TITLE: GCN CIRCULAR NUMBER: 12305 SUBJECT: Swift-BAT Trigger 501485 is noise DATE: 11/08/26 09:06:44 GMT FROM: Scott Barthelmy at NASA/GSFC B.-B. Zhang (PSU), S. D. Barthelmy (GSFC), V. D'Elia (ASDC), C. Gronwall (PSU), P. T. O'Brien (U Leicester), J. P. Osborne (U Leicester) and D. M. Palmer (LANL) report on behalf of the Swift Team: At 08:30:08 UT, the Swift Burst Alert Telescope (BAT) triggered on noise (trigger=501485). Swift slewed immediately. The BAT on-board calculated location is RA, Dec 264.165, -27.375, which is RA(J2000) = 17h 36m 40s Dec(J2000) = -27d 22' 29" 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 08:32:04.8 UT, 116.1 seconds after the BAT trigger. No source was detected in 1.2 ks 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 119 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. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the sub-image. The 8'x8' region for the list of sources generated on-board covers 100% of the BAT error circle. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the region. No correction has been made for the large, but uncertain extinction expected. Due to the marginal significance of the detection, the lack of an XRT source, the poor positional match with any known gamma-ray source, and the increase in the noise level caused by Sco X-1 and other galactic center sources in the Field of View, we believe that this trigger was probably a noise fluctuation. Burst Advocate for this burst is B.-B. Zhang (bbzhang AT psu.edu). 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.)