TITLE:   GCN CIRCULAR
NUMBER:  20296
SUBJECT: GRB 161219B: Swift detection of a burst with an optical afterglow
DATE:    16/12/19 19:07:03 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

A. D'Ai (INAF-IASFPA), J. A. Kennea (PSU),
H. A. Krimm (CRESST/NSF/USRA), N. P. M. Kuin (UCL-MSSL),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 18:48:39 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 161219B (trigger=727541).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 91.717, -26.790 which is 
   RA(J2000) = 06h 06m 52s
   Dec(J2000) = -26d 47' 23"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single triangle-
shaped structure with a duration of about 10 sec.  The peak count rate
was ~3900 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 18:50:27.5 UT, 108.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 91.7140, -26.7915 which
is equivalent to:
   RA(J2000)  = 06h 06m 51.35s
   Dec(J2000) = -26d 47' 29.4"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 11 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 (3.06 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4
(+2.38/-1.96) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 111 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	06:06:51.41 = 91.71421
  DEC(J2000) = -26:47:29.2  = -26.79144
with a 90%-confidence error radius of about 0.5 arc sec. This position is 0.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
16.10 with a 1-sigma error of about  0.03. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.03. 

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.)

[GCN OPS NOTE(20dec16): Per author's request, the "-29:47:29.2"
in the 4th paragraph was changed to "-26:47:29.2".]