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GCN Circular 23482

Subject
GRB 181202A: Swift detection of a burst with an optical counterpart
Date
2018-12-02T06:48:16Z (5 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
J. K. Cannizzo (NASA/UMBC), J.D. Gropp (PSU),
K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 06:36:24 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 181202A (trigger=874334).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 280.745, +27.951 which is 
   RA(J2000) = 18h 42m 59s
   Dec(J2000) = +27d 57' 03"
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 10 sec.  The peak count rate
was ~1100 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 06:37:49.8 UT, 85.4 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 280.73563, 27.95894 which is equivalent
to:
   RA(J2000)  = 18h 42m 56.55s
   Dec(J2000) = +27d 57' 32.2"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 41 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 consistent with the Galactic value of 1.67
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting 86
seconds after the BAT trigger. There is a candidate afterglow in the rapidly
available 2.7'x2.7' sub-image at
  RA(J2000)  =	18:42:56.38 = 280.73490
  DEC(J2000) = +27:57:35.0  =  27.95973
with a 90%-confidence error radius of about 0.63 arc sec. This position is 3.4
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.67 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.14. 

Burst Advocate for this burst is J. K. Cannizzo (cannizzo AT milkyway.gsfc.nasa.gov). 
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.)
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