TITLE:   GCN CIRCULAR
NUMBER:  20446
SUBJECT: GRB 170113A: Swift detection of a burst with an optical counterpart
DATE:    17/01/13 10:15:42 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

P.A. Evans (U Leicester), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), H. A. Krimm (CRESST/NSF/USRA),
B. Mingo (U Leicester), P. T. O'Brien (U Leicester),
B. Sbarufatti (INAF-OAB/PSU), R. L. C. Starling (U Leicester) and
E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift Team:

At 10:04:05 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170113A (trigger=732526).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 61.648, -71.925, which is 
   RA(J2000)  =  04h 06m 36s
   Dec(J2000) = -71d 55' 28"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple peaks
with a duration of about 30 sec.  The peak count rate
was ~2100 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 10:05:04.2 UT, 58.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 61.73063, -71.94326 which
is equivalent to:
   RA(J2000)  = 04h 06m 55.35s
   Dec(J2000) = -71d 56' 35.7"
with an uncertainty of 4.0 arcseconds (radius, 90% containment). This
location is 113 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 1.01e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 67 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	04:06:55.85 =  61.73272
  DEC(J2000) = -71:56:34.5  = -71.94293
with a 90%-confidence error radius of about 0.62 arc sec. This position is 3.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.39 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.11. 

Burst Advocate for this burst is P.A. Evans (pae9 AT star.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.)