TITLE: GCN CIRCULAR NUMBER: 13238 SUBJECT: Swift Trigger 519874 is probably a noise event DATE: 12/04/11 01:31:44 GMT FROM: David Palmer at LANL S. D. Barthelmy (GSFC), D. N. Burrows (PSU), P. D'Avanzo (INAF-OAB), N. Gehrels (NASA/GSFC), S. T. Holland (STScI), E. A. Hoversten (PSU), J. A. Kennea (PSU), A. Y. Lien (NASA/GSFC/ORAU), F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU), C. A. Swenson (PSU) and B.-B. Zhang (PSU) report on behalf of the Swift Team: At 01:16:39 UT, the Swift Burst Alert Telescope (BAT) triggered and located a marginal peak (6.51 sigma) in the resulting image (trigger=519874). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 35.550, +59.746 which is RA(J2000) = 02h 22m 12s Dec(J2000) = +59d 44' 46" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows nothing significant at the time of the trigger. The XRT began observing the field at 01:18:11.4 UT, 92.5 seconds after the BAT trigger. No source was detected in 218 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 150 seconds with the White filter starting 95 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 large, but uncertain extinction expected. Due to the marginal nature of the BAT detection, and the lack of any detection by XRT and UVOT despite the prompt slew, we believe that this event was most likely a statistical fluctuation and not an astrophysical source.