Identification of two categories of optically bright gamma-ray bursts

Abstract

We present the results of a systematic analysis of the intrinsic optical afterglow light curves for a complete sample of gamma-ray bursts (GRBs) observed in the period from 1997 February to 2005 August. These light curves are generally well sampled, with at least four detections in the R band. The redshifts of all the bursts in the sample are available. We derive the intrinsic R-band afterglow light curves (luminosity vs. time within the cosmic proper rest frame) for these GRBs and discover that they essentially follow two universal tracks beyond 2 hours after the GRBs triggered. The optical luminosities at 1 day show a clear bimodal distribution, peaking at 1.4 × 10⁴⁶ ergs s^-1 for the luminous group and 5.3 × 10⁴⁴ ergs s^-1 for the dim group. About 75% of the GRBs are in the luminous group, and the other 25% belong to the dim group. While the luminous group has a widely distributed range of redshifts, the bursts in the dim group all appear at redshifts lower than 1.1. © 2006. The American Astronomical Society. All rights reserved.