TITLE: GCN CIRCULAR NUMBER: 17929 SUBJECT: Swift trigger 643949 is V404 Cyg DATE: 15/06/15 18:55:32 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), H. A. Krimm (CRESST/GSFC/USRA), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), A. Maselli (INAF-IASFPA) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 18:31:38 UT, the Swift Burst Alert Telescope (BAT) triggered and located V404 Cyg. Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 306.020, +33.850 which is RA(J2000) = 20h 24m 05s Dec(J2000) = +33d 50' 59" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual with an image trigger, the available BAT light curve shows no significant structure. The XRT began observing the field at 18:34:37.3 UT, 179.1 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source with an enhanced position: RA, Dec 306.0162, 33.8673 which is equivalent to: RA(J2000) = 20h 24m 03.89s Dec(J2000) = +33d 52' 02.4" with an uncertainty of 2.4 arcseconds (radius, 90% containment). This location is 63 arcseconds from the BAT onboard position, within the BAT error circle. 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 0.8 arcseconds from that of a known transient low-mass X-ray binary V* V404 Cyg. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 8.10 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 183 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 100% of the XRT 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 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.