TITLE: GCN CIRCULAR NUMBER: 18472 SUBJECT: GRB 151021A: Skynet PROMPT-CTIO/DSO17 observations of the optical afterglow DATE: 15/10/26 12:27:55 GMT FROM: Adam S. Trotter at UNC-Chapel Hill/PROMPT/Skynet A. Trotter, D. Reichart, J. Haislip, A. Smith, D. Caton, L. Hawkins, J. Moore, N. Frank, M. Maples, E. Johnson, R. Joyner, J. Martin, C. Salemi, J. A. Crain, K. Ivarsen, A. LaCluyze, and M. Nysewander report: Skynet observed the Swift BAT/XRT localization of GRB 151021A (Melandri et al., GCN 18425, Swift trigger=660671) with with one 16" telescope (PROMPT 5; I band) of the PROMPT array at CTIO, Chile, and with the 17" telescope at the Appalachian State University Dark Sky Observatory (DSO17; B band) in NC, USA. Starting at 2015-10-21 01:31:14 UT and continuing until 03:00 UT (t=2m-91m post-trigger), Skynet took a total of 38 exposures ranging from 10-160s. In the PROMPT I-band images, we clearly detect a fading optical afterglow at the position reported by de Ugarte Postigo et al. (GCN 18426), with I~14 at t=2m at I~19 at t=75m. A DSO17 B-band image taken at t=2.2m shows no source to a 3-sigma limit of B>15.7. A preliminary light curve is at: http://www.skynet.unc.edu/grb/grb151021a.png Limiting magnitudes are in the Vega System, calibrated to 4 APASS DR9 stars in the field. Magnitudes have not been corrected for the small line-of-sight Milky Way dust extinction, with expected E(B-V)=0.009 (Schlafly & Finkbeiner 2011). No further Skynet observations are scheduled.