GCN Circular 9879

M. Perri (ASDC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU), V. D'Elia (ASDC),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
F. E. Marshall (NASA/GSFC), M. H. Siegel (PSU),
M. Stamatikos (OSU/NASA/GSFC), M. A. Stark (PSU) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 01:01:06 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090904 (trigger=361830).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 100.882, +50.206 which is 
   RA(J2000) = 06h 43m 32s
   Dec(J2000) = +50d 12' 23"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a small peak at T_zero
and then two large peaks at T+130 and T+175 sec with a duration
of at least 200 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~T+130 sec after the trigger. 

The XRT began observing the field at 01:05:38.6 UT, 271.7 seconds after
the BAT trigger. XRT found a bright, uncatalogued, fading X-ray source located
at RA, Dec 100.8816, +50.2040 which is equivalent to:
   RA(J2000)  = 06h 43m 31.58s
   Dec(J2000) = +50d 12' 14.4"
with an uncertainty of 5.2 arcseconds (radius, 90% containment). This
location is 7.3 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 XRT prompt light curve shows a large flare peaking about 30s 
after the start of the XRT observations. 

The initial flux in the 2.5 s image was 1.71e-09 erg cm^-2 s^-1 (0.2-10
keV). 


UVOT took a finding chart exposure of 150 seconds with the White filter
starting 141 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. Because part of the 2.7'x2.7'
sub-image was not received, the overlap with the XRT error circle is uncertain. 
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
0.10. 




Burst Advocate for this burst is M. Perri (perri AT asdc.asi.it). 
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.)