TITLE:   GCN CIRCULAR
NUMBER:  13262
SUBJECT: Swift Trigger 520928 is probably not a burst
DATE:    12/04/26 12:01:18 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
A. P. Beardmore (U Leicester), S. Campana (INAF-OAB),
M. M. Chester (PSU), P. A. Evans (U Leicester), B. Gendre (ASDC),
C. Guidorzi (U Ferrara), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL),
O. M. Littlejohns (U Leicester), A. Maselli (INAF-IASFPA),
P. T. O'Brien (U Leicester), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU), R. L. C. Starling (U Leicester),
G. Tagliaferri (INAF-OAB) and T. N. Ukwatta (MSU) report on behalf of
the Swift Team:

At 11:38:15 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a potential source which could be a GRB (trigger=520928).  
Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 75.720, -26.314 which is 
   RA(J2000) = 05h 02m 53s
   Dec(J2000) = -26d 18' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). As is usual with an image trigger, the 
currently available BAT light curve does not show much structure. 

The XRT began observing the field at 11:40:56.3 UT, 160.5 seconds after
the BAT trigger. No source was detected in 300 s of promptly downlinked
data. We are waiting for the full dataset to detect and localise the
XRT counterpart. 

No UVOT processed data products are available at this time. 

Due to the marginal detection (7.05 sigma) in the image, the lack
of a detectable rate increase, and the non-detection by XRT and
UVOT despite the prompt slew, it is likely that this is merely
a statistical fluctuation and not an astrophysical source. 
However, the immediately available XRT data covers only 70%
of the BAT error circle.  A definitive determination of the
reality of the source will require the download of the full dataset.