TITLE:   GCN CIRCULAR
NUMBER:  9881
SUBJECT: GRB 090904B: Swift detection of a burst
DATE:    09/09/04 01:50:16 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@lheamail.gsfc.nasa.gov>

V. D'Elia (ASDC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), S. T. Holland (CRESST/USRA/GSFC),
J. A. Kennea (PSU), C. B. Markwardt (CRESST/GSFC/UMD),
F. E. Marshall (NASA/GSFC), C. Pagani (PSU), M. Perri (ASDC),
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 01:24:18 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090904B (trigger=361831).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 264.191, -25.212 which is 
   RA(J2000)  =  17h 36m 46s
   Dec(J2000) = -25d 12' 43"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple overlapping peaks
starting at T-5 sec with a duration of about 60 sec.  The peak count rate
was ~1800 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 01:26:32.3 UT, 134.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 264.1854, -25.2138
which is equivalent to:
   RA(J2000)  = 17h 36m 44.50s
   Dec(J2000) = -25d 12' 49.7"
with an uncertainty of 2.4 arcseconds (radius, 90% containment). This
location is 19 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 (3.28e+21
cm^-2, Kalberla et al. 2005), with an excess column of 4.8
(+5.33/-4.01) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 137 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 100% of the XRT error circle. The 8'x8'
region for the list of sources generated on-board covers 100% of the
XRT error circle.  Because of the density of catalogued stars,further
analysis is required to report an upper limit for any afterglow in the
region. 

We note that this is a separate burst from the "A" burst.  
But there may be some confusion resulting from overlapping GCN Notices. 

Burst Advocate for this burst is V. D'Elia (delia 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.)