TITLE: GCN CIRCULAR NUMBER: 17799 SUBJECT: Swift Trigger 639636: a possible GRB DATE: 15/05/02 11:51:03 GMT FROM: Scott Barthelmy at NASA/GSFC A. Maselli (INAF-IASFPA), S. D. Barthelmy (GSFC), J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 11:30:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located a possible GRB (trigger=639636). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 350.788, +7.897, which is RA(J2000) = 23h 23m 09s Dec(J2000) = +07d 53' 50" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image triggers (7.4 sigma), there is nothing significant in the real-time light curve. The XRT began observing the field at 11:33:13.7 UT, 151.3 seconds after the BAT trigger. No source was detected in the 2.5-s promptly available image. We are waiting for the full dataset to detect and localise the XRT counterpart. UVOT took a finding chart exposure of 104 seconds with the White filter starting 502 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 16% 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 0.11. Burst Advocate for this burst is A. Maselli (maselli AT ifc.inaf.it). 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.)