TITLE:   GCN CIRCULAR
NUMBER:  14551
SUBJECT: Trigger 555096: Swift detection of a possible burst
DATE:    13/05/04 02:29:39 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

J. R. Cummings (NASA/UMBC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), N. Gehrels (NASA/GSFC),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU),
E. Troja (NASA/GSFC/UMCP) and T. N. Ukwatta (MSU) report on behalf of
the Swift Team:

At 02:05:34 UT, the Swift Burst Alert Telescope (BAT) triggered and
located trigger=555096.  Swift slewed immediately to the transient. 
The BAT on-board calculated location is 
RA, Dec 272.365, -16.344 which is 
   RA(J2000) = 18h 09m 27s
   Dec(J2000) = -16d 20' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed two FRED peaks
with a duration of about 25 sec.  The peak count rate was ~1617 counts/sec 
(15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 02:07:00.2 UT, 85.9 seconds after
the BAT trigger. No source was detected in 1.1 ks 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 88 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 large, but uncertain extinction expected. 

The event is unusual in that it is a long burst in BAT with no 
detected X-ray counterpart after an immediate slew.  We also note
that this location is in the Galactic bulge near the plane
(lon = 13.65, lat=1.56).  Ground analysis of the detector
plane histogram finds only a marginal image detection (5.9 sigma). 

For these reasons, we will not be able to determine the reality
or nature of this event until ground-linked data is available. 

Burst Advocate for this burst is J. R. Cummings (jayc AT milkyway.gsfc.nasa.gov). 
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.)