GCN Circular 3128

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
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), 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
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)].  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
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).

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.