[vsnet-grb-info 7161] GRB 081228: Swift detection of a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8742
SUBJECT: GRB 081228: Swift detection of a burst
DATE:    08/12/28 01:33:10 GMT
FROM:    David Burrows at PSU/Swift  <dxb15 at psu.edu>

K. L. Page (U Leicester), W. H. Baumgartner (GSFC/UMBC),
P. J. Brown (PSU), D. N. Burrows (PSU),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), C. B. Markwardt (CRESST/GSFC/UMD) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 01:17:40 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 081228 (trigger=338338).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 39.467, +30.843 which is 
   RA(J2000) = 02h 37m 52s
   Dec(J2000) = +30d 50' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
with a duration of about 2.5 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~0.5 sec after the trigger. 

The XRT began observing the field at 01:18:58.7 UT, 78.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 39.46043, 30.85393 which is equivalent
to:
   RA(J2000)  = 02h 37m 50.50s
   Dec(J2000) = +30d 51' 14.1"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 44 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
8.8e+20 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of  37 seconds with the White filter
starting 82 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 25% of
the BAT 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
BAT 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
0.16. 

Burst Advocate for this burst is K. L. Page (kpa AT star.le.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.)