TITLE: GCN CIRCULAR NUMBER: 15148 SUBJECT: GRB 130831A: Skynet/PROMPT-SSO detections DATE: 13/08/31 21:08:52 GMT FROM: Adam S. Trotter at UNC-Chapel Hill/PROMPT/Skynet A. Trotter, J. Haislip, A. LaCluyze, J. Moore, N. Frank, D. Reichart, T. Berger, H. T. Cromartie, R. Egger, A. Foster, K. Ivarsen, M. Maples, M. Nysewander, and J. A. Crain report: Skynet observed the Swift/BAT localization of GRB 130831A (Hagen et al., GCN 15139, Swift trigger #568849) with four 17" telescopes of the newly-commissioned PROMPT array at Siding Spring Observatory, NSW, Australia. Starting at 2013-08-31, 13:39 UT and continuing until 17:41 UT (t=35.5m-4.65h post-trigger), it took 615 80-s exposures in the BVRI bands. We detect a bright fading afterglow in all four bands at RA 23:54:29.8, Dec 29:25:46.4 (J2000.0), within the error circle of the Enhanced Swift-XRT localization (Evans et al., GCN 15147). A preliminary light curve is at: http://skynet.unc.edu/grb/grb130831a.png Magnitudes are in the Vega system, calibrated to 6 APASS stars in the field, and have not been corrected for the modest expected line-of-sight Milky Way dust extinction E(B-V)=0.046 (Schlegel et al. 1998). Skynet observations are ongoing.