TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3128 SUBJECT: GRB 050315: Variability Analysis of Swift BAT Time History DATE: 05/03/20 23:58:18 GMT FROM: Don Lamb at U.Chicago GRB 050315: Variability Analysis of Swift BAT Time History T. Q. Donaghy, T. Sakamoto, D. Q. Lamb, E. E. Fenimore, D. E. Reichart, on behalf of the Swift BAT Team report: We have calculated the variability measure V (Fenimore and Ramirez-Ruiz 2000; Reichart et al. 2001) for GRB 050315 (Parsons et al., GCN Circular 3094), using the Swift BAT time history data for this burst. We find values V = 0.017 +/- 0.027, 0.068 +/- 0.022, 0.086 +/- 0.041, and 0.064 +/- 0.016 (68 percent confidence regions) in the 15-25 keV, 25-50 keV, 50-100 keV, and 25-100 keV energy bands, respectively. These values of V are the same within the statistical uncertainties; this is consistent with the results of Reichart et al. (2001), who found no significant trend of V with energy for BATSE bursts. Analyzing the Swift BAT spectral data for GRB 050315, we find a peak photon energy flux in 1 s of (9.2 +/- 0.6) x 10-8 erg cm-2 s-1 (68% confidence region) in the 33.9 - 339.1 keV energy band [which corresponds to the 100-1000 keV energy band in the rest frame of the source for a redshift of z = 1.949 (Kelson and Berger, GCN Circular 3101)]. This peak photon energy flux corresponds to an isotropic- equivalent peak luminosity in the 100-1000 keV energy band in the rest frame of the source of L_obs = (3.2 +/- 0.2) x 10^51 erg s-1. Using V in the 25-100 keV energy band as a luminosity (and therefore a redshift) estimator, we estimate an isotropic-equivalent luminosity in 100-1000 keV in the rest frame of the source of log L_V (erg s-1) = 51.46 +/- 0.747 and a redshift z_V = 1.94 (+2.02/-0.97) for the burst. These values are consistent with the observed values log L_obs (erg s-1) = 51.51 (+0.02/-0.04) (90% confidence region) and z = 1.949 (Kelson and Berger, GCN Circular 3101). These results are encouraging. They suggest that analyses of the Swift BAT time history data for a large sample of bursts will provide a decisive test of the validity of the variability measure V as a luminosity (and therefore a redshift) estimator, and if the analyses confirm its validity, they will make possible a better calibration of the estimator.