TITLE:   GCN CIRCULAR
NUMBER:  23364
SUBJECT: GRB 181022A: Swift detection of a burst
DATE:    18/10/22 17:53:32 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

F. E. Marshall (NASA/GSFC), S. D. Barthelmy (GSFC),
A. D'Ai (INAF-IASFPA), J.D. Gropp (PSU), J. A. Kennea (PSU),
H. A. Krimm (NSF/USRA), S. J. LaPorte (PSU),
M. J. Moss (George Washington University), K. L. Page (U Leicester),
D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 17:30:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 181022A (trigger=868312).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 56.795, -29.332, which is 
   RA(J2000)  =  03h 47m 11s
   Dec(J2000) = -29d 19' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 10 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 17:31:49.9 UT, 96.8 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 56.7920,
-29.3797 which is equivalent to:
   RA(J2000)  = 03h 47m 10.09s
   Dec(J2000) = -29d 22' 46.8"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 171 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 (8.80 x
10^19 cm^-2, Willingale et al. 2013), with an excess column of 4.1
(+3.17/-2.70) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting 97
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 none of the XRT
error circle. 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 18.0 mag. No correction has been made for the expected extinction
corresponding to E(B-V) of 0.01. 

This source lies within the current (Sector 4) field-of-view of TESS camera 2. 

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.)