TITLE: GCN CIRCULAR NUMBER: 17901 SUBJECT: Swift Trigger 642453: a possible GRB DATE: 15/06/04 03:18:19 GMT FROM: David Palmer at LANL V. D'Elia (ASDC), S. D. Barthelmy (GSFC), J. R. Cummings (NASA/UMBC), L. Izzo (URoma/ICRA), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 03:00:10 UT, the Swift Burst Alert Telescope (BAT) triggered and located a possible GRB (trigger=642453). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 238.343, -46.622 which is RA(J2000) = 15h 53m 22s Dec(J2000) = -46d 37' 19" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual for an image trigger, there is no activity visible in the immediately-available lightcurve. The XRT began observing the field at 03:02:28.4 UT, 138.1 seconds after the BAT trigger. No source was detected in 35 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 96 seconds with the White filter starting 141 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 0.95. Due to the lack of a rate increase trigger, and the non-detection of a counterpart in the relatively short XRT observation, we cannot confirm that this is a real astrophysical source at this time. A determination of the reality of this event will require the full downlinked dataset and possibly further XRT observations. Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.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.)