TITLE: GCN CIRCULAR NUMBER: 18951 SUBJECT: GRB 160131A: Swift detection of a burst with an optical counterpart DATE: 16/01/31 08:32:26 GMT FROM: Scott Barthelmy at NASA/GSFC K. L. Page (U Leicester) and S. D. Barthelmy (GSFC) report on behalf of the Swift Team: At 08:20:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 160131A (trigger=672236). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 78.182, -7.067 which is RA(J2000) = 05h 12m 44s Dec(J2000) = -07d 04' 02" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a large FRED-like peak structure with a duration of about 100 sec. The peak count rate was ~8100 counts/sec (15-350 keV), at ~16 sec after the trigger. The XRT began observing the field at 08:21:41.6 UT, 69.7 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 78.1700, -7.0489 which is equivalent to: RA(J2000) = +05h 12m 40.80s Dec(J2000) = -07d 02' 56.0" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 77 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. The initial flux in the 2.5 s image was 1.29e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 78 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 05:12:40.34 = 78.16810 DEC(J2000) = -07:02:59.1 = -7.04975 with a 90%-confidence error radius of about 0.61 arc sec. This position is 7.5 arc sec. from the center of the XRT error circle. The estimated magnitude is 13.88 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.11. Burst Advocate for this burst is K. L. Page (klp5 AT leicester.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.)