Gamma photometric redshifts for long gamma-ray bursts

Abstract

It is known that the soft tail of the gamma-ray bursts' spectra show excesses from the exact power-law dependence. In this article we show that this departure can be detected in the peak flux ratios of different BATSE DISCSC energy channels. This effect allows to estimate the redshift of the bright long gamma-ray bursts in the BATSE Catalog. A verification of these redshifts is obtained for the 8 GRB which have both BATSE DISCSC data and measured optical spectroscopic redshifts. There is good correlation between the measured and estimated redshifts, and the average error is Delta z ~ 0.33. The method is similar to the photometric redshift estimation of galaxies in the optical range, hence it can be called as ``gamma photometric redshift estimation''. The estimated redshifts for the long bright gamma-ray bursts are up to z =~ 4. For the the faint long bursts - which should be up to z =~ 20 - the redshifts cannot be determined unambiguously with this method.