GCN Circular 21410

A. Cholden-Brown (PSU), D. N. Burrows (PSU),
P. A. Evans (U Leicester), J. A. Kennea (PSU), S. J. LaPorte (PSU),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester) and B. Sbarufatti (INAF-OAB/PSU) report on
behalf of the Swift Team:

At 12:01:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170804A (trigger=766194).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 6.433, -64.748 which is 
   RA(J2000) = 00h 25m 44s
   Dec(J2000) = -64d 44' 52"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 40 sec.  The peak count rate
was ~1400 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 12:03:39.9 UT, 122.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 6.39106, -64.78341 which
is equivalent to:
   RA(J2000)  = 00h 25m 33.85s
   Dec(J2000) = -64d 47' 00.3"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 142 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.79 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.4
(+3.30/-2.37) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 5.37e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 131 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	00:25:34.45 =	6.39354
  DEC(J2000) = -64:47:03.0  = -64.78416
with a 90%-confidence error radius of about 0.63 arc sec. This position is 6.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.01 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 A. Cholden-Brown (aaronb AT swift.psu.edu). 
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.)