Euclidean vs. non-Euclidean Gamma-Ray Bursts

We classify gamma-ray bursts (GRBs) according to their observed durations and physical properties of their spectra. We find that long/hard bursts (of duration T90 > 2.5 s, and typical photon energy Ep > ∼ 0.8 MeV corresponding to BATSE’s energy fluence hardness H 32 > 3) show the strongest deviation from the three-dimensional Euclidean brightness distribution. The majority of GRBs, i.e., short bursts (T90 < 2.5 s) and long/soft bursts (with T90 > 2.5 s, and H e 32 < 3) show little, if any, deviations from the Euclidean distribution. These results contradict the prediction of extragalactic GRB models that the most distant bursts should be the most affected by cosmological energy redshift and time-dilation (long/soft GRBs). The strongly non-Euclidean GRB subclass has very hard spectra of typical photon energy above ∼ 1 MeV, i.e., outside the ideal energy range for optimal detection by BATSE. Selection effects due to BATSE inefficient triggering and/or intrinsic burst properties may cause the strong deviation from the Euclidean distribution for the hardest class of long GRBs. Our results imply strong luminosity and spectral evolution of GRB sources placed at extragalactic distances. Whether there co-exist multiple GRB populations with different (Galactic or extragalactic) origins remains an open question. Submitted to the Astrophysical Journal Letters: November 24, 1997 Revised: 27 January 1998; accepted: 13 February 1998 Subject Headings: Gamma-rays: bursts, theory