TITLE:   GCN CIRCULAR
NUMBER:  33206
SUBJECT: GRB 230123A: Swift detection of a burst
DATE:    23/01/23 10:36:16 GMT
FROM:    Kim Page at U.of Leicester  <kimlpage1978@gmail.com>

M. A. Williams (PSU), R. A. J. Eyles-Ferris (U Leicester),
J.D. Gropp (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (U Tampa) and K. L. Page (U Leicester) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 10:19:44 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230123A (trigger=1150429).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 40.937, -65.676 which is 
   RA(J2000) = 02h 43m 45s
   Dec(J2000) = -65d 40' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). The BAT light curve showed a single-peaked
structure with a duration of about 10 sec.  The peak count rate 
was ~800 counts/sec (15-350 keV), at ~1 sec after the trigger. 
The immediately available BAT light curve does not include data 
from ~T+8 s to ~T+100 s. There might be additional burst emission 
in that time interval. 

The XRT began observing the field at 10:21:13.6 UT, 88.8 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 40.85977, -65.68672
which is equivalent to:
   RA(J2000)  = 02h 43m 26.34s
   Dec(J2000) = -65d 41' 12.2"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 120 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 (2.95 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 1.3
(+1.44/-1.26) x 10^22 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 92 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 12% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
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
0.022. 

Burst Advocate for this burst is M. A. Williams (mjw6837 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/)