GCN Circular 21822

B. Sbarufatti (PSU), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC),
C. Gronwall (PSU), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), M. H. Siegel (PSU) and A. Tohuvavohu (PSU)
report on behalf of the Swift Team:

At 00:55:46 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170906B (trigger=770958). Swift slewed immediately to the burst. 
The BAT on-board calculated location is
RA, Dec 232.221, -28.251 which is
   RA(J2000) = 15h 28m 53s
   Dec(J2000) = -28d 15' 03.6"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve is not immediately available
for an unknown reason. 

The XRT began observing the field at 00:57:04.2 UT, 78.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 232.2037, -28.2563
which is equivalent to:
   RA(J2000)  = 15h 28m 48.90s
   Dec(J2000) = -28d 15' 22.8"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 58 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 (1.69 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 1.1
(+1.25/-0.71) x 10^22 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 80 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
0.42. 

Burst Advocate for this burst is B. Sbarufatti (boris.sbarufatti 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.)