GCN Circular 9754

C. B. Markwardt (CRESST/GSFC/UMD), S. D. Barthelmy (GSFC),
M. M. Chester (PSU), A. de Ugarte Postigo (INAF-OAB),
N. Gehrels (NASA/GSFC), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
N. P. M. Kuin (MSSL), W.B Landsman (GSFC), J. Mao (INAF-OAB),
R. Margutti (Univ Bicocca&OAB), P. T. O'Brien (U Leicester),
K. L. Page (U Leicester), P. Romano (INAF-IASFPA) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 17:31:14 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090809 (trigger=359530).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 328.645, -0.083 which is 
   RA(J2000)  =  21h 54m 35s
   Dec(J2000) = -00d 04' 59"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a weak peak
with a duration of about 10 sec.  The peak count rate
was ~900 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 17:32:58.5 UT, 104.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 328.67772, -0.08465 which is equivalent
to:
   RA(J2000)  = 21h 54m 42.65s
   Dec(J2000) = -00d 05' 04.7"
with an uncertainty of 4.5 arcseconds (radius, 90% containment). This
location is 117 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 (6.08e+20
cm^-2, Kalberla et al. 2005), with an excess column of 2.5
(+2.71/-2.28) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of  86 seconds with the White 
filter  starting 107 seconds after the BAT trigger. A possible 
afterglow candidate has  been found in the list of sources generated 
on-board, located at RA=328.68008  (21:54:43.22), dec=-0.08385 
(-00:05:01.9), with an estimated White magnitude = 19.9 +/-0.4.  The 
2.7'x2.7' sub-image does not include the XRT position. No  correction 
has been made for the expected extinction corresponding to E(B-V) of  
0.10. 

Burst Advocate for this burst is C. B. Markwardt (Craig.Markwardt AT nasa.gov). 
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.)