[vsnet-grb-info 17387] Trigger 668877: Swift detection of V 404 Cyg

[GCN Circulars]

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  <scott at milkyway.gsfc.nasa.gov>

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.