TITLE: GCN CIRCULAR NUMBER: 25655 SUBJECT: ZTF19abvizsw: COATLI Optical Observations DATE: 19/09/04 14:39:06 GMT FROM: Rosa Leticia Becerra Godinez at Inst. de Astronoma,UNAM Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Nat Butler (ASU), Diego Gonz��lez (UNAM), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Margarita Pereyra (UNAM), and Eleonora Troja (GSFC) report: We observed the field of ZTF19abvizsw/AT2019pim (Kool et al., GCN Circ. 25616; Perley et al., GCN Circ. 25643) with the COATLI 50-cm telescope and interim imager at the Observatorio Astron��mico Nacional on the Sierra de San Pedro M��rtir (http://coatli.astroscu.unam.mx) from 2019-09-04 07:32 to 12:10, obtaining a total of 3.04 hours of exposure in the w filter. We detect the source at w = 21.60 +/- 0.10. Our w magnitudes are calibrated against the Pan-STARRS1 catalog, are on an approximate AB system, and are not corrected for Galactic extinction in the direction of the GRB. Applying the color transformation of Becerra et al. (2019, ApJ, 872, 118) to the g-r color of Perley et al., we predict r = 21.50 +/- 0.10. Thus, we do not see fading compared to the measurement of r = 21.62 +/- 0.09 reported by Perley et al. but do confirm their measurement of fading compared to the discovery magnitude of r = 19.45 +/- 0.11 reported by Kool et al. Further observations are planned. We thank the COATLI technical team and the staff of the Observatorio Astron��mico Nacional.