Explaining the Gamma-Ray Burst E_peak Distribution

Abstract

The characteristic photon energy for Gamma Ray Bursts, E_peak, has a remarkably narrow distribution for bursts of similar peak flux, with values between 150 and 600 keV for most faint bursts. This result is surprising within the framework of internal shock models, since spectral shifts associated with the jet's blue shift (by a Lorentz factor of Gamma) and the cosmological red shift (by a factor of 1+z) should cause substantial smearing in the distribution of the spectral peak in the jet's co-moving frame, E_rest. For the general case where the luminosity (L) varies as Gamma^N and E_rest varies as Gamma^M, then the observed E_peak will vary as L^{(M+1)/N}(1+z)^{-1}. For two independent set of 20 and 84 bursts, E_peak(1+z) varies as a power law of the luminosity with an index of (M+1)/N=0.36+-0.03. With this measured value, the above functional dependence of E_peak on L and z results in E_peak being roughly constant for bursts of similar peak flux, P_256. Thus, the kinematic smearing will be small, hence allowing the E_peak distribution to be narrow. This model also predicts that bright bursts will have high E_peak values because they all have some combination of high luminosity (and hence a large blue shift Gamma) and a nearby distance (and hence a small cosmological red shift). Quantitatively, E_peak should vary roughly as P_256^0.36, and this model prediction is strikingly confirmed with BATSE data by Mallozzi et al. A prediction of this model is that GRBs at very high red shift z~10 should all appear with E_peak at ~200 keV. A further prediction of this model is that normal bursts with P_256 below the BATSE trigger threshold will appear as x-ray flashes with E_peak~70 keV; just as is reported by Kippen et al. and Heise et al.