TITLE: GCN CIRCULAR NUMBER: 15139 SUBJECT: GRB 130831A: Swift detection of a burst with an optical counterpart DATE: 13/08/31 13:21:11 GMT FROM: David Palmer at LANL L. M. Z. Hagen (PSU), M. M. Chester (PSU), J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), D. Grupe (PSU), J. A. Kennea (PSU), V. Mangano (PSU), F. E. Marshall (NASA/GSFC), C. J. Mountford (U Leicester), P. T. O'Brien (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU), E. Troja (NASA/GSFC/UMCP) and B.-B. Zhang (UAH) report on behalf of the Swift Team: At 13:04:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 130831A (trigger=568849). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 358.649, +29.448 which is RA(J2000) = 23h 54m 36s Dec(J2000) = +29d 26' 53" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 50 sec. The peak count rate was ~10000 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 13:06:22.3 UT, 125.8 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 358.62398, 29.42849 which is equivalent to: RA(J2000) = 23h 54m 29.76s Dec(J2000) = +29d 25' 42.6" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 105 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 4.80 x 10^20 cm^-2 (Kalberla et al. 2005). The initial flux in the 2.5 s image was 8.87e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 190 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 23:54:29.89 = 358.62453 DEC(J2000) = +29:25:46.2 = 29.42950 with a 90%-confidence error radius of about 0.74 arc sec. This position is 5.8 arc sec. from the center of the XRT error circle. The estimated magnitude is 14.56 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.04. Burst Advocate for this burst is L. M. Z. Hagen (lea.zernow.hagen AT gmail.com). 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.)