GCN Circular 21640

A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB),
K. L. Page (U Leicester), T. Sakamoto (AGU), B. Sbarufatti (PSU),
R. L. C. Starling (U Leicester) and E. Troja (NASA/GSFC/UMCP) report
on behalf of the Swift Team:

At 09:11:51 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170822A (trigger=768939).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 94.382, +55.019 which is 
   RA(J2000) = 06h 17m 32s
   Dec(J2000) = +55d 01' 07"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex 
structure with a duration of about 70 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 09:14:34.2 UT, 163.2 seconds after
the BAT trigger. The position determined from promptly downlinked data
differs significantly from the on-board position, suggesting that the
XRT may have centroided on a cosmic ray; the initial XRT position
notice should be treated with caution. Using promptly downlinked data
we find a fading, uncatalogued X-ray source with an enhanced position:
RA, Dec 94.3869, 55.0040 which is equivalent to:
   RA(J2000)  = 06h 17m 32.86s
   Dec(J2000) = +55d 00' 14.5"
with an uncertainty of 2.0 arcseconds (radius, 90% containment). This
location is 54 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 (1.91 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 2.6
(+2.37/-2.09) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 171 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
0.17. 

Burst Advocate for this burst is A. Melandri (andrea.melandri AT brera.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/too.html.)