TITLE: GCN CIRCULAR NUMBER: 28880 SUBJECT: ZTF20acozryr/AT2020yxz (GRB 201103B): Flattening of the optical light curve DATE: 20/11/11 11:54:49 GMT FROM: Gregory SungHak Paek at SNU Gregory S.H. Paek (SNU), Myungshin Im (SNU), Joonho Kim (SNU), Gu Lim (SNU), Mankeun Jeong (SNU), Chung-Uk Lee (KASI), Seung-Lee Kim (KASI), Hyun-Il Sung (KASI), Taewoo Kim (DOAO), and Wonseok Kang (DOAO) and GECKO team We observed the ZTF20acozryr/AT2020yxz (Coughlin et al., GCN 28841) in BVRI with various optical telescopes of the GW EM-Counterpart Korean Observatory (GECKO). The facilities used for the observations are the 1.6-m class telescopes of Korea Microlensing Telescope Network (KMTNet) located in Siding Spring Observatory (SSO) and South African Astronomical Observatory (SAAO), the 1-m telescope in Deokheung Optical Astronomy Observatory (DOAO), the 1-m telescope the Mt. Lemmon Optical Astronomy Observatory (LOAO), and the 0.36-m KIAS Chamnun Telescope (KCT) and RASA36 telescope in Chile. We calibrated flux with PS1 and APASS catalog, and the magnitudes are in AB system. The magnitudes are not corrected for galactic extinction. Preliminary R-band magnitudes are given for the most recent a few epoch observations. Obs.&Tel. DATE-OBS(UT) Filter Mag Error t-t0(days since burst) ----------- ----------------------- ------ ------- ------- ---------------------- DOAO 2020-11-08T19:51:31 R 21.6 0.1 5.07 KCT 2020-11-10T04:27:38 R 21.8 0.3 6.43 KMTNet-SAAO 2020-11-10T21:37:55 R 21.8 0.1 7.1 t0 = 2020-11-03T18:06:45.36 (Ursi et al., GCN 28831) About a week after GRB occurs, the brightness seems to remain nearly constant, suggesting a possible onset of a supernova component accompanied with GRB. We thank all the operators at GECKO facilities for performing the observation. Gravitational-wave EM Counterpart Korean Observatory (GECKO) is a network of 10+ 0.5m to 1m class telescopes over the world.