Are γ -ray bursts at cosmological distances optically thin?

Abstract

The observed spatial distribution of gamma-ray bursts indicates that they probably originate at cosmological distances. At this distance scale their variability time scale and flux above MeV imply an initial optical depth to pair production greater than or similar to 10(10). This appears to be in conflict with their highly nonthermal spectra. We show that this difficulty can be removed if axion bursts from supernovae are converted to gamma rays over cosmological distances. Nonthermal bursts with the relevant flux, duration, variability, and spectra are obtained just for the range of axion masses of 10(-5)-10(-4) eV that accounts for the cold dark matter in the Universe. The observed rate of gamma-ray bursts implies that axions should be converted efficiently to photons in only one out of approximately 10(4) supernovae.