TITLE: GCN CIRCULAR NUMBER: 8582 SUBJECT: Trigger 335971: Swift possible detection of a burst DATE: 08/11/29 21:57:53 GMT FROM: David Palmer at LANL J. Mao (INAF-OAB), P. J. Brown (PSU), J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), C. Guidorzi (INAF-OAB), S. T. Holland (CRESST/USRA/GSFC), R. Margutti (Univ Bicocca&OAB), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), P. Romano (INAF-IASFPA), R. L. C. Starling (U Leicester), G. Tagliaferri (INAF-OAB) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 21:41:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located a possible GRB, trigger=335971. Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 324.330, +21.516 which is RA(J2000) = 21h 37m 19s Dec(J2000) = +21d 30' 59" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve does not show any significant peak, which is not unusual for an image trigger. The XRT began observing the field at 21:43:58.4 UT, 137.7 seconds after the BAT trigger. No source was detected in the promptly available XRT data. We are waiting for the full dataset to detect and localise the XRT counterpart. UVOT took a finding chart exposure of nominal 150 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 overlap of the sub-image and the BAT error circle is 25%. The typical limiting magnitude for the sub-image is 19.5. The overlap of the 8'x8' region for the list of sources generated on-board and the BAT error circle is 100%. The typical limiting magnitude for the source list is 18. No correction has been made for the expected extinction corresponding to E(B-V) of 0.10. Due to the marginal nature of the BAT detection, and the lack of a confirmed counterpart, we are not sure whether this is a real burst, although a final confirmation will require the downloaded data from the next Malindi pass.