Статья

High Quality Early Time Light Curves of GRB 060206: Implications for Gamma Ray Burst Environments and Energetics

Monfardini, ACurrent Address: Il Centro per la Ricerca Scientifica e Tecnologica (ITC-IRST) and Istituto Nazionale di Fisica Nucleare (INFN), Trento, via Sommarive, 18 38050 Povo (TN), ItalyKobayashi, SAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKGuidorzi, CAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKCarter, DAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKMundell, C GAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKBersier, D FAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKGomboc, ACurrent address: Fakulteta za Matematiko in Fiziko (FMF), University of Ljubljana, Jadranska 19, 1000 Ljubljana, SloveniaMelandri, AAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKMottram, C JAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKSmith, R JAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKSteele, I AAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UK

2006en

ABI

Аннотация

The 2-m robotic Liverpool Telescope (LT) reacted promptly to the high-redshift (z=4.048) gamma-ray burst GRB 060206. The afterglow was identified automatically and multicolor r'i'z' imaging was triggered without human intervention. Combining our data with those obtained from later follow-ups provides a well-sampled optical light-curve from 5 minutes to >2d after the gamma event. The light-curve is highly structured with at least three bumps evident in the first 75 minutes, including a major rebrightening (Delta_r'=-1.6 at t=3000s), interpreted as late energy injection. At early time (t=440s), we find evidence for fast (Delta t_(rest)<4s<<t) variability, indicating on-going internal-engine activity. We emphasise that a low redshift GRB (z<1) with similar intrinsic properties would have been interpreted completely differently due to undersampling of the light curve in the rest frame at early times; the light-curve behaviour of GRB 060206 should therefore not be considered peculiar. Finally, although the observed late-time steepening of the optical light curve resembles a jet break if taken in isolation, the lack of a corresponding change in the X-ray slope rules out a jet-break interpretation. Traditionally, GRB jet breaks have been inferred from optical data in the absence of simultaneous X-ray data. We suggest therefore that current estimates of the jet opening angle distribution might be biased by events like GRB060206. Consequently, the GRB explosion energy distribution and event rates may have to be revised.

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