TITLE:   GCN CIRCULAR
NUMBER:  20603
SUBJECT: GRB 170205A: Swift detection of a burst with an optical counterpart
DATE:    17/02/05 12:58:40 GMT
FROM:    David Burrows at PSU/Swift  <burrows@astro.psu.edu>

E. Troja (NASA/GSFC/UMCP), D. N. Burrows (PSU),
A. Cholden-Brown (PSU), H. A. Krimm (CRESST/NSF/USRA),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC) and
K. L. Page (U Leicester) report on behalf of the Swift Team:

At 12:39:09 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170205A (trigger=736843).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 262.151, -0.053 which is 
   RA(J2000) = 17h 28m 36s
   Dec(J2000) = -00d 03' 08"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a duration of about 25 sec.  The peak count rate
was ~5000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 12:40:10.9 UT, 61.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
262.1696, -0.0626 which is equivalent to:
   RA(J2000)  = 17h 28m 40.70s
   Dec(J2000) = -00d 03' 45.3"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 75 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 in excess of the Galactic value (1.97 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 6.4
(+5.93/-4.46) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 7.05e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 314 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	17:28:40.69 = 262.16953
  DEC(J2000) = -00:03:47.3  =  -0.06313
with a 90%-confidence error radius of about 0.62 arc sec. This position is 1.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.62 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.35. 

Burst Advocate for this burst is E. Troja (eleonora.troja 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.)