TITLE: GCN CIRCULAR NUMBER: 18777 SUBJECT: Trigger 668877: Swift detection of V 404 Cyg DATE: 15/12/30 22:37:54 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), V. D'Elia (ASDC), C. Gronwall (PSU), L. Izzo (URoma/ICRA), H. A. Krimm (CRESST/GSFC/USRA), A. Y. Lien (GSFC/UMBC) and T. Sakamoto (AGU) report on behalf of the Swift Team: At 22:26:00 UT, the Swift Burst Alert Telescope (BAT) triggered and located V 404 Cyg (trigger=668877). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 305.945, +33.850, which is RA(J2000) = 20h 23m 47s Dec(J2000) = +33d 50' 59" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical with image triggers, there is nothing obvious in the real-time light curve. The XRT began observing the field at 22:28:09.1 UT, 128.7 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source located at RA, Dec 306.01806, 33.86606 which is equivalent to: RA(J2000) = 20h 24m 04.33s Dec(J2000) = +33d 51' 57.8" with an uncertainty of 3.8 arcseconds (radius, 90% containment). . This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. This position is 3.1 arcseconds from a known X-ray source: 1SXPS J202404.2+335155. This source is in the Swift XRT 1SXPS catalogue with a mean 0.3-10 keV count-rate of 0.0183 +/- 0.0015 ct/sec; see http://www.swift.ac.uk/1SXPS/1SXPSJ202404.2%2B335155 for details of these previous observations. A power-law fit to a spectrum formed from promptly downlinked event data does not constrain the column density. UVOT took a finding chart exposure of 150 seconds with the White filter starting 132 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 none of the XRT error circle. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT 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.