TITLE: GCN CIRCULAR NUMBER: 36556 SUBJECT: GRB 240529A: Swift detection of a burst with an optical counterpart DATE: 24/05/29 03:38:03 GMT FROM: David Palmer at LANL <palmer@lanl.gov> R. A. J. Eyles-Ferris (U Leicester), R. Gupta (NASA/GSFC), A. Y. Lien (U Tampa), K. L. Page (U Leicester), D. M. Palmer (LANL) and T. M. Parsotan (GSFC) report on behalf of the Neil Gehrels Swift Observatory Team: At 02:58:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 240529A (trigger=1231488). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 335.322, +51.555 which is RA(J2000) = 22h 21m 17s Dec(J2000) = +51d 33' 19" 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 at least 20 sec since we are missing data after ~8 sec. The peak count rate was ~4000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 03:06:22.0 UT, 471.2 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 335.35696, 51.56207 which is equivalent to: RA(J2000) = 22h 21m 25.67s Dec(J2000) = +51d 33' 43.5" with an uncertainty of 3.5 arcseconds (radius, 90% containment). This location is 82 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 https://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 4.06 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 128 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 22:21:26.04 = 335.35851 DEC(J2000) = +51:33:43.4 = 51.56205 with a 90%-confidence error radius of about 1.10 arc sec. This position is 2.7 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.84. No correction has been made for the large, but uncertain, extinction expected. Burst Advocate for this burst is R. A. J. Eyles-Ferris (raje1 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/36556. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...