TITLE:   GCN CIRCULAR
NUMBER:  13982
SUBJECT: GRB 121123A: Swift detection of a burst with optical counterpart
DATE:    12/11/23 10:17:59 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

E. A. Helder (PSU), S. D. Barthelmy (GSFC), V. D'Elia (ASDC),
P. A. Evans (U Leicester), F. E. Marshall (NASA/GSFC),
C. J. Mountford (U Leicester), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 10:02:41 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 121123A (trigger=539358).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 307.362, -11.860 which is 
   RA(J2000)  =  20h 29m 27s
   Dec(J2000) = -11d 51' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
structure with a duration of about 25 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 10:04:44.3 UT, 123.0 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 307.3179, -11.8590 which is equivalent to:
   RA(J2000)  = +20h 29m 16.30s
   Dec(J2000) = -11d 51' 32.4"
with an uncertainty of 5.4 arcseconds (radius, 90% containment). This
location is 155 arcseconds from the BAT onboard position, within the
BAT error circle. No event data are yet available to determine the
column density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 1.30e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 131 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	20:29:16.31 = 307.31794
  DEC(J2000) = -11:51:35.4  = -11.85983
with a 90%-confidence error radius of about 0.67 arc sec. This position is 3.0
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.34 with a 1-sigma error of about  0.17. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.05. 

Burst Advocate for this burst is E. A. Helder (helder 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/too.html.)