TITLE:   GCN CIRCULAR
NUMBER:  20123
SUBJECT: GRB 161104A: Swift detection of a short burst
DATE:    16/11/04 10:00:19 GMT
FROM:    Kim Page at U.of Leicester  <klp5@leicester.ac.uk>

B. Mingo (U Leicester), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), P.A. Evans (U Leicester),
K. L. Page (U Leicester) and T. N. Ukwatta (LANL) report on behalf of
the Swift Team:

At 09:42:26 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 161104A (trigger=720568).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 77.881, -51.452 which is 
   RA(J2000) = 05h 11m 31s
   Dec(J2000) = -51d 27' 07"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
structure with a duration of about 0.3 sec.  The peak count rate
was ~6000 counts/sec (15-350 keV), at ~0.1 sec after the trigger. 

The XRT began observing the field at 09:43:24.9 UT, 58.3 seconds after
the BAT trigger. The position determined from promptly downlinked data
is incorrect because the XRT centroided on a cosmic ray. 
Using promptly downlinked data we find an uncatalogued X-ray source 
located at RA, Dec 77.89236, -51.45898 which is equivalent to:
   RA(J2000)  = 05h 11m 34.17s
   Dec(J2000) = -51d 27' 32.3"
with an uncertainty of 4.5 arcseconds (radius, 90% containment). This
location is 35 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. 

UVOT took a finding chart exposure of 101 seconds with the White filter
starting 67 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 expected
extinction corresponding to E(B-V) of 0.02. 

Burst Advocate for this burst is B. Mingo (bm188 AT 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.)