GCN Circular 30600

A. Y. Lien (GSFC/UMBC), C. Gronwall (PSU), J.D. Gropp (PSU),
N. J. Klingler (GSFC/UMBC/CRESSTII), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 10:03:40 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210807A (trigger=1064221).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 76.677, +58.245 which is 
   RA(J2000) = 05h 06m 43s
   Dec(J2000) = +58d 14' 43"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). Because this is an image trigger,
the actual light curve structure requires further ground 
data analysis. The 112-s image trigger indicates that 
this is a long burst. 

The XRT began observing the field at 10:07:02.3 UT, 201.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 76.6815, 58.2503 which is equivalent to:
   RA(J2000)  = 05h 06m 43.56s
   Dec(J2000) = +58d 15' 01.1"
with an uncertainty of 11.1 arcseconds (radius, 90% containment). This
location is 20 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 4.08e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 210 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	05:06:43.90 =  76.68293
  DEC(J2000) = +58:14:59.2  =  58.24978
with a 90%-confidence error radius of about 0.61 arc sec. This position is 3.3
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.00 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.409. 

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/)