GCN Circular 30610

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 22:57:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210807C (trigger=1064421).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 143.551, -51.409 which is 
   RA(J2000) = 09h 34m 12s
   Dec(J2000) = -51d 24' 31"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Although the lightcurve before T+8s is
not currently available due to a telemetry dropout, the later lightcurve
shows a complex structure extending at least to T+90s.  The peak count 
rate in the available lightcurve was ~800 counts/sec (15-350 keV), 
at ~20 sec after the trigger. 

The XRT began observing the field at 23:00:05.5 UT, 177.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 143.53455, -51.40237 which
is equivalent to:
   RA(J2000)  = 09h 34m 08.29s
   Dec(J2000) = -51d 24' 08.5"
with an uncertainty of 7.1 arcseconds (radius, 90% containment). This
location is 43 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data does not constrain the column density. 

The initial flux in the 2.5 s image was 2.31e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the V filter starting
180 seconds after the BAT trigger. No credible afterglow candidate has been
found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the
XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. The list of sources is typically complete to about 18.0 mag. No
correction has been made for the large, but uncertain, extinction expected. 

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/)