TITLE:   GCN CIRCULAR
NUMBER:  21193
SUBJECT: Swift Trigger 755873: a possible GRB
DATE:    17/06/04 22:00:29 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

K. L. Page (U Leicester), S. D. Barthelmy (GSFC),
N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 21:30:52 UT, the Swift Burst Alert Telescope (BAT) triggered on a possible
GRB (trigger=755873).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 200.808, +64.198, which is 
   RA(J2000)  =  13h 23m 14s
   Dec(J2000) = +64d 11' 52"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a large (~2000 cnts/sec)
ramp up to the trigger time, then several overlapping peaks lasting ~50 sec
with a maximum of ~4000 cnts/sec.  There is also a 3000 cnt/sec drop at T+80 sec. 
This plus an anomalously large ratio in the rate-trigger significance and
the image significance makes this event uncertain. 

The XRT began observing the field at 21:32:52.9 UT, 120.5 seconds after
the BAT trigger. No source was detected in 990 s of promptly downlinked
data. We are waiting for the full dataset to detect and localise the
XRT counterpart. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 124 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of
the BAT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
BAT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.02. 

This event occurred as Swift was on the fringes of the SAA, 
and the intensities of the apparent BAT lightcurve and image 
strength are not in simple agreement.  Due to the non-detection
of an afterglow by the XRT, we cannot confidently determine
whether this is an astrophysical event. 

Final determination of the reality of this event will require
the full downlinked dataset. 

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)