[vsnet-grb-info 17521] GRB 160131A: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18951
SUBJECT: GRB 160131A: Swift detection of a burst with an optical counterpart
DATE:    16/01/31 08:32:26 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

K. L. Page (U Leicester) and S. D. Barthelmy (GSFC) report on behalf
of the Swift Team:

At 08:20:31 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160131A (trigger=672236).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 78.182, -7.067 which is 
   RA(J2000)  =  05h 12m 44s
   Dec(J2000) = -07d 04' 02"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a large FRED-like peak
structure with a duration of about 100 sec.  The peak count rate
was ~8100 counts/sec (15-350 keV), at ~16 sec after the trigger. 

The XRT began observing the field at 08:21:41.6 UT, 69.7 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 78.1700, -7.0489 which is equivalent to:
   RA(J2000)  = +05h 12m 40.80s
   Dec(J2000) = -07d 02' 56.0"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 77 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 1.29e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 78 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	05:12:40.34 =  78.16810
  DEC(J2000) = -07:02:59.1  =  -7.04975
with a 90%-confidence error radius of about 0.61 arc sec. This position is 7.5
arc sec. from the center of the XRT error circle. The estimated magnitude is
13.88 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.11. 

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.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.)