TITLE: GCN CIRCULAR NUMBER: 13886 SUBJECT: GRB 121024A: Swift detection of a burst with an optical counterpart DATE: 12/10/24 03:12:57 GMT FROM: Scott Barthelmy at NASA/GSFC C. Pagani (U Leicester), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), C. Gronwall (PSU), J. A. Kennea (PSU), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sakamoto (AGU), B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU) and T. N. Ukwatta (MSU) report on behalf of the Swift Team: At 02:56:12 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 121024A (trigger=536580). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 70.467, -12.269 which is RA(J2000) = 04h 41m 52s Dec(J2000) = -12d 16' 06" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a few overlapping peaks with a duration of about 15 sec. The peak count rate was ~3500 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 02:57:45.5 UT, 93.0 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 70.4715, -12.2904 which is equivalent to: RA(J2000) = +04h 41m 53.16s Dec(J2000) = -12d 17' 25.4" with an uncertainty of 5.5 arcseconds (radius, 90% containment). This location is 78 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. The initial flux in the 2.5 s image was 1.10e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 158 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 04:41:53.28 = 70.47201 DEC(J2000) = -12:17:26.8 = -12.29079 with a 90%-confidence error radius of about 0.78 arc sec. This position is 2.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.38 with a 1-sigma error of about 0.16. No correction has been made for the expected extinction corresponding to E(B-V) of 0.10. Burst Advocate for this burst is C. Pagani (cp232 AT star.le.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.)