TITLE:   GCN CIRCULAR
NUMBER:  23158
SUBJECT: GRB 180821A: Swift detection of a burst
DATE:    18/08/21 16:53:28 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

A. Tohuvavohu (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU),
H. A. Krimm (NSF/USRA), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU),
M. H. Siegel (PSU) and K. K. Simpson (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 16:40:34 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180821A (trigger=854578).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 7.763, -38.619 which is 
   RA(J2000) = 00h 31m 03s
   Dec(J2000) = -38d 37' 07"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for an image trigger, the
immediately-available BAT lightcurve shows no obvious variation. 

The XRT began observing the field at 16:44:04.8 UT, 210.3 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 7.78833, -38.62660
which is equivalent to:
   RA(J2000)  = 00h 31m 09.20s
   Dec(J2000) = -38d 37' 35.8"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 76 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. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

The initial flux in the 2.5 s image was 3.67e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 218 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 19.6 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 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.02. 

Burst Advocate for this burst is A. Tohuvavohu (aaronb AT swift.psu.edu). 
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/too.html.)