TITLE:   GCN CIRCULAR
NUMBER:  8945
SUBJECT: GRB 090307: Swift-BAT refined analysis of trigger 345551
DATE:    09/03/07 17:07:38 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@lheamail.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC),
P. A. Evans (U Leicester), E. E. Fenimore (LANL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL),
A. M. Parsons (GSFC), T. Sakamoto (GSFC/UMBC), G. Sato (ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-60 to T+303 sec from recent telemetry downlinks,
we report further analysis of posible GRB 090307 (trigger #345551)
(Evans, et al., GCN Circ. 8943).  The BAT ground-calculated position is
RA, Dec = 245.007, -28.647 deg, which is 
   RA(J2000)  =  16h 20m 01.7s 
   Dec(J2000) = -28d 38' 48.2" 
with an uncertainty of 3.3 arcmin, (radius, sys+stat, 90% containment).
We note that this position is based on a weak peak in the image domain (5.5 sigma).
This is lower than our regular detections because this is a Subthreshold
trigger.  The partial coding was 87%.
 
The mask-weighted light curve shows a weak pulse starting at ~T-7 sec and
ending at ~T+20 sec.  T90 (15-350 keV) is 22 +- 8 sec (estimated error
including systematics).
 
The time-averaged spectrum from T-7.4 to T+16.6 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.08 +- 0.39.  The fluence in the 15-150 keV band is 2.6 +- 0.6 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+5.57 sec in the 15-150 keV band
is 0.2 +- 0.1 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. 
 
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/345551/BA/

Given the weak nature of this event and the as yet unconfirmed nature
of this trigger by the XRT follow-up observations, we can not rule out
that this trigger is due something other than a GRB.  There are further
XRT follow-up observations in the observing plan.