GCN Circular 26601

M. G. Bernardini (INAF-OAB), S. D. Barthelmy (GSFC),
P. D'Avanzo (INAF-OAB), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. J. Klingler (PSU), S Laha (GSFC/UMBC/CRESST),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
A. Melandri (INAF-OAB), M. J. Moss (GWU) and A. Tohuvavohu (U Toronto)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 00:01:19 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 191228A (trigger=946476).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 5.339, -8.670, which is 
   RA(J2000)  =  00h 21m 21s
   Dec(J2000) = -08d 40' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve is typically plain for an image trigger. 

The XRT began observing the field at 00:03:23.2 UT, 123.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 5.34755, -8.68313 which is
equivalent to:
   RA(J2000)  = 00h 21m 23.41s
   Dec(J2000) = -08d 40' 59.3"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 56 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 https://www.swift.ac.uk/sper.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (3.80 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.4
(+5.17/-3.93) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 0.1 s image was 1.26e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 131 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. 
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
0.04. 

Burst Advocate for this burst is M. G. Bernardini (grazia.bernardini AT brera.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/)