TITLE: GCN CIRCULAR NUMBER: 20993 SUBJECT: Trigger 745966: Swift detection of 4U1850-03 DATE: 17/04/06 12:53:01 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), A. Cholden-Brown (PSU), A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC), A. Y. Lien (GSFC/UMBC), B. Mingo (U Leicester), K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 12:29:01 UT, the Swift Burst Alert Telescope (BAT) triggered and located 4U1850-03 (trigger=745966). Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 282.038, -2.439, which is RA(J2000) = 18h 48m 09s Dec(J2000) = -02d 26' 18" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows multiple peaks with a total duration of about 300 sec. The peak count rate was ~1800 counts/sec (15-350 keV). The XRT began observing the field at 12:33:02.2 UT, 241.1 seconds after the BAT trigger. No source was detected in the 2.5-s promptly available image. It is likely that the source was very bright and the instrument went directly to Windowed Timing mode, preventing us from getting a prompt position. UVOT took a finding chart exposure of 150 seconds with the White filter starting 245 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 100% 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 25% of the BATT 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.