The addition of duration does not improve the luminosity relations for Gamma Ray Bursts

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Conference Proceeding

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Firmani et al. (2006) proposed a new Gamma Ray Burst (GRB) luminosity relation that showed a significant improvement over the Liso - E peak relation. (Liso is the isotropic peak luminosity and Epeak is the photon energy of the spectral peak for the burst.) The new proposed relation simply modifies the Epeak value by multiplying it by a power of T0.45, where T0.45 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 Liso, T0.45 and Epeak, 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 Liso - Epeak 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., T90, T50, T 90/Npeak, the fluence divided by the peak flux, T0 .30, and T0.60) in place of T0.45. The quality of each alternative duration measure is evaluated with the root mean square of the scatter between the observed and fitted logarithmic Liso values. Although we find some durations yield slightly better results than T0 .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 Liso - Epeak relation. We also present a simple and direct derivation of the Firmani relation from both the Liso - E peak and Amati (2002) 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. © 2009 American Institute of Physics.

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AIP Conference Proceedings

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