GCN Circular 10256

C. Pagani (PSU), A. P. Beardmore (U Leicester),
P. A. Evans (U Leicester), C. Gronwall (PSU),
S. T. Holland (CRESST/USRA/GSFC), N. P. M. Kuin (MSSL),
C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester),
K. L. Page (U Leicester), A. Rowlinson (U Leicester),
M. Stamatikos (OSU/NASA/GSFC), G. Tagliaferri (INAF-OAB) and
L. Vetere (PSU) report on behalf of the Swift Team:

At 09:49:57 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 091208B (trigger=378559).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 29.410, +16.883 which is 
   RA(J2000) = 01h 57m 39s
   Dec(J2000) = +16d 52' 57"
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 15 sec.  The peak count rate
was ~9100 counts/sec (15-350 keV), at ~8 sec after the trigger. 

The XRT began observing the field at 09:51:53.1 UT, 115.2 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 29.3920,
16.8899 which is equivalent to:
   RA(J2000)  = 01h 57m 34.09s
   Dec(J2000) = +16d 53' 23.6"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 66 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 in excess of the Galactic value (4.85e+20
cm^-2, Kalberla et al. 2005), with an excess column of 3.3
(+1.52/-1.28) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 119 seconds after the BAT trigger.    There is an
afterglow in the rapidly available 2.7'x2.7' sub-image at 
   RA(J2000)  =    01:57:34.06 = 29.39193 
   DEC(J2000) =   +16:53:22.7  = 16.88963  with a 90%-confidence
error radius of about 0.6 arc sec. This position  is 0.67 arc sec. 
from the center of the XRT error circle. The estimated  magnitude is
17.9. No correction has been made for the expected extinction
corresponding to E(B-V) of 0.05.     There is a bright afterglow
candidate consistent with the XRT position. 

Burst Advocate for this burst is C. Pagani (pagani AT astro.psu.edu). 
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.)