GCN Circular 19987

A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf
of the Swift Team:

At 23:07:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 161004B (trigger=715246).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 112.157, -39.895 which is 
   RA(J2000) = 07h 28m 38s
   Dec(J2000) = -39d 53' 40"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single FRED peak
with substructure and  a duration of about 20 sec.  The peak count rate
was ~12000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 23:09:12.7 UT, 73.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 112.15290, -39.89858 which
is equivalent to:
   RA(J2000)  = 07h 28m 36.70s
   Dec(J2000) = -39d 53' 54.9"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 17 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. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

The initial flux in the 2.5 s image was 5.23e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 82 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 expected extinction corresponding to E(B-V) of
0.54. 

Burst Advocate for this burst is A. D'Ai (antonino.dai AT ifc.inaf.it). 
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.)