TITLE: GCN CIRCULAR NUMBER: 22799 SUBJECT: GRB 180620A: RATIR Optical Afterglow Detection DATE: 18/06/20 09:16:28 GMT FROM: Nat Butler at Az State U Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (STScI), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UVI), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (U. Wash.), and Vicki Toy (UMD) report: We observed the field of GRB 180620A (Evans, et al., GCN 22798) with the Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the 1.5m Harold Johnson Telescope at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir from 2018/06 20.36 to 2018/06 20.38 UTC (0.03 to 0.47 hours after the BAT trigger), obtaining a total of 0.36 hours exposure in the r and i bands. For a source within the Swift-XRT error circle, in comparison with the USNO-B1 and 2MASS catalogs, we obtain the following detections: r = 18.01 +/- 0.01 i = 17.71 +/- 0.01 These magnitudes are in the AB system and are not corrected for Galactic extinction in the direction of the GRB. We observe the rise and initial decline of the afterglow. We thank the staff of the Observatorio Astronómico Nacional in San Pedro Mártir. Further observations are planned.