Rotation Measure Evolution of the Repeating Fast Radio Burst Source FRB 121102

The repeating fast radio burst source FRB 121102 has been shown to have an exceptionally high and variable Faraday rotation measure (RM), which must be imparted within its host galaxy likely by or local environment. In the redshifted ($z=0.193$) reference frame, RM decreased from $1.46\times10^5$~rad~m$^{-2}$ $1.33\times10^5$~rad~m$^{-2}$ between January August 2017, showing day-timescale variations of $\sim200$~rad~m$^{-2}$. Here we present sixteen RMs detections with Arecibo 305-m telescope, Effelsberg 100-m, Karl G. Jansky Very Large Array, providing a record 121102's over 2.5-year timespan. Our observations show decreasing trend in RM, although is not linear, dropping average 15\% year$^{-1}$ $\sim9.7\times10^4$~rad~m$^{-2}$ at most recent epoch 2019. Erratic, short-term $\sim10^3$~rad~m$^{-2}$ week$^{-1}$ were also observed MJDs 58215--58247. A decades-old neutron star embedded still-compact supernova remnant near massive black hole accretion torus proposed explain RMs. We compare theoretical models describing evolution for FRBs originating remnant. age unknown, find that agree ages $\sim6-17$~years time first available measurements 2017. draw comparisons Galactic center magnetar, PSR J1745--2900.