TITLE:   GCN CIRCULAR
NUMBER:  23309
SUBJECT: GRB 181010A: Swift detection of a burst with an optical counterpart
DATE:    18/10/10 06:07:32 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

A. Melandri (INAF-OAB), S. D. Barthelmy (GSFC),
M. J. Moss (George Washington University), K. L. Page (U Leicester)
and B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 05:55:51 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 181010A (trigger=866434).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 52.557, -23.035, which is 
   RA(J2000)  =  03h 30m 14s
   Dec(J2000) = -23d 02' 06"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple peaks
with a total duration of about 25 sec.  The peak count rate
was ~2200 counts/sec (15-350 keV), at ~8 sec after the trigger. 

The XRT began observing the field at 05:57:24.7 UT, 93.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 52.56984, -23.03721 which is equivalent
to:
   RA(J2000)  = 03h 30m 16.76s
   Dec(J2000) = -23d 02' 14.0"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 43 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 (1.89 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 5.9
(+2.46/-2.18) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 96 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	03:30:16.89 =  52.57036
  DEC(J2000) = -23:02:15.4  = -23.03760
with a 90%-confidence error radius of about 0.78 arc sec. This position is 3.0
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.37 with a 1-sigma error of about  0.16. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.03. 

Burst Advocate for this burst is A. Melandri (andrea.melandri AT brera.inaf.it). 
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.)