TITLE: GCN CIRCULAR NUMBER: 20688 SUBJECT: Trigger 738728: Swift triggered on noise near NGC 4096 DATE: 17/02/16 19:39:35 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), F. E. Marshall (NASA/GSFC), B. Mingo (U Leicester), K. L. Page (U Leicester) and E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift Team: At 19:21:57 UT, the Swift Burst Alert Telescope (BAT) triggered on noise near NGC 4096 (trigger=738728). The image significance was 5.86 sigma. Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 181.474, +47.650, which is RA(J2000) = 12h 05m 54s Dec(J2000) = +47d 38' 60" 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 19:24:12.9 UT, 135.5 seconds after the BAT trigger. No source was detected in 411 s of promptly downlinked data. UVOT took a finding chart exposure of 150 seconds with the White filter starting 139 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.