GCN Circular 22028

K. L. Page (U Leicester), A. P. Beardmore (U Leicester),
J.D. Gropp (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
B. Sbarufatti (PSU) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 23:07:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 171020A (trigger=780845).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 39.262, +15.186 which is 
   RA(J2000) = 02h 37m 03s
   Dec(J2000) = +15d 11' 10"
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 50 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~2 sec after the trigger. 
Note that the BAT light curve might be contaminated by Swift J0243.6+6124,
which has a period of 9.86 s (ATel #10809). 

The XRT began observing the field at 23:09:35.2 UT, 144.4 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 39.2482,
15.2038 which is equivalent to:
   RA(J2000)  = 02h 36m 59.57s
   Dec(J2000) = +15d 12' 13.5"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 80 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 1.55
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 147 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
0.19. 

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk). 
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.)