"Anomalous" Optical Gamma-Ray Burst Afterglows Are Common: Two z ~ 4 Bursts, GRB 060206 and GRB 060210

Abstract

We report on two z ~ 4 gamma-ray bursts, GRB 060206 and GRB 060210, for which we have obtained well-sampled optical light curves. Both light curves show unusual behavior. GRB 060206 experienced a slow early decay, followed by a rapid increase in brightness by factor 2.5 about 1 hr after the burst. Its afterglow then faded in a broken power-law fashion, with a smooth break at tb = 0.6 days, but with additional, less dramatic bumps and wiggles. The afterglow of GRB 060210 is also unusual: the light curve was more or less flat between 60 and 300 s after the burst, followed by a 70% increase at 600 s after the burst, after which the light curve declined as a ~t-1.3 power law. We argue that anomalous optical afterglows are likely to be the norm and that such rapid optical variations should be seen in many bursts, given good enough sampling. Given that, some of the usual procedures, such as deriving the jet opening angle from fitting a smooth function to the optical light curve, might often have a poor statistical significance. We propose that the rapid rise at ~3000 s in the optical for GRB 060206 and the optical bump at ~700 s in GRB 060210 might be due to the turn-on of the external shock. The existence and timing of such features could provide us with valuable additional information about the bursts.