GCN Circular 18815

A. Melandri (INAF-OAB), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC),
P. D'Avanzo (INAF-OAB), P.A. Evans (U Leicester), C. Gronwall (PSU)
and K. L. Page (U Leicester) report on behalf of the Swift Team:

At 11:24:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160104A (trigger=669319).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 76.804, +11.354 which is 
   RA(J2000) = 05h 07m 13s
   Dec(J2000) = +11d 21' 14"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed multiple weak peaks
with a duration of about 20 sec.  The peak count rate was ~1929 counts/sec 
(15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 11:25:30.0 UT, 79.3 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 76.7965,
11.3239 which is equivalent to:
   RA(J2000)  = 05h 07m 11.17s
   Dec(J2000) = +11d 19' 25.9"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 111 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.71
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 85 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 0.26. 

Burst Advocate for this burst is A. Melandri (andrea.melandri AT brera.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.)