TITLE:   GCN CIRCULAR
NUMBER:  29521
SUBJECT: GRB 210217A: Swift detection of a burst with an optical counterpart
DATE:    21/02/17 23:36:33 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

K. K. Simpson (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. J. Klingler (PSU), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), M. J. Moss (GWU),
D. M. Palmer (LANL), B. Sbarufatti (PSU) and M. H. Siegel (PSU) report
on behalf of the Neil Gehrels Swift Observatory Team:

At 23:25:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210217A (trigger=1033264).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 97.610, +68.715 which is 
   RA(J2000) = 06h 30m 26s
   Dec(J2000) = +68d 42' 53"
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 5 sec.  The peak count rate
was ~3400 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 23:27:20.1 UT, 97.8 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 97.58675, 68.72497 which
is equivalent to:
   RA(J2000)  = 06h 30m 20.82s
   Dec(J2000) = +68d 43' 29.9"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 47 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 8.69
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 103 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	06:30:19.93 =  97.58305
  DEC(J2000) = +68:43:30.6  =  68.72516
with a 90%-confidence error radius of about 0.63 arc sec. This position is 6.2
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.72 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.103. 

Burst Advocate for this burst is K. K. Simpson (kira.simpson1984 AT gmail.com). 
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/)