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 within its local environment. In the redshifted (z = 0.193) source reference frame, the RM decreased from 1.46 x 10(5) rad m(-2) to 1.33 x 10(5) rad m(-2) between 2017 January and August, showing day-timescale variations of similar to 200 rad m(-2). Here we present 16 FRB 121102 RMs from burst detections with the Arecibo 305 m radio telescope, the Effelsberg 100 m, and the Karl G. Jansky Very Large Array, providing a record of FRB 121102's RM over a 2.5 yr time span. Our observations show a decreasing trend in RM, although the trend is not linear, dropping by an average of 15% year(-1) and is similar to 9.7 x 10(4) rad m(-2) at the most recent epoch of 2019 August. Erratic, short-term RM variations of similar to 10(3) rad m(-2) week(-1) were also observed between MJDs 58215-58247. A decades-old neutron star embedded within a still-compact supernova remnant or a neutron star near a massive black hole and its accretion torus have been proposed to explain the high RMs. We compare the observed RMs to theoretical models describing the RM evolution for FRBs originating within a supernova remnant. FRB 121102's age is unknown, and we find that the models agree for source ages of similar to 6-17 yr at the time of the first available RM measurements in 2017. We also draw comparisons to the decreasing RM of the Galactic center magnetar, PSR J1745-2900.

Automated summary

Monitoring of the RM variations of FRB 121102 shows a decreasing trend, although not linear. Additionally, significant short-term RM fluctuations were observed, potentially explained by the presence of a neutron star in a supernova remnant or near a massive black hole.