TITLE:   GCN CIRCULAR
NUMBER:  14007
SUBJECT: GRB 121128A: Swift detection of a burst with an optical counterpart
DATE:    12/11/28 05:18:00 GMT
FROM:    Samantha Oates at MSSL  <sro@mssl.ucl.ac.uk>

S. R. Oates (UCL-MSSL), S. D. Barthelmy (GSFC),
H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL),
C. B. Markwardt (NASA/GSFC), K. L. Page (U Leicester),
M. H. Siegel (PSU) and M. Stamatikos (OSU/NASA/GSFC) report on behalf
of the Swift Team:

At 05:05:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 121128A (trigger=539866).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 300.583, +54.321 which is 
   RA(J2000) = 20h 02m 20s
   Dec(J2000) = +54d 19' 17"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 40 sec.  The peak count rate
was ~20,000 counts/sec (15-350 keV), at ~20 sec after the trigger. 

The XRT began observing the field at 05:06:54.3 UT, 77.2 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 300.60067, 54.30060 which
is equivalent to:
   RA(J2000)  = 20h 02m 24.16s
   Dec(J2000) = +54d 18' 02.2"
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This
location is 82 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 2.08
x 10^21 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.58e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 85 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	20:02:24.01 = 300.60004
  DEC(J2000) = +54:17:59.2  =  54.29977
with a 90%-confidence error radius of about 0.61 arc sec. This position is 5.5
arc sec. from the center of the XRT error circle. The estimated magnitude is
16.11 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.31. 

Burst Advocate for this burst is S. R. Oates (sro AT mssl.ucl.ac.uk). 
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.)