2011a). Its duration is shorter than the rest of the sample (T 90 ≈ 450 s), but it shows many of the characteristics of this class, including intense emission episodes and significant periods of quiescence between emission episodes.…”

“…Some ex-hibit clear temporal coincidence between optical and γ-ray features, suggesting a prompt origin (Vestrand et al 2005;Vestrand et al 2006;Racusin et al 2008;Guidorzi et al 2011a;Kopač et al 2013).…”

Limits on Optical Polarization During the Prompt Phase of GRB 140430a

“…13 For discussion on X-ray flares see e.g., O'Brien et al 2006;Chincarini et al 2007;Margutti et al 2010. 14 For optical flares/rebrightenings see e.g., Akerlof et al 1999;Blake et al 2005;Vestrand et al 2005;Page et al 2007;Mundell et al 2007a;Racusin et al 2008;Thöne et al 2010;Guidorzi et al 2011;Gendre et al 2012Gendre et al , 2013Kopač et al 2013;Virgili et al 2013;Elliott et al 2014;Greiner et al 2014;Vestrand et al 2014;Nappo et al 2014. 15 For X-ray flashes see e.g., Boër et al 2006;Krühler et al 2009;Guidorzi et al 2009. On 2014 April 30, at t 0 = 20 : 33 : 36 UT, the Burst Alert Telescope (BAT; Barthelmy et al 2005) onboard the Swift satellite triggered on the long GRB 140430A and immediately slewed to the burst (Siegel et al 2014).…”

Radio Flares From Gamma-Ray Bursts

“…Despite ten years of accurate GRB localizations disseminated automatically in real-time from Swift (Gehrels et al 2004) and rapid, ground-based follow-up by autonomous robotic optical telescopes (e.g., Monfardini et al 2006;Guidorzi et al 2011;Virgili et al 2013), only a small fraction (∼5%) of early-time optical light curves show clear evidence of optical RS emission (Mészáros & Rees 1999;Sari & Piran 1999b;Kobayashi 2000;Roming et al 2006;Gomboc et al 2008;Kopač et al 2013;Japelj et al 2014). Mundell et al (2007) suggested that this lack of bright optical RS emission may be explained if the typical synchrotron frequency of the RS already lies at radio fre-quencies at early time.…”