GCN Circular 19833

M. H. Siegel (PSU), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf
of the Swift Team:

At 22:29:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160821B (trigger=709357).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 279.987, +62.393, which is 
   RA(J2000)  =  18h 39m 57s
   Dec(J2000) = +62d 23' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single spike
structure with a duration of about 0.5 sec.  The peak count rate
was ~5000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 22:30:19.1 UT, 66.0 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
279.9780, 62.3920 which is equivalent to:
   RA(J2000)  = 18h 39m 54.71s
   Dec(J2000) = +62d 23' 31.3"
with an uncertainty of 2.5 arcseconds (radius, 90% containment). This
location is 15 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 consistent with the Galactic value of 5.75
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.45e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 76 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' covers 100% of
the XRT error region. The typical 3-sigma upper limit has
been about 19.6 mag. No correction has been made
for the expected extinction corresponding to E(B-V) of 0.05. 

Burst Advocate for this burst is M. H. Siegel (siegel 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.)