TITLE:   GCN CIRCULAR
NUMBER:  21450
SUBJECT: GRB 170810A: Swift detection of a burst with a bright optical counterpart
DATE:    17/08/10 22:39:41 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

S. L. Gibson (U Leicester), D. N. Burrows (PSU), J. A. Kennea (PSU),
S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the
Swift Team:

At 22:01:41 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170810A (trigger=767284).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 187.950, +3.647 which is 
   RA(J2000) = 12h 31m 48s
   Dec(J2000) = +03d 38' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of 160 sec. The peak count rate 
was ~2600 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 22:02:57.5 UT, 76.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
187.9389, 3.6603 which is equivalent to:
   RA(J2000)  = 12h 31m 45.33s
   Dec(J2000) = +03d 39' 37.0"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 62 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 (2.23 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.2
(+1.98/-1.79) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 4.45e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 876 seconds after the BAT trigger. There is a candidate afterglow in
the list of sources generated on-board at
  RA(J2000)  =	12:31:45.31 = 187.93881
  DEC(J2000) = +03:39:38.9  =	3.66080
with a 90%-confidence error radius of about 1.10 arc sec. This position is 3.5
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.87. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.03. 

An earlier UVOT finding chart exposure, starting 85 seconds after the 
BAT trigger, showed that the counterpart was much brighter at that time. 
However, this brightness precluded automatic analysis.  
Results for this early bright detection will be presented later. 

The original GCN notice of this burst suggested an association with the
galaxy NGC4496A.  However, the apparent separation from this galaxy,
0.295 deg, implies that these two objects are not related. 

Burst Advocate for this burst is S. L. Gibson (slg44 AT le.ac.uk). 
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.)