GCN Circular 30664

A. P. Beardmore (U Leicester), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. J. Klingler (GSFC/UMBC/CRESSTII), N. P. M. Kuin (UCL-MSSL),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and A. Tohuvavohu (U Toronto) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 17:38:00 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210820A (trigger=1069551).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 20.629, +4.632 which is 
   RA(J2000) = 01h 22m 31s
   Dec(J2000) = +04d 37' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 30 sec.  The peak count rate
was ~500 counts/sec (15-350 keV), at ~8 sec after the trigger. 

The XRT began observing the field at 17:40:05.7 UT, 125.0 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 20.6376, 4.6071 which is equivalent to:
   RA(J2000)  = 01h 22m 33.02s
   Dec(J2000) = +04d 36' 25.6"
with an uncertainty of 11.1 arcseconds (radius, 90% containment). This
location is 94 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 3.75e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
130 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	01:22:33.48 =  20.63952
  DEC(J2000) = +04:36:28.2  =	4.60783
with a 90%-confidence error radius of about 0.76 arc sec. This position is 7.4
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.53 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.031. 

Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.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/)