GCN Circular 21405

S. J. LaPorte (PSU), A. Cholden-Brown (PSU), J. A. Kennea (PSU),
F. E. Marshall (NASA/GSFC), 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 17:30:27 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170803A (trigger=766081).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 174.950, -16.311 which is 
   RA(J2000) = 11h 39m 48s
   Dec(J2000) = -16d 18' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multiple-peaked
structure with a duration of about 5 sec.  The peak count rate
was ~15000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 17:32:27.0 UT, 120.1 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 174.93532, -16.31155 which is
equivalent to:
   RA(J2000)  = 11h 39m 44.48s
   Dec(J2000) = -16d 18' 41.6"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 50 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 (3.54 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3
(+2.47/-2.15) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 88 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	11:39:44.11 = 174.93381
  DEC(J2000) = -16:18:37.9  = -16.31052
with a 90%-confidence error radius of about 0.75 arc sec. This position is 6.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.12 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.04. 

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.)