TITLE: GCN CIRCULAR NUMBER: 9851 SUBJECT: GRB 090831B: Swift detection of a possible burst DATE: 09/08/31 18:26:54 GMT FROM: David Palmer at LANL M. De Pasquale (UCL-MSSL), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), P. A. Evans (U Leicester), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC), C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL), P. Romano (INAF-IASFPA), M. Stamatikos (OSU/NASA/GSFC), R. L. C. Starling (U Leicester) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 18:01:14 UT, the Swift Burst Alert Telescope (BAT) triggered and located possible GRB 090831B (trigger=361477). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 3.619, +27.970 which is RA(J2000) = 00h 14m 29s Dec(J2000) = +27d 58' 12" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows nothing significant as is typical for an image trigger. The XRT began observing the field at 18:03:44.9 UT, 149.9 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 150 seconds with the White filter starting 153 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 0.03. Due to the lack of detection by XRT, we cannot confirm that this is a true astrophysical source. Further analysis of the downlinked data (available in ~3 hours) will be required. Burst Advocate for this burst is M. De Pasquale (mdp AT mssl.ucl.ac.uk). 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.)