TITLE:   GCN CIRCULAR
NUMBER:  10138
SUBJECT: GRB 091109: Swift detection of a burst with optical afterglow
DATE:    09/11/09 05:11:55 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

S. R. Oates (UCL-MSSL), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), N. Gehrels (NASA/GSFC),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
C. B. Markwardt (CRESST/GSFC/UMD), K. L. Page (U Leicester),
D. M. Palmer (LANL), M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC)
and L. Vetere (PSU) report on behalf of the Swift Team:

At 04:57:43 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 091109 (trigger=375246).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 309.265, -44.176 which is 
   RA(J2000) = 20h 37m 04s
   Dec(J2000) = -44d 10' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a spiky
structure with a duration of about 25 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~20 sec after the trigger. 

The XRT began observing the field at 05:00:14.1 UT, 150.7 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 309.25854, -44.15853 which
is equivalent to:
   RA(J2000)  = 20h 37m 2.05s
   Dec(J2000) = -44d 09' 30.7"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 65 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 (2.99e+20
cm^-2, Kalberla et al. 2005), with an excess column of 3 (+1.84/-1.63)
x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 155 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	20:37:01.81 = 309.25755
  DEC(J2000) = -44:09:29.6  = -44.15821
with a 90%-confidence error radius of about 0.69 arc sec. This position is 2.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.92 with a 1-sigma error of about 0.18. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.03. 

Burst Advocate for this burst is S. R. Oates (sro AT mssl.ucl.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.)