[vsnet-grb-info 16648] Swift trigger 643949 is V404 Cyg

[GCN Circulars]

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  <scott at milkyway.gsfc.nasa.gov>

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.