Effects of Spectral Shape on Cosmological Models of the BATSE Gamma‐Ray Burst Intensity Distribution

Abstract

We have used the photon energy spectra of 100 bright gamma-ray bursts (GRBs) obtained from data from the Large Area Detectors of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory to examine the effects of spectral shape on cosmological models of the burst number-intensity distribution (log N – log P), where N is the number of bursts with peak photon flux greater than P. Since it has become common to use an assumed burst photon spectrum in computations of this type, we have examined the consequences of this assumption by using a range of observed burst spectra to create theoretical intensity distributions. We used a conventional Friedmann cosmology to create the models, and assumed that there is no burst source rate density evolution and that the sources are monoluminous (standard candles). This enabled us to focus on the effects of spectral shape on the log N – log P model parameters (peak luminosity and redshift). The shape of the burst spectrum has an influence on the maximum redshift consistent with the BATSE data. The variation in the burst photon spectra causes the model maximum redshifts to vary by an average of ~20%. However, the maximum redshift can differ among log N – log P models incorporating different spectra by more than a factor of 2. Thus, the use of an assumed burst spectrum in modeling the number-intensity distribution yields results that are moderately dependent upon the assumed spectral shape.