TITLE: GCN CIRCULAR NUMBER: 22859 SUBJECT: GRB 180626A: COATLI Optical Detection DATE: 18/06/26 17:10:10 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 180626A (Evans et al., GCN 22850) 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 08:21:50.6 to 11:18:41.7 UTC (from 33.7 seconds after the trigger or 17.9 seconds after the alert to 2.95 hours after the trigger), obtaining a total of 7020 seconds of exposure in the w filter. Within the enhanced XRT error region (Osborne et al., GCN Circ. 22852), we detect the afterglow previously reported by Evans et al. (GCN Circ. 22850), Martone et al. (GCN Circ. 22854), and Breeveld et al. (GCN Circ. 22857) with w = 21.4 +/- 0.9 In our first set of 30 images each of 5 seconds of exposure, from 08:21:50.6 to 08:27:10.1 UTC, we do not detect the afterglow with a 10-sigma limiting magnitude of w > 19.2 These magnitudes are calibrated against the Pan-STARRS1 catalog and 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 (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.