TITLE: GCN CIRCULAR NUMBER: 15371 SUBJECT: GRB 131024B: Swift detection of a burst DATE: 13/10/24 21:56:41 GMT FROM: Hans Krimm at NASA-GSFC H. A. Krimm (CRESST/GSFC/USRA), M. M. Chester (PSU), S. T. Holland (STScI), J. A. Kennea (PSU), V. Mangano (PSU), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), A. Maselli (INAF-IASFPA), J. L. Racusin (NASA/GSFC), B. Sbarufatti (INAF-OAB/PSU), C. A. Swenson (PSU), E. Troja (NASA/GSFC/UMCP) and B.-B. Zhang (UAH) report on behalf of the Swift Team: At 21:35:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 131024B (trigger=575754). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 144.403, +44.240 which is RA(J2000) = 09h 37m 37s Dec(J2000) = +44d 14' 23" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multiple-peak structure with a duration of about 30 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 21:37:21.8 UT, 110.5 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 144.5032, 44.2715 which is equivalent to: RA(J2000) = 09h 38m 00.76s Dec(J2000) = +44d 16' 17.6" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 282 arcseconds from the BAT onboard position. 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 (1.27 x 10^20 cm^-2, Kalberla et al. 2005), with an excess column of 2.6 (+1.51/-1.36) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 5.91e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 278 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. Burst Advocate for this burst is H. A. Krimm (krimm 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/too.html.) [GCN OPS NOTE(24oct13): The Admin changed the burst name from an "A" to a "B" in the first sentence.]