Identification of two categories of optically bright gamma-ray bursts and a model-independent luminosity indicator

Abstract

We identify two categories of optically bright gamma-ray bursts (GRBs) with the intrinsic optical afterglow light curves for a complete sample of gamma-ray bursts (GRBs) detected from Feb. 1997 to Aug. 2005. The light curves follow two universal tracks at 2 hours in the rest frame since GRB triggers, and the optical luminosities at 1 day show a bimodal distribution. While the luminous group (∼ 75% of the GRBs in our sample) has a wide range of redshift distribution, the bursts in the dim group all appear at a redshift lower than 1.1. We make a blind search for the relations of gamma-ray energy to observabales of prompt gamma-rays and optical afterglows by a multi-variable regression analysis method without imposing any theoretical models and assumptions for the bursts in our sample. We find a tight relation of the isotropic gamma-ray energy to the the peak energy and the optical light curve break time in the rest frame. Regarding this relation as a luminosity indicator, we explore the possible constraints on the cosmological parameters. An approach also proposed to calibrate the GRB luminosity indicators without introducing a low-redshift GRB sample based on the Bayesian theory. © 2006 American Institute of Physics.