TITLE:   GCN CIRCULAR
NUMBER:  19979
SUBJECT: GRB 161004A: Swift detection of a burst with a possible optical counterpart
DATE:    16/10/04 13:10:14 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

P.A. Evans (U Leicester), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), A. A. Breeveld (UCL-MSSL),
D. N. Burrows (PSU), A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC),
J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), B. Mingo (U Leicester),
K. L. Page (U Leicester), P. Romano (INAF-IASFPA), T. Sakamoto (AGU),
B. Sbarufatti (INAF-OAB/PSU) and E. Troja (NASA/GSFC/UMCP) report on
behalf of the Swift Team:

At 12:58:28 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 161004A (trigger=715084).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 263.171, -0.928, which is 
   RA(J2000)  =  17h 32m 41s
   Dec(J2000) = -00d 55' 41"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a couple of peaks
with a total duration of about 5 sec.  The peak count rate
was ~1800 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 12:59:49.3 UT, 80.7 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 263.15298, -0.94831 which is equivalent
to:
   RA(J2000)  = 17h 32m 36.72s
   Dec(J2000) = -00d 56' 53.9"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 97 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.88 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 4.1
(+3.92/-2.99) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 85 seconds after the BAT trigger. There is a candidate afterglow in
the list of sources generated on-board at
  RA(J2000)  =	17:32:36.59 = 263.15244
  DEC(J2000) = -00:57:02.2  =  -0.95061
with a 90%-confidence error radius of about 1.10 arc sec. This position is 6.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.03. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.25. 

Burst Advocate for this burst is P.A. Evans (pae9 AT star.le.ac.uk). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)