GCN Circular 18739

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
F. E. Marshall (NASA/GSFC), L. M. McCauley (PSU) and
K. L. Page (U Leicester) report on behalf of the Swift Team:

At 22:47:14 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151228B (trigger=668641).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 344.441, +8.076, which is 
   RA(J2000)  =  22h 57m 46s
   Dec(J2000) = +08d 04' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows FRED-like peak
structure with a duration of about 40 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 22:48:04.4 UT, 49.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
344.4247, 8.0816 which is equivalent to:
   RA(J2000)  = 22h 57m 41.93s
   Dec(J2000) = +08d 04' 53.7"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 61 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 5.74
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 6.74e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 57 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =  22:57:41.85 = 344.42437
  DEC(J2000) = +08:04:54.0  =   8.08166
with a 90%-confidence error radius of about 0.80 arc sec. This position is 3.8
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.74 with a 1-sigma error of about  0.17. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.06. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien AT 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.)