TITLE: GCN CIRCULAR NUMBER: 18538 SUBJECT: GRB 151031A: Swift detection of a burst DATE: 15/10/31 06:06:57 GMT FROM: David Palmer at LANL E. Sonbas (Adiyaman Univ.), V. D'Elia (ASDC), L. Izzo (URoma/ICRA), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA) and D. M. Palmer (LANL) report on behalf of the Swift Team: At 05:50:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 151031A (trigger=662330). Swift slewed to the burst after a 6 minute delay due to viewing constraints. The BAT on-board calculated location is RA, Dec 83.199, -39.092 which is RA(J2000) = 05h 32m 48s Dec(J2000) = -39d 05' 28" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 10 sec. The peak count rate was ~2500 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 05:57:43.9 UT, 433.6 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 83.19532, -39.12049 which is equivalent to: RA(J2000) = 05h 32m 46.88s Dec(J2000) = -39d 07' 13.8" with an uncertainty of 4.1 arcseconds (radius, 90% containment). This location is 103 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 2.62 x 10^20 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 439 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 none of the XRT error circle. 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 0.03. Burst Advocate for this burst is E. Sonbas (edasonbas AT yahoo.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.)