TITLE:   GCN CIRCULAR
NUMBER:  32774
SUBJECT: GRB 221016A: Swift detection of a burst
DATE:    22/10/16 23:53:22 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

K. L. Page (U Leicester), J. A. Kennea (PSU),
N. J. Klingler (GSFC/UMBC/CRESSTII), A. Y. Lien (U Tampa),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL),
A. Tohuvavohu (U Toronto) and M. A. Williams (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 23:39:24 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 221016A (trigger=1129775).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 38.949, -34.624 which is 
   RA(J2000) = 02h 35m 48s
   Dec(J2000) = -34d 37' 26"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
with tail structure with a duration of about 30 sec.  The peak count rate
was ~3500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 23:42:24.0 UT, 179.8 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 38.94479, -34.61039 which is equivalent
to:
   RA(J2000)  = 02h 35m 46.75s
   Dec(J2000) = -34d 36' 37.4"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 50 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 https://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 in excess of the Galactic value (3.53 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.8
(+3.14/-2.70) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White 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 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.026. 

Burst Advocate for this burst is K. L. Page (kimlpage1978 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/)