TITLE: GCN CIRCULAR NUMBER: 11370 SUBJECT: GRB 101024A: Swift detection of a burst with an optical afterglow DATE: 10/10/24 12:03:25 GMT FROM: Phil Evans at U of Leicester M. De Pasquale (UCL-MSSL), A. P. Beardmore (U Leicester), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), S. T. Holland (CRESST/USRA/GSFC), P. T. O'Brien (U Leicester), C. Pagani (U Leicester), C. J. Saxton (UCL-MSSL), B. Sbarufatti (INAF-OAB/IASFPA), M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC) and R. L. C. Starling (U Leicester) report on behalf of the Swift Team: At 11:39:29.82 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 101024A (trigger=437016). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 66.475, -77.248 which is RA(J2000) = 04h 25m 54s Dec(J2000) = -77d 14' 50" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The lightcurve shows 4 distinct peaks, with the brightest peaking at T0+10s. Duration was about 23s. The XRT began observing the field at 11:40:46.8 UT, 77.0 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 66.5064, -77.2648 which is equivalent to: RA(J2000) = 04h 26m 1.54s Dec(J2000) = -77d 15' 53.2" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 66 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 http://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (6.52 x 10^20 cm^-2, Kalberla et al. 2005), with an excess column of 2.2 (+1.39/-1.27) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 81 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 04:26:01.73 = 66.50719 DEC(J2000) = -77:15:55.9 = -77.26552 with a 90%-confidence error radius of about 0.64 arc sec. This position is 2.1 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.38 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.09. Burst Advocate for this burst is M. De Pasquale (mdp@mssl.ucl.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.)