TITLE:   GCN CIRCULAR
NUMBER:  25427
SUBJECT: GRB 190821A: Swift detection of a burst
DATE:    19/08/21 17:31:00 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien@nasa.gov>

A. D'Ai (INAF-IASFPA), E. Ambrosi (INAF-IASFPA), V. D'Elia (SSDC),
J.D. Gropp (PSU), J. A. Kennea (PSU), N. J. Klingler (PSU),
H. A. Krimm (NSF), S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), M. J. Moss (GWU),
K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU),
K. K. Simpson (PSU) and A. Tohuvavohu (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 17:10:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 190821A (trigger=921722).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 250.077, -34.022 which is 
   RA(J2000) = 16h 40m 18s
   Dec(J2000) = -34d 01' 16"
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 ~2200 counts/sec (15-350 keV), at ~30 sec after the trigger. 

The XRT began observing the field at 17:11:21.8 UT, 78.0 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 250.05464, -34.00719 which
is equivalent to:
   RA(J2000)  = 16h 40m 13.11s
   Dec(J2000) = -34d 00' 25.9"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 85 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 http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (3.35 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 3.2
(+2.68/-2.37) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 2.95e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 88 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. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the expected extinction corresponding
to E(B-V) of 0.54. 

Burst Advocate for this burst is A. D'Ai (antonino.dai AT inaf.it). 
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/)