TITLE:   GCN CIRCULAR
NUMBER:  8964
SUBJECT: GRB 090309: Swift detection of a burst
DATE:    09/03/09 23:50:52 GMT
FROM:    David Burrows at PSU/Swift  <dxb15@psu.edu>

P. A. Evans (U Leicester), W. H. Baumgartner (GSFC/UMBC),
D. N. Burrows (PSU), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), W.B Landsman (GSFC),
F. E. Marshall (NASA/GSFC), C. Pagani (PSU),
K. L. Page (U Leicester), D. M. Palmer (LANL), A. M. Parsons (GSFC),
B. A. Rowlinson (U Leicester), T. Sakamoto (NASA/UMBC),
M. H. Siegel (PSU), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU)
report on behalf of the Swift Team:

At 23:29:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090309 (trigger=345945).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 284.981, -25.264 which is 
   RA(J2000) = 18h 59m 55s
   Dec(J2000) = -25d 15' 50"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a peak with a 
with a duration of about 3 sec.  There is a hint of a secondary peak 
around T+25 sec with a duration of 10 sec.  The peak count rate
was ~1400 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 23:30:32.5 UT, 79.3 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 284.9730, -25.2611
which is equivalent to:
   RA(J2000)  = 18h 59m 53.51s
   Dec(J2000) = -25d 15' 39.8"
with an uncertainty of 3.3 arcseconds (radius, 90% containment). This
location is 28 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 consistent with the Galactic value of
1.07e+21 cm^-2 (Kalberla et al. 2005). 


UVOT took a finding chart exposure of 150 seconds with the White filter
starting 81 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 typical upper limit for this image is approximately
white = 19 mag. No correction has been made for the expected extinction
corresponding to E(B-V) of 0.23. 

Burst Advocate for this burst is P. A. Evans (pae9 AT star.le.ac.uk). 
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.)