The nu F nu Peak Energy Distributions of Gamma-Ray Bursts Observed by BATSE

Abstract

The majority of gamma-ray bursts exhibit a peak in their nuF_nu photon energy spectra at an energy E_p that is in the range of ~20-2000 keV available to the Large Area Detectors of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. If gamma-ray burst sources are at cosmological distances (d >~ 1 Gpc), then the spectra of dim bursts should be redshifted to lower energies relative to those of bright bursts. The magnitude of the shift is a function of the cosmological redshifts z of both the dim and bright burst sources and hence yields the range of redshift available to the bursts; this range is further constrained by considering cosmological model fits to the burst number-intensity distribution. We have produced photon energy spectra for ~400 bursts using data from BATSE to investigate if this expected shift in the nuF_nu peak is observed. We find that the mean peak energies of the burst spectra are correlated with intensity: lower intensity groups of burst spectra exhibit a lower average peak energy, although the distributions of E_p are quite wide. Denoting the redshift of an event with an observed peak photon flux P (photons cm^-2 s^-1) by z_p, we find that the maximum range consistent with the bursts of our sample is (1+z_1)/(1+z_100) = 1.86 (+0.36, -0.24).