TITLE: GCN CIRCULAR NUMBER: 13885 SUBJECT: Swift Trigger 536502 is likely a noise fluctuation in coincidence with SAX J1818.6-1703 DATE: 12/10/22 18:46:47 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), C. J. Mountford (U Leicester), P. Romano (INAF-IASFPA) and M. C. Stroh (PSU) report on behalf of the Swift Team: At 18:08:06 UT, the Swift Burst Alert Telescope (BAT) triggered (536502) on a noise fluctuation in the image domain which we likely believe was a chance coincidence with the source SAX J1818.6-1703 in the on-board catalog. As is typical for image triggers (64 sec, image significance=5.81 sigma), the real-time TDRSS lightcurve does not show anything significant. The XRT began observing the field at 18:16:45.8 UT, 519.7 seconds after the BAT trigger. No source was detected in 441 s of promptly downlinked data, which covered 87% of the BAT error circle. We are waiting for the full dataset to detect and localise any possible XRT counterpart. UVOT took a finding chart exposure of 150 seconds with the White filter starting 546 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.