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The observed spatial distribution of $\gamma-$ray bursts indicates that theyprobably originate at cosmological distances. At this distance scale theirvariability timescale and flux above MeV imply an initial optical-depth to pairproduction $> 10^{10}$. This appears to be in conflict with their highlynon-thermal spectra. We show that this difficulty can be removed if axionbursts from supernovae are converted to $\gamma-$rays over cosmologicaldistances. Nonthermal bursts with the relevant flux, duration, variability andspectra are obtained just for the range of axion masses of $10^{-5}-10^{-4}{\rmeV}$ that accounts for the cold dark matter in the universe. The observed rateof $\gamma-$ray bursts implies that axions should be converted efficiently tophotons in only one out of $\sim 10^{4}$ supernovae.Comment: 6 pages, TEX, to appear in Physical Review

physical review. d. particles, fields, gravitation, and cosmology/physical review. d. particles and fields

Are<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>γ</mml:mi></mml:math>-ray bursts at cosmological distances optically thin?

Areγ-ray bursts at cosmological distances optically thin?