GCN Circular 20302

V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), T. Sakamoto (AGU),
B. Sbarufatti (INAF-OAB/PSU) and T. N. Ukwatta (LANL) report on behalf
of the Swift Team:

At 08:33:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 161220A (trigger=727610).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 350.817, -47.502 which is 
   RA(J2000) = 23h 23m 16s
   Dec(J2000) = -47d 30' 07"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a FRED-like 
structure with a duration of about 12 sec.  The peak count rate
was ~1285 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 08:34:39.2 UT, 69.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 350.84774,
-47.49773, which is equivalent to:
   RA(J2000)  = 23h 23m 23.46s
   Dec(J2000) = -47d 29' 51.8"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 76 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 1.19
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.94e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 78 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. Results from the list of sources
generated on-board are not available at this time. No correction has been made
for the expected extinction corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.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.)