TITLE:   GCN CIRCULAR
NUMBER:  8344
SUBJECT: GRB 081008: Swift detection of a burst with optical afterglow
DATE:    08/10/08 20:12:50 GMT
FROM:    Jamie A. Kennea at PSU/Swift-XRT  <kennea@astro.psu.edu>

J. L. Racusin (PSU), W. H. Baumgartner (GSFC/UMBC), P. J. Brown (PSU),
D. N. Burrows (PSU), P. A. Evans (U Leicester), D. Grupe (PSU),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
J. A. Kennea (PSU), V. La Parola (INAF-IASFPA),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
J. P. Osborne (U Leicester), A. M. Parsons (GSFC),
M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester),
G. Tagliaferri (INAF-OAB), T. N. Ukwatta (GSFC/GWU) and
L. Vetere (PSU) report on behalf of the Swift Team:

At 19:58:09 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 081008 (trigger=331093).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 279.987, -57.464 which is 
   RA(J2000) = 18h 39m 57s
   Dec(J2000) = -57d 27' 50"
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 150 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 19:59:36.5 UT, 87.1 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 279.9587, -57.4312 which is equivalent to:
   RA(J2000)  = 18h 39m 50.08s
   Dec(J2000) = -57d 25' 52.3"
with an uncertainty of 5.3 arcseconds (radius, 90% containment). This
location is 130 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. 

UVOT took a finding chart exposure of 150 seconds with the White (160-650 nm)
filter starting 96 seconds after the BAT trigger. There is a candidate
afterglow in the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	18:39:50.11 = 279.9588
  DEC(J2000) = -57:25:52.3  = -57.4312
with a 1-sigma error radius of about 1.6 arc sec. This position is 0.3 arcseconds
from the center of the XRT error circle. The estimated magnitude is 15.0 with a
1-sigma error of about 0.5 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.10. 

Burst Advocate for this burst is J. L. Racusin (racusin 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.)