TITLE: GCN CIRCULAR NUMBER: 28761 SUBJECT: GRB 201024A: Swift detection of a burst with an optical afterglow DATE: 20/10/24 03:10:12 GMT FROM: David Palmer at LANL F. E. Marshall (NASA/GSFC), M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB), J.D. Gropp (PSU), N. J. Klingler (PSU), N. P. M. Kuin (UCL-MSSL), S. Laha (GSFC/UMBC/CRESST), A. Melandri (INAF-OAB), D. M. Palmer (LANL), T. Sbarrato (INAF-OAB) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 02:48:59 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201024A (trigger=1001514). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 125.948, +3.377 which is RA(J2000) = 08h 23m 47s Dec(J2000) = +03d 22' 38" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single peak structure with a duration of about 5 sec. The peak count rate was ~1200 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 02:50:14.0 UT, 74.9 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source with an enhanced position: RA, Dec 125.9521, 3.3537 which is equivalent to: RA(J2000) = 08h 23m 48.50s Dec(J2000) = +03d 21' 13.4" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 85 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 3.64 x 10^20 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 6.41e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 495 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 08:23:48.55 = 125.95229 DEC(J2000) = +03:21:14.5 = 3.35403 with a 90%-confidence error radius of about 0.62 arc sec. This position is 0.7 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.85 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.03. Burst Advocate for this burst is F. E. Marshall (marshall 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/)