TITLE:   GCN CIRCULAR
NUMBER:  13573
SUBJECT: GRB 120804A: Swift detection of a short burst
DATE:    12/08/04 01:07:03 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

A. Y. Lien (NASA/GSFC/ORAU), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), S. T. Holland (STScI), J. A. Kennea (PSU),
K. L. Page (U Leicester), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 00:54:14 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120804A (trigger=529686).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 233.929, -28.761 which is 
   RA(J2000) = 15h 35m 43s
   Dec(J2000) = -28d 45' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a fast double-peaked
structure with a duration of about 1.5 sec.  The peak count rate
was ~15000 counts/sec (15-350 keV), at ~0.5 sec after the trigger. 

The XRT began observing the field at 00:55:47.8 UT, 93.6 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 233.94815, -28.78102 which is
equivalent to:
   RA(J2000)  = 15h 35m 47.56s
   Dec(J2000) = -28d 46' 51.7"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 94 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 (9.29 x
10^20 cm^-2, Kalberla et al. 2005), with an excess column of 1.9
(+1.89/-1.67) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 96 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 none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT 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 0.23. 

Burst Advocate for this burst is A. Y. Lien (yarleen AT gmail.com). 
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.)