GCN Circular 21901

A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC), P. A. Evans (U Leicester),
J.D. Gropp (PSU), J. A. Kennea (PSU), H. A. Krimm (NSF/USRA),
S. J. LaPorte (PSU), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), B. Sbarufatti (PSU) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 14:28:48 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170921A (trigger=773509).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 26.849, +30.874 which is 
   RA(J2000) = 01h 47m 24s
   Dec(J2000) = +30d 52' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 20 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 14:30:25.8 UT, 97.3 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 26.8484,
30.9069 which is equivalent to:
   RA(J2000)  = 01h 47m 23.63s
   Dec(J2000) = +30d 54' 24.8"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 118 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 (5.49 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 1.4
(+0.67/-0.55) x 10^22 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 100 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 none of
the XRT error circle. 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 0.05. 

Burst Advocate for this burst is A. D'Ai (antonino.dai AT ifc.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.)