GCN Circular 17196

J. R. Cummings (NASA/UMBC), V. D'Elia (ASDC), N. Gehrels (NASA/GSFC),
A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 06:02:52 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 141220A (trigger=621915).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 195.050, +32.144 which is 
   RA(J2000) = 13h 00m 12s
   Dec(J2000) = +32d 08' 37"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a duration of about 10 sec.  The peak count rate
was ~5000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 06:04:31.8 UT, 99.2 seconds after
the BAT trigger. The position determined from promptly downlinked data
differs significantly from the on-board position, suggesting that the
XRT may have centroided on a cosmic ray; the initial XRT position
notice should be treated with caution. Using promptly downlinked data
we find a fading, uncatalogued X-ray source located at RA, Dec
195.06735, 32.13361 which is equivalent to:
   RA(J2000)  = 13h 00m 16.16s
   Dec(J2000) = +32d 08' 01.0"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 64 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 1.33
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 105 seconds after the BAT trigger. The 2.7'x2.7' sub-image shows a
likely optical afterglow near the XRT position. We are unable to confirm or
measure a magnitude due to the loss of star tracker lock. 

We note that the XRT position may have an additional error of order
10 arcseconds due to star tracker lock.  A more accurate location
will be generated when the full dataset is available. 

Burst Advocate for this burst is J. R. Cummings (jayc 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.)