2002ApJ...578..304S

We show that the source population of long gamma-ray bursts (GRBs) has declined by at least a factor of 12 (at the 90% confidence level) since the early stages of the universe (z∼2-3). This result has been obtained using the combined BATSE and Ulysses GRB brightness distribution and the detection of four GRBs with known redshifts brighter than 10⁵² ergs.s^–1 in the 50-300 keV range at their peak. The data indicate that the decline of the GRB source population is as fast as, or even faster than, the measured decline of the star formation rate. Models for the evolution of neutron star binaries predict a significantly larger number of apparently bright GRBs than observed. Thus, our results give independent support to the hypernova model, which naturally explains the fast decline in the progenitor population.