Source Density Evolution of Gamma-Ray Bursts

Recent optical afterglow observations of gamma-ray bursts (GRBs) indicate a setting and distance scale that many relate to star formation regions. In this paper, we use ⟨V/Vmax⟩ and a set of artificial trigger thresholds to probe several potential GRB source density evolutionary scenarios. In particular, we compare a uniform subset of BATSE 4B data to cosmological scenarios where GRBs evolve as the comoving density, the star formation rate, the QSO rate, and the SN Type Ic rate. Standard candle bursts with power-law spectra and a universe without vacuum energy were assumed. Our results significantly favor a comoving density model, which implies that GRB source density evolution is weaker than expected in these evolutionary scenarios. GRB density might still follow star formation rates given proper concurrent GRB luminosity evolution, significant beaming, significant error in standard candle assumptions, or if a significant modification of star formation rate estimates were to occur.