TITLE:   GCN CIRCULAR
NUMBER:  22256
SUBJECT: GRB 171216A: Swift detection of a burst
DATE:    17/12/16 03:23:29 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

F. E. Marshall (NASA/GSFC), J. R. Cummings (NASA/UMBC),
A. Deich (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU),
A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 03:06:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 171216A (trigger=797441).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 212.078, -50.473 which is 
   RA(J2000) = 14h 08m 19s
   Dec(J2000) = -50d 28' 22"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of ~20 sec.  The peak count rate
was ~600 counts/sec (15-350 keV), at ~4 sec after the trigger. 

The XRT began observing the field at 03:08:54.7 UT, 115.2 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
211.9924, -50.4851 which is equivalent to:
   RA(J2000)  = 14h 07m 58.17s
   Dec(J2000) = -50d 29' 06.3"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 200 arcseconds from the BAT onboard position. 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 2.84
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 5.37e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 123 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. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the expected extinction corresponding
to E(B-V) of 0.26. 

Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.nasa.gov). 
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.)