GCN Circular 9746

J. K. Cannizzo (NASA/UMBC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
P.A. Curran (MSSL-UCL), A. de Ugarte Postigo (INAF-OAB),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC),
C. Guidorzi (U Ferrara), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (MSSL),
J. Mao (INAF-OAB), R. Margutti (Univ Bicocca&OAB),
K. L. Page (U Leicester), T. Sakamoto (NASA/UMBC),
M. H. Siegel (PSU), M. Stamatikos (NASA/ORAU),
T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 15:00:27 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090807 (trigger=359378).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 273.744, +10.281 which is 
   RA(J2000)  =  18h 14m 59s
   Dec(J2000) = +10d 16' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT TDRSS light curve does not show anything
as is typical for image triggers. 

The XRT began observing the field at 15:03:00.5 UT, 153.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright, fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 273.7438,
10.2665 which is equivalent to:
   RA(J2000)  = 18h 14m 58.51s
   Dec(J2000) = +10d 15' 59.3"
with an uncertainty of 2.4 arcseconds (radius, 90% containment). This
location is 52 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 (8.6e+20
cm^-2, Kalberla et al. 2005), with an excess column of 2 (+1.68/-1.50)
x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 8.55e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of nominal 150 seconds with the 
White filter  starting 163 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
3-sigma  upper limit is about 19.2 mag. 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 18.0 mag. No
correction  has been made for the expected extinction corresponding
to E(B-V) of  0.18. 

Burst Advocate for this burst is J. K. Cannizzo (cannizzo AT milkyway.gsfc.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.)