TITLE:   GCN CIRCULAR
NUMBER:  29158
SUBJECT: GRB 201223A: Swift detection of a burst with a bright optical counterpart
DATE:    20/12/23 18:12:13 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

J.D. Gropp (PSU), S. D. Barthelmy (GSFC), J. A. Kennea (PSU),
N. J. Klingler (PSU), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 17:58:26 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 201223A (trigger=1014316).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 132.849, +71.174 which is 
   RA(J2000) = 08h 51m 24s
   Dec(J2000) = +71d 10' 26"
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 50 sec.  The peak count rate
was ~1600 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 17:59:40.7 UT, 73.7 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 132.78615, 71.17926 which
is equivalent to:
   RA(J2000)  = 08h 51m 08.68s
   Dec(J2000) = +71d 10' 45.3"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 75 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 3.13
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 78 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	08:51:09.51 = 132.78963
  DEC(J2000) = +71:10:47.4  =  71.17982
with a 90%-confidence error radius of about 0.61 arc sec. This position is 3.8
arc sec. from the center of the XRT error circle. The estimated magnitude is
16.16 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.05. 

Burst Advocate for this burst is J.D. Gropp (jdg44 AT psu.edu). 
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/)