TITLE: GCN CIRCULAR NUMBER: 23331 SUBJECT: Swift Trigger 866757: is probably not a GRB DATE: 18/10/12 19:23:41 GMT FROM: David Palmer at LANL P. D'Avanzo (INAF-OAB), D. N. Burrows (PSU), J. A. Kennea (PSU), D. M. Palmer (LANL), B. Sbarufatti (PSU), M. H. Siegel (PSU) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 19:06:28 UT, the Swift Burst Alert Telescope (BAT) triggered due to rising rates during the approach to the SAA and found a low-significance peak in the resulting image (trigger=866757). Swift did not slew to the location due to a Moon constraint. The BAT on-board calculated location is RA, Dec 264.743, -12.473 which is RA(J2000) = 17h 38m 58s Dec(J2000) = -12d 28' 22" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows the pre-SAA rise in count rates but does not show a peak around T=0. Due to a Moon observing constraint, Swift cannot slew to the BAT position until 02:59 UT on 2018 October 16. No XRT or UVOT follow-ups are planned. Due to the low significance of the on-board peak detection (6.5 sigma, falling to 6.2 sigma in ground analysis), the lack of a visible peak in the BAT lightcurve, and the fact that Swift was approaching the SAA at the time, we believe that this is most likely a chance fluctuation in the image plane and not a GRB. Further analysis of the full downlinked dataset will be needed to determine the reality of this event. Burst Advocate for this burst is P. D'Avanzo (paolo.davanzo AT brera.inaf.it). 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.)