GCN Circular 19668
WeiKang Zheng, Alexei V. Filippenko (UC Berkeley), and Melissa Graham (University of Washington) report on behalf of the KAIT GRB team: We observed the field of GRB 160703A (Cenko et al., GCN 19645) with the 10 m Keck-I telescope on July 3 starting at 13:17:55 UT, or 1.125 hours after the burst. A 180 s g'-band image and a 150 s R-band image were taken simultaneously with LRIS. We detect the optical afterglow (Tachibana et al., GCN 19652; Kuroda et al., GCN 19653; Hagen & Cenko, GCN 19656) in both the g' and R images. The afterglow is well separated in our images from the nearby source N2JN062852 noted by Tachibana et al. (GCN 19652). We measure the afterglow to be 1.58" west and 1.30" north of N2JN062852. A finding chart is posted at website URL http://w.astro.berkeley.edu/~zwk/grb/GRB160703A/GRB160703A_keck_lris_g_fc.png . We find that the afterglow had a brightness of R=17.9 mag at 1.125 hours after the burst, calibrated with the USNO B1.0 R2 system.