TITLE: GCN CIRCULAR NUMBER: 22846 SUBJECT: GRB 180624A: RATIR Optical Observations DATE: 18/06/25 22:16:14 GMT FROM: Alan M. Watson at Instituto de Astronomia UNAM Alan M. Watson (UNAM), Eleonora Troja (GSFC), Nat Butler (ASU), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (STScI), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UVI), 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 180624A (Gibson et al., GCN Circ. 22832) 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 25.26 to 2018/06 25.47 UTC (16.42 to 21.45 hours after the BAT trigger), obtaining a total of 3.91 hours exposure in the r and i bands. We detect a source in the enhanced Swift-XRT error circle (Goad et al., GCN Circ. 22835) with the following magnitudes: r = 21.03 +/- 0.02 i = 20.45 +/- 0.01 These magnitudes are calibrated against the SDSS DR9 catalog, are in the AB system, and are not corrected for Galactic extinction in the direction of the GRB. We note that the source has faded significantly from the earlier observations of Guidorzi et al. (GCN Circ. 22839) of r = 19.9 +/- 0.1 at 0.54 hours and Schady (GCN Circ. 22840) of r = 20.2 +/- 0.1 at 15.07 hours. We also see the source fade by about 0.5 mag in each our bands during the course of our observations. We confirm the observation of Rossi et al. (GCN Circ. 22845) that the source is a couple of arcsec to the SE of the SDSS galaxy J211223.25-022015.9. We thank the staff of the Observatorio Astronómico Nacional in San Pedro Mártir.