TITLE: GCN CIRCULAR NUMBER: 30057 SUBJECT: GRB 210517A: KCT optical upper limit DATE: 21/05/20 06:11:09 GMT FROM: Gregory SungHak Paek at SNU Gregory S.H. Paek (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), Gu Lim (SNU ARC/SNU), on behalf of GECKO collaboration We searched for the optical counterpart of GRB 210517A (V. E'Elia et al. GCN #30032) with the 0.36-m class KCT (KIAS Chamnun Telescope) in DeepSkyChile, one of the facilities of the GW EM-Counterpart Korean Observatory (GECKO). We observed the center of Swift/XRT localization (RA, Dec = 358.2245 -39.1020) +2.94 hours after report, but could not find any possible transient within the field of view and at the reported location of optical afterglow (N. R. Tanvir et al. GCN 30037). We calibrated flux with the APASS catalog and used an AB magnitude system. Depth means 5 sigma upper limit for a point source detection. The magnitudes are not corrected for galactic extinction. ------ Filter Date-obs[UT] Exp.time[s] Depth I 2021-05-17T08:24:35.583 180*3 18.435 R 2021-05-17T08:37:00.750 180*3 19.581 V 2021-05-17T08:49:25.333 180*3 20.005 B 2021-05-17T09:01:50.250 180*3 20.047 Gravitational-wave EM Counterpart Korean Observatory (GECKO) is a network of 10+ 0.5m to 1m class telescopes over the world.