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/) View this GCN Circular online at https://gcn.nasa.gov/circulars/37110. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...