TITLE: GCN CIRCULAR NUMBER: 21109 SUBJECT: GRB 170519A: RATIR Optical and NIR Observations DATE: 17/05/19 13:08:22 GMT FROM: Nat Butler at UC berkeley 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), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (ASU), and Vicki Toy (UMD) report: We observed the field of GRB 170519A (Ukwatta, et al., GCN 21106) 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 2017/05 19.22 to 2017/05 19.32 UTC (1.8 minutes to 2.44 hours after the BAT trigger), obtaining a total of 1.41 hours exposure in the r and i bands and 0.60 hours exposure in the Z, Y, J, and H bands. The optical transient (GCN 21106; Izzo, et al., GCN 21108) is clearly detected. In comparison with the SDSS DR9 and 2MASS catalogs, we obtain: r 16.72 +/- 0.01 i 16.50 +/- 0.01 Z 16.34 +/- 0.01 Y 16.25 +/- 0.01 J 16.15 +/- 0.01 H 15.87 +/- 0.01 These magnitudes are in the AB system and are not corrected for Galactic extinction in the direction of the GRB. The lightcurve rises in all bands initially, peaking at i~16 at about 15 minutes after the GRB trigger, then fading as t^(-0.8). We thank the staff of the Observatorio Astronómico Nacional in San Pedro Mártir.