Identification of Two Categories of Optically Bright Γ-ray Bursts

We present the results of a systematical analysis of the intrinsic optical afterglow light curves for a complete sample of gamma-ray bursts (GRBs) observed in the period from Feb. 1997 to Aug. 2005. 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 lightcurves (luminosity versus time within the cosmic proper rest frame) for these GRBs, and discover a fact that they essentially follow two universal tracks after 2 hours since the GRB triggers. The optical luminosities at 1 day show a clear bimodal distribution, peaking at 1.4 × 10 ergs s for the luminous group and 5.3×10 ergs s 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 wide range of redshift distribution, the bursts in the dim group all appear at a redshift lower than 1.1. Subject headings: gamma rays: bursts—gamma rays: observations—methods: statistical