GCN Circular 23349

M. J. Moss (George Washington University), A. D'Ai (INAF-IASFPA),
V. D'Elia (SSDC), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC),
K. L. Page (U Leicester), D. M. Palmer (LANL) and A. Tohuvavohu (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 19:00:33 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 181020A (trigger=867987).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 13.967, -47.380 which is 
   RA(J2000) = 00h 55m 52s
   Dec(J2000) = -47d 22' 47"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 30 sec.  The peak count rate
was ~12000 counts/sec (15-350 keV), at ~7 sec after the trigger. 

The XRT began observing the field at 19:01:29.0 UT, 55.6 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 13.9837, -47.3796 which is equivalent to:
   RA(J2000)  = 00h 55m 56.09s
   Dec(J2000) = -47d 22' 46.6"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 40 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 6.04e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
121 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	00:55:55.78 =  13.98240
  DEC(J2000) = -47:22:51.8  = -47.38105
with a 90%-confidence error radius of about 0.76 arc sec. This position is 6.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.51 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is M. J. Moss (mikejmoss3 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.)