GCN Circular 8687

E. A. Hoversten (PSU), W. H. Baumgartner (GSFC/UMBC),
D. N. Burrows (PSU), M. M. Chester (PSU), J. R. Cummings (NASA/UMBC),
J. A. Kennea (PSU), V. Mangano (INAF-IASFPA),
C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC),
P. T. O'Brien (U Leicester), D. M. Palmer (LANL),
J. L. Racusin (PSU), P. Romano (INAF-IASFPA),
B. A. Rowlinson (U Leicester), M. C. Stroh (PSU),
G. Tagliaferri (INAF-OAB), T. N. Ukwatta (GSFC/GWU) and
L. Vetere (PSU) report on behalf of the Swift Team:

At 16:21:11 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 081221 (trigger=337889).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 15.805, -24.526 which is 
   RA(J2000) = 01h 03m 13s
   Dec(J2000) = -24d 31' 32"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed two peaks, at 
T+0 to T+10 seconds and a much larger one at T+15 to T+40 seconds.  
The peak count rate was ~20000 counts/sec (15-350 keV), at 22 sec 
after the trigger. 

The XRT began observing the field at 16:22:20.2 UT, 68.4 seconds after
the BAT trigger. XRT found a bright, fading, uncatalogued X-ray source located
at RA, Dec 15.7934, -24.5484 which is equivalent to:
   RA(J2000)  = 01h 03m 10.41s
   Dec(J2000) = -24d 32' 54.2"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
location is 89 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

The initial flux in the 0.1 s image was 7.36e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White  
filter starting 78 seconds after the BAT trigger, and a finding  
chart exposure of 250 seconds with the u filter starting 290 seconds 
 after the 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 3-sigma upper limit at the XRT position is 
19.9  mag in White, 19.8 mag in u. No correction has been made for 
the expected extinction  corresponding to E(B-V) of 0.02. 

We note that this burst is coincident with a Fermi GBM burst (trigger 251569273). 

Burst Advocate for this burst is E. A. Hoversten (hoversten 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.)