High-quality early-time light curves of GRB 060206: Implications for gamma-ray burst environments and energetics

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 (Δt_rest < 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.