TITLE: GCN CIRCULAR NUMBER: 22806 SUBJECT: GRB 180620A: COATLI Detection and Light Curve DATE: 18/06/20 15:16:55 GMT FROM: Alan M. Watson at Instituto de Astronomia UNAM Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L. Becerra (UNAM), William H. Lee (UNAM), Carlos Román-Zúñiga (UNAM), Alexander Kutyrev (GSFC), and Eleonora Troja (GSFC) report: We observed the field of GRB 180620A (Evans et al., GCN 22798) with the COATLI 50-cm telescope and interim imager at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir (http://coatli.astroscu.unam.mx) from 2018-06-20 08:35:37 to 11:17:31 (from 39.2 seconds after the trigger or 21.1 seconds after the alert to 2.7 hours after the trigger), obtaining a total of 6250 seconds of exposure in the w filter. We detect the source reported by Butler et al. (GCN Circ. 22799) with w = 18.00 +/- 0.02 This magnitude is in the USNO system and is not corrected for Galactic extinction in the direction of the GRB. Our light curve shows a sharp initial fall from 16.5 mag to 17.3 mag, followed by a slower rise to 16.7 mag (also seen by Butler et al.), followed by a monotonic fall to 19.0 mag. We thank the COATLI technical team (Fernando Ángeles, Oscar Chapa, Salvador Cuevas, Alejandro Farah, Jorge Fuentes, Rosalía Langarica, Fernando Quirós, and Carlos Tejada) and the staff of the Observatorio Astronómico Nacional.