Article

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

A. MonfardiniAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKS. KobayashiAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKC. GuidorziAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKD. CarterAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKC. G. MundellAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKD. F. BersierAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKA. GombocAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKA. MelandriAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKC. J. MottramAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKR. J. SmithAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UKI. A. SteeleAstrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, UK

2006en

ABI

Abstract

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 the multicolor r'i'z' imaging program 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 more than 2days 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 (Δr'~-1.6 at t~3000 s), interpreted as late energy injection. At early time (t~440 s), we find evidence for fast (Δtrest<4 s<<t) variability, indicating ongoing internal-engine activity. We emphasize 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 behavior 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 therefore suggest that current estimates of the jet-opening angle distribution might be biased by events like GRB 060206. Consequently, the GRB explosion energy distribution and event rates may have to be revised.

Identifiers

DOI: 10.1086/506170

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Cited by 30 references

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