DOES THE ADDITION OF A DURATION IMPROVE THE Liso-Epeak RELATION FOR GAMMA-RAY BURSTS?

Firmani et al. proposed a new gamma ray burst (GRB) luminosity relation that showed a significant improvement over the L-iso-E-peak relation (where L-iso is the isotropic peak luminosity and E-peak is the photon energy of the spectral peak for the burst). The new proposed relation simply modifies the E-peak value by multiplying it by a power of T-0.45, where T-0.45 is a particular measure of the GRB duration. We begin by reproducing the results of Firmani for his 19 bursts. We then test the Firmani relation for the same 19 bursts, except that we use independently measured values for L-iso, T-0.45, and E-peak, and we find that the relation deteriorates substantially. We further test the relation by using 60 GRBs with measured spectroscopic redshifts, and find a relation that has a comparable scatter as the original L-iso-E-peak relation. That is, a much larger sample of bursts does not reproduce the small scatter as reported by Firmani et al. Finally, we investigate whether the Firmani relation is improved by the use of any of 32 measures of duration (e. g., T-90, T-50, T-90/N-peak, the fluence divided by the peak flux, T-0.30, and T-0.60) in place of T-0.45. The quality of each alternative duration measure is evaluated with the root mean square of the scatter between the observed and fitted logarithmic L-iso values. Although we find that some durations yield slightly better results than T-0.45, the differences between the duration measures are minimal. We find that the addition of a duration does not add any significant improvement to the L-iso-E-peak relation. We also present a simple and direct derivation of the Firmani relation from both the L-iso-E-peak and Amati relations. In all, we conclude that the Firmani relation neither has an independent existence nor does it provide any significant improvement on previously known relations that are simpler.