TITLE:   GCN CIRCULAR
NUMBER:  20885
SUBJECT: GRB 170317A: Swift detection of a burst with an optical counterpart
DATE:    17/03/17 10:01:55 GMT
FROM:    Kim Page at U.of Leicester  <klp5@leicester.ac.uk>

S. J. LaPorte (PSU), A. P. Beardmore (U Leicester),
B. Mingo (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 09:45:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170317A (trigger=742866).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 93.088, +50.520 which is 
   RA(J2000) = 06h 12m 21s
   Dec(J2000) = +50d 31' 13"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a FRED
structure with a duration of about 10 sec.  The peak count rate
was ~3000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 09:47:22.4 UT, 83.3 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 93.06317, 50.49431 which is equivalent
to:
   RA(J2000)  = 06h 12m 15.16s
   Dec(J2000) = +50d 29' 39.5"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 109 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 consistent with the Galactic value of 2.53
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 86 seconds after the BAT trigger. There is a candidate afterglow in
the list of sources generated on-board at
  RA(J2000)  =	06:12:14.78 =  93.06160
  DEC(J2000) = +50:29:34.8  =  50.49300
with a 90%-confidence error radius of about 1.10 arc sec. This position is 5.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.05. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.19. 

Burst Advocate for this burst is S. J. LaPorte (extragsam AT gmail.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.)