GCN Circular 23366

F. E. Marshall (NASA/GSFC), A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC),
J. A. Kennea (PSU), S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC),
M. J. Moss (George Washington University) and D. M. Palmer (LANL)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 02:04:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 181023A (trigger=868427).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 248.329, +19.564 which is 
   RA(J2000) = 16h 33m 19s
   Dec(J2000) = +19d 33' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual with an image trigger, the available
BAT light curve shows no significant structure. 

The XRT began observing the field at 02:07:17.8 UT, 195.7 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 248.32979, 19.57140 which is equivalent
to:
   RA(J2000)  = 16h 33m 19.15s
   Dec(J2000) = +19d 34' 17.0"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 26 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 (6.04 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.4
(+2.82/-2.40) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 199 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.05. 

Burst Advocate for this burst is F. E. Marshall (marshall 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.)