GCN Circular 18778
S. D. Barthelmy (GSFC), V. D'Elia (ASDC), L. Izzo (URoma/ICRA), A. Y. Lien (GSFC/UMBC) and T. Sakamoto (AGU) report on behalf of the Swift Team: At 22:44:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located V 404 Cyg (trigger=668879). Swift was already observing this source. The BAT on-board calculated location is RA, Dec 306.016, +33.850, which is RA(J2000) = 20h 24m 04s Dec(J2000) = +33d 51' 00" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows activity at ~T-50 sec and at T_zero, but there is gap in the data from T_zero to T+100 sec. The XRT began observing the field at 22:48:41.67 UT, 230.1 seconds after the BAT trigger. No source was detected in 175 s of promptly downlinked data. We are waiting for the full dataset to detect and localise the XRT counterpart. UVOT took a finding chart exposure of 250 seconds with the U filter starting 230 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. Results from the 2.7'x2.7' sub-image are not available at this time. The overlap of the 8'x8' region for the list of sources generated on-board and the XRT error circle is uncertain. No correction has been made for extinction. This is the second trigger from this source (GCN 18777). It triggered while Swift was observing as a result of the first trigger. As such XRT and UVOT have data before and during T_zero for this second trigger.