TITLE:   GCN CIRCULAR
NUMBER:  8057
SUBJECT: GRB 080804: Swift detection of a burst with optical afterglow
DATE:    08/08/04 23:37:27 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@lheamail.gsfc.nasa.gov>

J. L. Racusin (PSU), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), M. M. Chester (PSU),
N. Gehrels (NASA/GSFC), D. Grupe (PSU),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
C. Pagani (PSU), D. M. Palmer (LANL), M. C. Stroh (PSU) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 23:20:14 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080804 (trigger=319016).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 328.701, -53.195 which is 
   RA(J2000)  =  21h 54m 48s
   Dec(J2000) = -53d 11' 40"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple overlapping
peaks with a duration of about 20 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 23:21:53.7 UT, 99.0 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 328.66583, -53.18532 which
is equivalent to:
   RA(J2000)  = 21h 54m 39.80s
   Dec(J2000) = -53d 11' 07.2"
with an uncertainty of 3.0 arcseconds (radius, 90% containment). This
location is 83 arcseconds from the BAT onboard position, within the BAT
error circle. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
1.63e+20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 5.50e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of nominal 100 seconds with the White
(160-650 nm) filter starting 109 seconds after the BAT trigger. There is a
candidate afterglow in the list of sources generated on-board at
  RA(J2000)  =  21:54:40.2 = 328.6675
  DEC(J2000) = -53:11:05   = -53.18472
with a 1-sigma error radius of about 1.0 arc sec. The estimated white
magnitude is 16.8 with a 1-sigma error of about 0.5 mag. No correction
has been made for a Galactic extinction corresponding to a reddening of
0.02 mag along the line of sight. 

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.)