Identification of Two Categories of Optically Bright Gamma-ray Bursts

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#10 ergs s 1 for the luminous group and 5.3#10 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. Subject headings: gamma rays: bursts — gamma rays: observations — methods: statistical Online material: machine-readable table