GCN Circular 18917
S. R. Oates, J. C. Tello (IAA-CSIC Granada), P. Tristram (Mt. John Univ. Observatory), I. Bond (Massey Univ.), S. Takahiro (Osaka Univ.), P. Yock (Univ. of Auckland) and A. J. Castro-Tirado (IAA-CSIC & ISA-UMA) report on behalf of a larger collaboration: We observed the field of GRB 160121A (Hagen et al., GCN 18912) with the 1.8m MOA-II telescope at Mt John University Observatory in New Zealand. A series of 60s exposures were taken with the R-band filter beginning 4.38 min (13:55:0 UT on 21-01-2016) after the Swift trigger 671231. We detect a single source at the edge of the Swift-XRT error circle (Hagen et al., GCN 18912), which we propose is the afterglow, at coordinates: RA (J2000.0) = 07:16:21.16 Dec (J2000.0) = -23:35:31.6 with an error of around 0.5". In the first exposure, ~292s after the trigger, we measure R ~19.7+\-0.2. The light curve appears to increase slightly in brightness, plateauing around 500s with a magnitude ~19. The light curve begins to decay around 1200s after the trigger. Magnitudes and upper limits are calibrated against USNO-B1 catalogue and are not corrected for the significant Galactic extinction corresponding to a reddening of E_B-V = 0.53 mag (Schlafly & Finkbeiner 2011).