TITLE:   GCN CIRCULAR
NUMBER:  20575
SUBJECT: GRB 170202A: Swift detection of a burst with an optical afterglow
DATE:    17/02/02 18:53:46 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

J. L. Racusin (NASA/GSFC), S. D. Barthelmy (GSFC), S. B. Cenko (GSFC),
A. Cholden-Brown (PSU), J. A. Kennea (PSU),
H. A. Krimm (CRESST/NSF/USRA), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 18:28:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170202A (trigger=736407).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 152.529, +5.002 which is 
   RA(J2000)  =  10h 10m 07s
   Dec(J2000) = +05d 00' 06"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows 4 main peaks
with a total duration of about 40 sec.  The peak count rate
was ~4800 counts/sec (15-350 keV), at ~16 sec after the trigger. 

The XRT began observing the field at 18:29:14.8 UT, 72.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 152.51422, 5.01213 which
is equivalent to:
   RA(J2000)  = 10h 10m 03.41s
   Dec(J2000) = +05d 00' 43.7"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 64 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 2.21e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 83 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	10:10:03.49 = 152.51455
  DEC(J2000) = +05:00:41.8  =	5.01161
with a 90%-confidence error radius of about 0.75 arc sec. This position is 0.5
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.46 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

Burst Advocate for this burst is J. L. Racusin (judith.racusin AT nasa.gov). 
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.)