Estimating redshifts for γ-ray bursts

Abstract

RECENT observations from the BATSE experiment on the Gamma Ray Observatory^1,2 demonstrate that the number of weak γ-ray bursts is smaller than expected for a uniform distribution of source distances, and that the distribution of weak bursts is isotropic. This suggests that the sources are at cosmological, rather than intragalactic, distances^3,4. A number of possible tests for estimating the distances to γ-ray bursters have been discussed³, but all are difficult to implement in practice. There is therefore no further evidence at present to support a cosmological distance to the bursters. Here I describe a test of distances that might be done with the BATSE data, which is equivalent to the formulation of the Hubble diagram for galactic redshifts. If the typical redshift of γ-ray bursters is of the order of unity, the weakest bursts should have softer spectra than the strongest. There is some evidence for such a correlation in the BATSE data⁵. If confirmed, this would provide the first spectroscopic indication that the burst sources lie at cosmological distances. There is also evidence that the redshift effect has been detected in the duration of a subset of the BATSE bursts⁶. If confirmed, this would be evidence that not only the photons but also the light curves are stretched by the cosmological redshift.