TITLE: GCN CIRCULAR NUMBER: 15365 SUBJECT: GRB 131024A: Swift detection of a burst DATE: 13/10/24 12:37:23 GMT FROM: David Palmer at LANL J. L. Racusin (NASA/GSFC), D. N. Burrows (PSU), N. Gehrels (NASA/GSFC), H. A. Krimm (CRESST/GSFC/USRA), A. Maselli (INAF-IASFPA), K. L. Page (U Leicester), D. M. Palmer (LANL), P. Romano (INAF-IASFPA), M. H. Siegel (PSU), E. Troja (NASA/GSFC/UMCP) and T. N. Ukwatta (MSU) report on behalf of the Swift Team: At 12:26:20 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 131024A (trigger=575707). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 290.464, -64.603 which is RA(J2000) = 19h 21m 51s Dec(J2000) = -64d 36' 08" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual with an image trigger, the available BAT light curve shows no significant structure. The XRT began observing the field at 12:28:34.2 UT, 133.5 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 290.48228, -64.60181 which is equivalent to: RA(J2000) = 19h 21m 55.75s Dec(J2000) = -64d 36' 06.5" with an uncertainty of 4.0 arcseconds (radius, 90% containment). This location is 28 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 consistent with the Galactic value of 4.45 x 10^20 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 150 seconds with the White filter starting 136 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 19.6 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 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.05. Burst Advocate for this burst is J. L. Racusin (judith.racusin AT 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.)