TITLE: GCN CIRCULAR NUMBER: 23861 SUBJECT: GRB 190204A: COATLI Optical Detection of the Fading Afterglow DATE: 19/02/04 12:54:35 GMT FROM: Alan M. Watson at Instituto de Astronomia UNAM Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L. Becerra (UNAM), Diego González (UNAM), William H. Lee (UNAM), Margarita Pereyra (UNAM), Carlos Román-Zúñiga (UNAM), Alexander Kutyrev (GSFC), and Eleonora Troja (GSFC) report: We observed the field of GRB 190204A (Lien et al., GCN Circ. 23852) 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 2019-02-04 05:46:35 to 2019-02-04 05:57:30 (from 34 to 689 seconds after the trigger), obtaining a total of 345 seconds of exposure in the w filter. At the position of the UVOT afterglow candidate (Lien et al., GCN Circ. 23852), we detect a source that fades from w = 13.5 to w = 17.5 over the course of our observations. A power-law fit suggest a temporal index of -1.44 +/- 0.03. Our w magnitudes are calibrated against the USNO-B1 catalog (adjusted to an approximate AB system) and are not corrected for Galactic extinction in the direction of the GRB. We thank the COATLI technical team and the staff of the Observatorio Astronómico Nacional.