TITLE:   GCN CIRCULAR
NUMBER:  35522
SUBJECT: GRB 240112C: Swift detection of a burst with an optical counterpart
DATE:    24/01/12 17:51:21 GMT
FROM:    David Palmer at LANL <palmer@lanl.gov>


R. A. J. Eyles-Ferris (U Leicester), J.D. Gropp (PSU),
S. Laha (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and
M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 17:37:24 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240112C (trigger=1208389).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 270.644, +24.487 which is 
   RA(J2000) = 18h 02m 35s
   Dec(J2000) = +24d 29' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Due to a telemetry gap, the BAT lightcurve
is not immediately available. 

The XRT began observing the field at 17:39:18.9 UT, 114.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 270.64225, 24.48830 which is equivalent
to:
   RA(J2000)  = 18h 02m 34.14s
   Dec(J2000) = +24d 29' 17.9"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 7.4 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. No spectrum from the promptly downlinked
event data is yet available to determine the column density. 

UVOT took a finding chart exposure of 115 seconds with the White filter
starting 120 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	18:02:34.59 = 270.64414
  DEC(J2000) = +24:29:21.2  =  24.48923
with a 90%-confidence error radius of about 0.75 arc sec. This position is 7.4
arc sec. from the center of the XRT error circle. The estimated magnitude is
16.73 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.108. 

Burst Advocate for this burst is R. A. J. Eyles-Ferris (raje1 AT leicester.ac.uk). 
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/)