TITLE: GCN CIRCULAR NUMBER: 17861 SUBJECT: GRB 150518A: Continued RATIR optical observations DATE: 15/05/22 20:44:31 GMT FROM: Owen Littlejohns at Az State U Owen Littlejohns (ASU), Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori Fox (UCB), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (ORAU/GSFC), Eleonora Troja (GSFC), Enrico Ramirez-Ruiz (UCSC), José A. de Diego (UNAM), Leonid Georgiev (UNAM), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC), and Harvey Moseley (GSFC) report: We continued to observe the field of GRB 150518A (Kawamuro, et al., GCN 17825) 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 for two further epochs. Our first full epoch now extends from 2015/05 20.15 to 2015/05 20.47 UTC (29.94 to 37.69 hours after the BAT trigger), obtaining a total of 6.04 hours exposure in the r, i and z bands. For the previously reported SDSS galaxy (Xu, et al., GCN 17829; Mao, et al., GCN 17830; Littlejohns, et al., GCN 17831), in comparison with the SDSS DR9, we obtain the following detections: r 21.27 +/- 0.04 i 21.02 +/- 0.04 z 20.53 +/- 0.24 The second epoch of RATIR observations ranged from 2015/05 21.21 to 2015/05 21.45 UTC (55.33 to 61.20 hours after the BAT trigger), obtaining a total of 3.59 hours exposure in the r and i bands, and 1.07 hours in the z band. For the same SDSS galaxy, again in comparison with the SDSS DR9, we obtain the following detections and upper limit (3-sigma): r 21.40 +/- 0.08 i 21.08 +/- 0.08 z > 17.70 The third epoch of RATIR observations was from 2015/05 22.15 to 2015/05 22.47 UTC (77.99 to 85.61 hours after the BAT trigger), obtaining a total of 6.04 hours exposure in the r, i and z bands. In this epoch, we obtain the following detections of the SDSS galaxy: r 21.34 +/- 0.04 i 21.08 +/- 0.04 z 20.80 +/- 0.35 These magnitudes are in the AB system and are not corrected for Galactic extinction in the direction of the GRB. Combining the r and i band observations from the second and third epochs, we find marginal evidence that the source has faded, changing by 0.07 +/-0.04 magnitudes when compared to the first epoch of RATIR observations (Littlejohns, et al., GCN 17831). During the full first epoch of observations, we also observed marginal (1-sigma) evidence for fading by 0.05 magnitudes in the r and i bands. Finally, we note that image subtraction between the third and first epochs, in both r and i band, yield positive residuals in the Swift-XRT error circle at the 1-sigma level. We thank the staff of the Observatorio Astronómico Nacional in San Pedro Mártir.