TITLE: GCN CIRCULAR NUMBER: 20101 SUBJECT: Swift Trigger 718919 is not an astrophysical event. DATE: 16/10/23 17:26:14 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), D. N. Burrows (PSU), N. Gehrels (NASA/GSFC), D. Malesani (DARK/NBI and DTU Space), K. L. Page (U Leicester), D. M. Palmer (LANL) and T. N. Ukwatta (LANL) report on behalf of the Swift Team: At 17:11:02 UT, the Swift Burst Alert Telescope (BAT) triggered on noise near to NGC 5023 (trigger=718919). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 197.822, +43.936, which is RA(J2000) = 13h 11m 17s Dec(J2000) = +43d 56' 09" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image triggers, there is nothing visible in the real-time telemetry light curve. We do not believe this trigger is astrophysical -- it is very likely to be a noise trigger. The XRT began observing the field at 17:13:14.8 UT, 131.9 seconds after the BAT trigger. No source was detected in 270 s of promptly downlinked data. UVOT took a finding chart exposure of 150 seconds with the White filter starting 135 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 0.02. BAT selected this trigger based on its proximity to the direction to a known nearby galaxy, despite its low significance (5.8 sigma). Based on the lack of a corresponding rate increase in BAT and the non-detection by XRT of any counterpart, we believe that this event is a statistical fluctuation in the image.