TITLE: GCN CIRCULAR NUMBER: 20137 SUBJECT: GRB 161104A: Gemini South optical observations DATE: 16/11/05 22:12:41 GMT FROM: Eleonora Troja at GSFC E. Troja (UMD/GSFC), A. Watson (UNAM), S. Covino (INAF), W. H. Lee (UNAM), N. Butler (ASU), J. Becerra-Gonzalez (UMD/GSFC), A. Lien (UMBC), T. Sakamoto (AGU) and N. Gehrels (NASA/GSFC) report: We observed the field of the short GRB 161104A (Mingo et al., GCN 20123) with the Gemini Multi-Object Spectrograph (GMOS) on the 8m Gemini South telescope. We acquired 6x120s exposures in the r-band beginning at 02:53 UT on 05 November 2016 (~17 hours after the Swift trigger) and under good observing conditions. The three sources reported by Nicuesa Guelbenzu et al. (GCN 20132) are detected with similar magnitudes, showing no significant variability between our observations and the GROND exposures taken 1 hour later. Within the enhanced XRT position (Goad et al., GCN 20124) we detect an additional faint, point-like source not reported by Nicuesa Guelbenzu et al. (GCN 20132). The position of this source is RA, Dec = 05:11:34.5, -51:27:33.90 (J2000), accurate to about 0.5" in each coordinate. We measure a preliminary magnitude of r~24.6 (AB). The lack of detection in the subsequent GROND observations suggests that the source significantly faded, however such rapid decay rate is unusual for a GRB optical afterglow. We thank the Gemini staff and observers for rapidly scheduling and executing these observations.