TITLE: GCN CIRCULAR NUMBER: 8153 SUBJECT: Swift trigger 322590: BAT detection of a possible burst DATE: 08/08/28 08:50:57 GMT FROM: Wayne Baumgartner at GSFC W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), O. Godet (U Leicester), C. Guidorzi (INAF-OAB), S. D. Hunsberger (PSU), V. Mangano (INAF-IASFPA), J. Mao (INAF-OAB), C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester), J. P. Osborne (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), A. M. Parsons (GSFC), D. Perez (U Leicester), B. Preger (ASDC), P. Romano (INAF-IASFPA), M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester), G. Tagliaferri (INAF-OAB), E. Troja (INAF-IASFPA) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 08:15:09 UT, the Swift Burst Alert Telescope (BAT) found trigger 322590. Swift slewed immediately to the trigger location. The BAT on-board calculated location is RA, Dec 323.467, +42.001 which is RA(J2000) = 21h 33m 52s Dec(J2000) = +42d 00' 05" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a marginal excess, perhaps single-peaked with a duration of about 1 sec. The peak count rate was ~500 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 08:16:19.1 UT, 69.7 seconds after the BAT trigger. No source was detected in 323 s of 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 224 seconds with the V filter starting 180 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 21:33:45.77 = 323.4407 DEC(J2000) = +42:01:49.4 = 42.0304 with a 1-sigma error radius of about 0.7 arc sec, but the DSS shows a star extremely close to this position. The estimated magnitude is 16.7 with a 1-sigma error of about 0.5 mag. No correction has been made for the expected extinction of about 1.7 magnitudes. Due to the weaknesss of the BAT detection and the lack of an XRT or of a reliable UVOT detection, we believe that this is not a GRB. However we cannot immediately rule out the possibility that this is a weak short burst. Further analysis with the Malindi data is required to determine the reality of this event. Burst Advocate for this burst is W. H. Baumgartner (wayne AT milkyway.gsfc.nasa.gov). 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.)