Necessity of evolution in cosmological γ-ray bursts

Abstract

RESULTS from the Burst and Transient Source Experiment (BATSE)1 on the Compton Gamma-Ray Observatory show a relative deficiency of faint γ-ray bursts without any detectable anisotropy. This gives strong support to suggestions that the bursts originate at cosmological distances2, such that cosmic expansion reduces the detectability of faint (and therefore distant) sources relative to a uniformly filled euclidean space. But the Pioneer Venus Orbiter (PVO) observations of brighter bursts show no such deficiency, indicating that the bursts that it sees are at redshifts less than about one. The best-fit cosmological model based on PVO data (peak luminosity L0 = 2 x 1050erg s-1) with no evolution of the source population predicts, when extrapolated to the fainter sources, a factor of 40 more events than BATSE sees. Even the 3a limit from PVO (L0 = 9.1 x 1051 ergs-1) is barely consistent with the BATSE result. Evolution of the cosmological γ-ray burst sources with epoch therefore seems necessary if the PVO and BATSE data are to be reconciled. The required evolutionary trend is for fewer or fainter bursts at larger redshift.