TITLE:   GCN CIRCULAR
NUMBER:  29420
SUBJECT: GRB 210207B: Swift detection of a burst with a bright optical counterpart
DATE:    21/02/07 22:03:29 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

A. Y. Lien (GSFC/UMBC), E. Ambrosi (INAF-IASFPA),
S. D. Barthelmy (GSFC), A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC),
J.D. Gropp (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
S. Laha (GSFC/UMBC/CRESST), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL), A. Tohuvavohu (U Toronto) and
E. Troja (NASA/GSFC/UMCP) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 21:52:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210207B (trigger=1031297).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 270.582, +53.694 which is 
   RA(J2000) = 18h 02m 20s
   Dec(J2000) = +53d 41' 39"
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 100 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~45 sec after the trigger. 

The XRT began observing the field at 21:53:22.0 UT, 73.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 270.6356, 53.6836 which is equivalent to:
   RA(J2000)  = 18h 02m 32.54s
   Dec(J2000) = +53d 41' 01.0"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 120 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 1.09e-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)  =	18:02:32.40 = 270.63501
  DEC(J2000) = +53:40:56.0  =  53.68223
with a 90%-confidence error radius of about 0.74 arc sec. This position is 5.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
13.72 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.041. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien AT nasa.gov). 
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/)