GCN Circular 13375
S. D. Barthelmy (GSFC), V. D'Elia (ASDC), C. B. Markwardt (NASA/GSFC), D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team: At 06:08:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located a possible near-threshold image peak near the location of the nearby galaxies NGC4405 and NGC 4383 (trigger=524949). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 186.424, +16.302, which is RA(J2000) = 12h 25m 42s Dec(J2000) = +16d 18' 09" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As it typical with image trigger (11.7 min) the TDRSS lightcurve does not show anything significant. The XRT began observing the field at 06:25:33.5 UT, 1029.9 seconds after the BAT trigger. No source was detected in 631 s of promptly downlinked data, which covered 82% of the BAT error circle. We are waiting for the full dataset to detect and localise the XRT counterpart if any. UVOT took a finding chart exposure of 150 seconds with the White filter starting 1033 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. As part of a sensitivity enhancement program for Swift, we now do follow-up observations on weak fluctuations near to a catalog of ~600 nearby galaxies and other objects. Most of these follow-ups are expected to be noise fluctuations. Given the lack of an XRT confirmation it is most likely that this source one of those false triggers.