GCN Circular 21011

M. H. Siegel (PSU), C. Gronwall (PSU), A. Y. Lien (GSFC/UMBC),
K. L. Page (U Leicester) and B. Sbarufatti (INAF-OAB/PSU) report on
behalf of the Swift Team:

At 07:43:33 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible GRB (trigger=748858).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 199.853, +15.471 which is 
   RA(J2000) = 13h 19m 25s
   Dec(J2000) = +15d 28' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual with an image trigger, the available
BAT light curve shows no significant structure. 

The XRT began observing the field at 07:45:58.1 UT, 145.1 seconds after
the BAT trigger. No source was detected in 844 s of promptly downlinked
data. We are waiting for the full dataset to detect and localise any
XRT counterpart. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 146 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
0.03. 

Given the relatively low significance of the image detection (7.1
sigma) and  the lack of detection in the initial XRT data, we cannot
confirm that  this is a true astrophysical event. A final
determination of the  nature of this trigger will require the full
downlinked dataset. 

Burst Advocate for this burst is M. H. Siegel (siegel AT swift.psu.edu). 
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.)