The Fluence Distribution of Gamma-Ray Bursts

In the use and interpretation of $\log{N}$--$\log{S}$ distributions for gamma-ray bursts, burst peak flux has typically been used for $S$. We consider here the use of the fluence as a measure of $S$, which may be a more appropriate quantity than the peak flux in such highly variable sources. We demonstrate how using the BATSE trigger data we can determine the selection effects on fluence. Then using techniques developed elsewhere to account for the important threshold effects and correlations. Applying the appropriate corrections to the distributions, we obtain a fluence distribution which shows a somewhat sharper break than the peak flux distribution, implying a possibly narrower fluence luminosity distribution. If bursts are at cosmological distances, these observations together indicate that evolution of the luminosity function is required.Comment: ApJ Letters, Accepted May 24, 1996. 16 pages LaTeX, 4 Postscript figures, uses AASTeX and psfig macros, paper also available at http://www-bigbang.stanford.edu/~ted/paper4.htm