[vsnet-grb-info 2112] GRB 050908: Swift-BAT refined analysis

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3951
SUBJECT: GRB 050908:  Swift-BAT refined analysis
DATE:    05/09/08 15:27:17 GMT
FROM:    Hans Krimm at NASA-GSFC  <krimm at milkyway.gsfc.nasa.gov>

G. Sato (ISAS), L. Barbier (GSFC), S. Barthelmy (GSFC),
P. Boyd (GSFC/UMBC), J. Cummings (GSFC/NRC), D. Hullinger (GSFC/UMD),
E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC),
T. Sakamoto (GSFC/NRC), J. Tueller (GSFC), W. Voges (MPE),
on behalf of the Swift-BAT team:

Using the data set from T-60 to T+120 sec from the recent telemetry
downlink, we report further analysis of Swift-BAT GRB 050908
(trigger #154112)  (Goad, et al., GCN 3942).  The refined BAT ground position
is (RA,Dec) = 20.451,-12.962 {01h 21m 48s, -12d 57' 45"} [deg; J2000]
+- 1.6 arcmin, (radius, 90% containment, statistical plus systematic).
This is 35 arcsec from the optical position reported by the KAIT GRB team
in Li et al. 2005, GCN 3945.  The partial coding was 80%.

The mask-weighted BAT light curve shows a roughly triangular shape with a
single peak rising from T-8 sec to peak at T+2 sec and return to background
levels at T+15 sec. The duration T90 (15-350 keV) is 20 +/- 2 sec.
(estimated error including systematics). There is an indication of hard to
soft spectral  evolution.  The soft emission decays more slowly and in fact
the duration in the 50-300 keV band is T90 = 7 +/- 1 seconds.

The power law index of the time-averaged spectrum fit over T-10.3 to
T+15.4 sec is 1.93 +- 0.17.   The fluence in the 15-150 keV band is
(5.1 +/- 0.5) x 10^-7 erg/cm2.  The 1-s peak photon flux measured from
T+1.66 sec to T+2.66 sec in the 15-150 keV band is (0.7 +/- 0.1) ph/cm2/s.
All the quoted errors are at the 90% confidence level.

Given the measured redshift for this burst, z=3.35 (Fugazza et al., GCN 3948;
Foley et al., GCN 3949) and assuming a cosmology with Omega_M = 0.3,
Omega_lamda = 0.7, H0 = 65, we derive Eiso for this burst of 1.36 x 10^52 ergs
in  65.25 - 652.50 keV at the GRB rest frame.