TITLE: GCN CIRCULAR NUMBER: 19820 SUBJECT: GRB 160816A: RATIR Optical and NIR Afterglow Confirmation DATE: 16/08/18 12:35:04 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 (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC), José A. de Diego (UNAM), Leonid Georgiev (UNAM), Jesús González (UNAM), Carlos Román- Zúñiga (UNAM), Neil Gehrels (GSFC), Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (ASU), and Vicki Toy (UMD) report: We observed the field of GRB 160816A (Mailyan, et al., GCN 19804; Racusin, et al., GCN 19802) 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 2016/08 18.17 to 2016/08 18.47 UTC (1.44 to 1.72 days after the GBM trigger), obtaining a total of 2.47 hours exposure in the r and i bands and 1.03 hours exposure in the Z, Y, J, and H bands. For the source reported in Troja, et al. (GCN 19809), in comparison with the USNO-B1 and 2MASS catalogs, we obtain the following detections and upper limit (3-sigma): r 22.74 +/- 0.15 i 22.40 +/- 0.12 Z 22.19 +/- 0.19 Y 22.18 +/- 0.25 J > 22.32 H 21.20 +/- 0.18 These magnitudes are in the AB system and are not corrected for Galactic extinction in the direction of the GRB. The source intensity decreased significantly between our two epochs, corresponding to fading with power-law index alpha~-1.3 referenced to the burst trigger. This confirms that the candidate reported by Guidorzi, et al. (GCN 19808) and Troja et al. (GCN 19809) is the GRB afterglow. Deep imaging of the GRB location reveals additional optical sources within the XRT localization (Kennea et al., GCN 19806), however none of them have faded. We thank the staff of the Observatorio Astronómico Nacional in San Pedro Mártir.