GCN Circular 26516

S. Laha (GSFC/UMBC/CRESST), D. N. Burrows (PSU), J.D. Gropp (PSU), N. P.
M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D.
M. Palmer (LANL) and A. Tohuvavohu (U Toronto) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 13:29:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 191220A (trigger=945344).  Swift slewed immediately to the
burst. The BAT on-board calculated location is
RA, Dec 281.391, +26.667 which is
   RA(J2000) = 18h 45m 34s
   Dec(J2000) = +26d 40' 02"
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 20 sec.  The peak count rate
was ~200 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 13:31:50.2 UT, 132.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 281.37924,
26.65599 which is equivalent to:
   RA(J2000)  = 18h 45m 31.02s
   Dec(J2000) = +26d 39' 21.6"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 54 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.

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.53
x 10^21 cm^-2 (Willingale et al. 2013).

The initial flux in the 2.5 s image was 4.03e-10 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 250 seconds with the U filter
starting 137 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 19.2 mag. The 8'x8' region for the list of sources
generated on-board covers 0.00% of the XRT error circle. No correction has
been made for the expected extinction corresponding to E(B-V) of 0.14.

Burst Advocate for this burst is S Laha (sib.laha 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/)