TITLE:   GCN CIRCULAR
NUMBER:  26279
SUBJECT: GRB 191123A: Swift detection of a burst with an optical counterpart
DATE:    19/11/23 11:05:32 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaronb@swift.psu.edu>

A. Tohuvavohu (U Toronto), J.D. Gropp (PSU), A. Y. Lien (GSFC/UMBC),
K. L. Page (U Leicester) and B. Sbarufatti (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 10:38:44 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 191123A (trigger=939079).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 215.261, +22.842 which is 
   RA(J2000) = 14h 21m 03s
   Dec(J2000) = +22d 50' 30"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 150 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 10:39:54.1 UT, 69.8 seconds after
the BAT trigger. The position determined from promptly downlinked data
differs significantly from the on-board position, suggesting that the
XRT may have centroided on a cosmic ray; the initial XRT position
notice should be treated with caution. Using promptly downlinked data
we find an uncatalogued X-ray source with an enhanced position: RA, Dec
215.2619, 22.8600 which is equivalent to:
   RA(J2000)  = 14h 21m 02.87s
   Dec(J2000) = +22d 51' 36.0"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 64 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 (3.01 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.9
(+2.28/-2.01) x 10^21 cm^-2 (90% confidence). 

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)  =	14:21:02.87 = 215.26196
  DEC(J2000) = +22:51:36.3  =  22.86008
with a 90%-confidence error radius of about 0.75 arc sec. This position is 9.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.13 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.04. 

Burst Advocate for this burst is A. Tohuvavohu (aaronb AT swift.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/)