TITLE:   GCN CIRCULAR
NUMBER:  29289
SUBJECT: GRB 210112A: Swift detection of a burst with an optical counterpart
DATE:    21/01/12 01:51:00 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

E. Ambrosi (INAF-IASFPA), S. D. Barthelmy (GSFC),
A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC), J. A. Kennea (PSU),
N. J. Klingler (PSU), S. Laha (GSFC/UMBC/CRESST),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 01:37:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210112A (trigger=1016881).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 218.999, +33.070 which is 
   RA(J2000)  =  14h 36m 00s
   Dec(J2000) = +33d 04' 13"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple peaks
with the larger peak at T+70 sec and duration of ~150 sec. The peak count rate
was ~20800 counts/sec (15-350 keV), at ~68 sec after the trigger. 

The XRT began observing the field at 01:38:18.0 UT, 74.2 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 219.0044, 33.0566 which is equivalent to:
   RA(J2000)  = 14h 36m 1.06s
   Dec(J2000) = +33d 03' 23.8"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
location is 50 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 0.1 s image was 7.10e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 82 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	14:36:01.45 = 219.00606
  DEC(J2000) = +33:03:15.4  =  33.05428
with a 90%-confidence error radius of about 0.66 arc sec. This position is 9.8
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.52 with a 1-sigma error of about  0.16. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.014. 

Burst Advocate for this burst is E. Ambrosi (elena.ambrosi AT inaf.it). 
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/)