TITLE: GCN CIRCULAR NUMBER: 21051 SUBJECT: GRB 170428A: RATIR Afterglow Confirmation DATE: 17/04/29 18:40:36 GMT FROM: Eleonora Troja at GSFC Eleonora Troja (GSFC), Nat Butler (ASU), Alan M. Watson (UNAM), 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), Jes��s Gonz��lez (UNAM), Carlos Rom��n-Z����iga (UNAM), Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (ASU), and Vicki Toy (UMD) report: We observed the field of GRB 170428A (Beardmore, et al., GCN 21042) with the Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the 1.5m Harold Johnson Telescope at the Observatorio Astron��mico Nacional on Sierra San Pedro M��rtir from 2017/04 29.41 to 2017/04 29.48 UTC (24.69 to 26.25 hours after the BAT trigger), obtaining a total of 0.90 hours exposure in the r and i bands and 0.35 hours exposure in the Z, Y, J, and H bands. For a source at the candidate afterglow position (Bolmer, et al., GCN 21050), in comparison with the SDSS DR9 and 2MASS catalogs, we obtain the following detection and upper limits (3-sigma): r > 22.3 i 22.1 +/- 0.4 Z > 20.9 Y > 20.6 J > 20.1 H > 19.9 These magnitudes are in the AB system and are not corrected for Galactic extinction in the direction of the GRB. Our observations show that the source reported by Bolmer, et al., (GCN 21050) significantly faded, and is therefore the afterglow of the short GRB170428A. We thank the staff of the Observatorio Astron��mico Nacional in San Pedro M��rtir.