Testing the Gamma-Ray Burst Energy Relationships

Abstract

Building on Nakar & Piran's analysis of the Amati relation relating gamma-ray burst peak energies E_p and isotropic energies E_iso, we test the consistency of a large sample of BATSE bursts with the Amati and Ghirlanda (which relates peak energies and actual gamma-ray energies E_gamma) relations. Each of these relations can be expressed as a ratio of the different energies that is a function of redshift (for both the Amati and Ghirlanda relations) and beaming fraction f_B (for the Ghirlanda relation). The most rigorous test, which allows bursts to be at any redshift, corroborates Nakar & Piran's result--88% of the BATSE bursts are inconsistent with the Amati relation--while only 1.6% of the bursts are inconsistent with the Ghirlanda relation if f_B=1. Even when we allow for a real dispersion in the Amati relation we find an inconsistency. Modelling the redshift distribution results in an energy ratio distribution for the Amati relation that is shifted by an order of magnitude relative to the observed distribution; any sub-population satisfying the Amati relation can comprise at most ~18% of our burst sample. A similar analysis of the Ghirlanda relation depends sensitively on the beaming fraction distribution for small values of f_B; for reasonable estimates of this distribution about a third of the burst sample is inconsistent with the Ghirlanda relation. Our results indicate that these relations are an artifact of the selection effects of the burst sample in which they were found; these selection effects may favor sub-populations for which these relations are valid.