TITLE:   GCN CIRCULAR
NUMBER:  37110
SUBJECT: GRB 240809A: Swift detection of a burst with a bright optical counterpart
DATE:    24/08/09 08:44:45 GMT
FROM:    P.A. Evans at U. Leicester <pae9@leicester.ac.uk>


P. A. Evans (U Leicester), R. A. J. Eyles-Ferris (U Leicester),
K. L. Page (U Leicester) and A. Tohuvavohu (U Toronto) report on
behalf of the Neil Gehrels Swift Observatory Team:

At 08:30:29 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240809A (trigger=1247745).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 237.551, -2.321 which is 
   RA(J2000) = 15h 50m 12s
   Dec(J2000) = -02d 19' 15"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 30 sec.  The peak count rate
was ~22500 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 08:31:57.9 UT, 88.3 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 237.5464, -2.3178 which is equivalent to:
   RA(J2000)  = 15h 50m 11.14s
   Dec(J2000) = -02d 19' 04.1"
with an uncertainty of 12 arcseconds (radius, 90% containment). This
location is 20 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 98 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	15:50:10.51 = 237.54381
  DEC(J2000) = -02:19:03.2  =  -2.31756
with a 90%-confidence error radius of about 0.61 arc sec. This position is 9.4
arc sec. from the center of the XRT error circle. The estimated magnitude is
14.47 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.249. 

Burst Advocate for this burst is P. A. Evans (pae9 AT leicester.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/)