Statistical Constraints On Non-Cosmological Subclasses Of GRBs

There still exists the possibility that the phenomenon of gamma-ray bursts (GRBs) is a mixture of events of different nature, even within the class of long (>2 s) bursts. We try to put statistical constraints on a possible non-cosmological component using the uniform GRB catalog of Stern & Tikhomirova obtained from an overall scan of the full 9.1 year BATSE 1024 ms data. The sample consists of 3906 GRBs and includes non-triggered bursts with peak fluxes down to 0.1 photons cm$^{-2}$ s$^{-1}$. We find no significant deviations from isotropy. The constraints on a non-cosmological population are still weak. The allowed contribution of a GRB subpopulation originating from an extended galactic halo is about 60% and the upper limit on an Eucledian component (e.g., nearby galactic or non-cosmological extragalactic GRBs) is 23%. The results concern mainly the class of long GRBs.