TITLE:   GCN CIRCULAR
NUMBER:  17944
SUBJECT: Swift Trigger 644520 is V404 Cyg
DATE:    15/06/18 01:00:29 GMT
FROM:    Jamie A. Kennea at PSU/Swift-XRT  <kennea@swift.psu.edu>

J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC), D. Malesani (DARK/NBI) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 00:44:05 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 306.012, +33.867 which is 
   RA(J2000) = 20h 24m 03s
   Dec(J2000) = +33d 52' 01"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 50 sec.  The peak count rate
was ~1300 counts/sec (15-350 keV), at ~20 sec after the trigger. 

The XRT began observing the field at 00:45:12.7 UT, 67.7 seconds after
the BAT trigger. XRT found a bright X-ray source located at RA, Dec
306.0163, 33.8645 which is equivalent to:
   RA(J2000)  = +20h 24m 3.91s
   Dec(J2000) = +33d 51' 52.2"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
position is 4.8 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. No event data are yet available to
determine the column density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 3.15e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 76 seconds after the BAT trigger. We detect V404 Cyg with a white
magnitude of 15.54 +- 0.14.  No correction has been made
for the large, but uncertain extinction expected.