TITLE: GCN CIRCULAR NUMBER: 13522 SUBJECT: Swift Trigger 528925 is probably a noise fluctuation DATE: 12/07/27 22:38:37 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), M. M. Chester (PSU), V. Mangano (INAF-IASFPA), A. Maselli (INAF-IASFPA) and D. M. Palmer (LANL) report on behalf of the Swift Team: At 22:24:46 UT, the Swift Burst Alert Telescope (BAT) triggered and located a possible source (trigger=528925). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 186.547, +18.066 which is RA(J2000) = 12h 26m 11s Dec(J2000) = +18d 03' 59" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for 1-min image triggers, the real-time TDRSS lightcurve does not show anything significant. This trigger has an image significance of 5.86 sigma, therefore this is an Interesting-Source subthreshold trigger. The XRT began observing the field at 22:27:12.8 UT, 146.3 seconds after the BAT trigger. No source was detected in 353 s of promptly downlinked data, which covered 90% of the BAT error circle. 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 151 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. Due to the low significance of the BAT image (5.86 sigma), the large offset from the nominal catalog match (0.161 degrees from NGC4394) and the non-detection any source with XRT, we believe that this is a statistical fluctuation, rather than any astrophysical source. As part of a campaign to increase sensitivity to nearby GRBs, we have lowered the threshold for positions in the vicinity of nearby galaxies such as NGC4394, which also increases the false-positive rate.