GCN Circular 3105

H. Krimm (GSFC/USRA), S. Barthelmy, L. Barbier (GSFC), J. Cummings (GSFC/NRC),
E. Fenimore (LANL), R. Fink, N. Gehrels (GSFC), S.T. Holland (GSFC/USRA),
D. Hullinger (GSFC/UMD), K. Hurley (UCB), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC),
S. Piranomonte (ASDC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS),
A. Smale (NASA HQ), M. Suzuki (Saitama), J. Tueller (GSFC)
on behalf of the Swift-BAT team:

Continued analysis of the full data set (now with telemetry gaps 
filled in) for the Swift-BAT GRB050315 (Parsons et al., GCN Circ 
3094, Sakamoto et al., GCN Circ 3099) yields a fluence of 4.2 x 10^-6 
erg/cm2 in the 15-350 keV band and a peak flux of 2.4 ph/cm2/sec 
(also 15-350 keV). The photon index of the 1-s peak spectrum (T0+24.6 
sec.) is 2.4 +- 0.3 (90% confidence).  The time averaged spectrum 
also shows a soft spectrum with a power law index of 2.3 +- 0.1 (90% 
confidence).   The Swift mission is still in its early orbit phase 
and the BAT spectral response continues to be refined.  Further 
spectral analysis is in progress.  In particular, since there is an 
uncertainty in the energy response at the lowest energies at the 
current time, it is not possible to conclude that this event is an 
X-ray flash.

The burst shows considerable structure in its light curve.  There is 
a precursor starting ~60 seconds before the trigger and continuing up 
until the main peak emission begins. The first of the two main peaks 
has a slow rise of about ten seconds and a gradual decline 
interrupted at TO ~22 seconds by a second peak with a much steeper 
rise.  The burst shows signs of spectral evolution (hard to soft) 
from the precursor through the two main peaks. There is no 
significant emission after the decay of the second peak.  The overall 
measures of burst duration give T90 = 96 +- 10 seconds and T50 = 25 
+- 5 seconds.  The errors in duration are estimated systematic errors.