TITLE:   GCN CIRCULAR
NUMBER:  18875
SUBJECT: GRB 160117B: Swift detection of a burst with optical counterpart
DATE:    16/01/17 14:14:09 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

E. Sonbas (Adiyaman Univ.), J. A. Kennea (PSU),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 13:59:27 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160117B (trigger=670800).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 132.190, -16.340 which is 
  RA(J2000) = 08h 48m 46s
  Dec(J2000) = -16d 20' 23"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 60 sec.  The peak count rate
was ~1300 counts/sec (15-350 keV), at ~7 sec after the trigger. 

The XRT began observing the field at 14:00:22.5 UT, 55.0 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 132.1953,
-16.3668 which is equivalent to:
   RA(J2000)  = 08h 48m 46.88s
   Dec(J2000) = -16d 22' 00.4"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 98 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 consistent with the Galactic value of 5.71
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 4.84e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 63 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	08:48:46.76 = 132.19484
  DEC(J2000) = -16:21:59.9  = -16.36664
with a 90%-confidence error radius of about 0.62 arc sec. This position is 3.2
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.32 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.07. 

Burst Advocate for this burst is E. Sonbas (edasonbas AT yahoo.com). 
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.)

[GCN OPS NOTE(17jan16): Per N.Cucchiara's request, the Trigger number
in the first paragraph was changed from 670793 to 670800.]