S.D. Barthelmy (GSFC), L. Barbier (GSFC), J. Cummings (GSFC/UMBC),
E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), J.L. Racusin (PSU),
T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU),
J. Tueller (GSFC), T. Ukwatta (GWU)
(i.e. the Swift-BAT team):
Using the data set from T-240 to T+962 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 070920B (trigger #291728)
(Racusin, et al., GCN Circ. 6808). The BAT ground-calculated position is
RA, Dec = 0.127, -34.844 deg which is
RA(J2000) = 00h 0m 30.5s
Dec(J2000) = -34d 50' 39"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.
The mask-weighted lightcurve shows a symmetrical peak starting at ~T-20 sec,
peaking at ~T+1 sec, and ending at ~T+17 sec. T90 (15-350 keV) is
20.2 +- 0.2 sec (estimated error including systematics).
The time-averaged spectrum from T-11.2 to T+13.4 sec is best fit by a power law
with an exponential cutoff. This fit gives a photon index 0.67 +- 0.58,
and Epeak of 41.3 +- 4.5 keV (chi squared 45.21 for 56 d.o.f.). For this
model the total fluence in the 15-150 keV band is 6.6 +- 0.5 x 10^-7 erg/cm2
and the 1-sec peak flux measured from T-0.49 sec in the 15-150 keV band is
0.8 +- 0.1 ph/cm2/sec. A fit to a simple power law gives a photon index
of 2.00 +- 0.10 (chi squared 67.16 for 57 d.o.f.). All the quoted errors
are at the 90% confidence level.
[GCN OPS NOTE(21sep07): Per author's request, the 'B' was added to the GRB
name in the first sentence.]