TITLE: GCN CIRCULAR NUMBER: 23260 SUBJECT: Swift Trigger 863053 is not an astrophysical event DATE: 18/09/24 00:30:54 GMT FROM: David Palmer at LANL S. D. Barthelmy (GSFC), C. Gronwall (PSU), A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL) and T. Sakamoto (AGU) report on behalf of the Neil Gehrels Swift Observatory Team: At 00:01:30 UT, the Swift Burst Alert Telescope (BAT) triggered on an image fluctuation which was misidentified as IGR 17511-3057 (trigger=863053) due to incorrect attitude information. Swift slewed immediately to the target. The BAT on-board calculated location of the image fluctuations is RA, Dec 267.574, -31.767, which is RA(J2000) = 17h 50m 18s Dec(J2000) = -31d 46' 01" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for an image trigger, there is nothing significant in the real-time light curve. The XRT began observing the field at 00:16:46.0 UT, 915.7 seconds after the BAT trigger. No source was detected in 305 s of promptly downlinked data. UVOT took a finding chart exposure of 150 seconds with the White filter starting 925 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 large, but uncertain, extinction expected. This event was due to the misidentification of an image fluctuation with a known source, as a result of a known race condition in the on-board software. It is not due to an astrophysical source.