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 1.46 10⁵ rad m⁻² to 1.33 10⁵ rad m⁻² between 2017 January and August, showing day-timescale variations of ∼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 ∼ 9.7 10⁴ rad m⁻² at the most recent epoch of 2019 August. Erratic, short-term RM variations of ∼10³ 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 ∼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.

重复的快速无线电脉冲串源FRB 121102已显示具有异常高且可变的法拉第旋转量度（RM），必须在其宿主星系内或可能由其本地环境赋予该法拉第旋转量度。在红移（z = 0.193）的源参考系中，RM在2017年1月至8月之间从1.46 10 ⁵ rad m ^-2降低到1.33 10 ⁵ rad m ^-2，显示日时标变化约为200 rad m ^-2。在这里，我们展示了用Arecibo 305 m射电望远镜，Effelsberg 100 m和Karl G. Jansky超大型阵列爆破检测得到的16个FRB 121102 RM，提供了2.5年时间内FRB 121102 RM的记录。我们的观察结果显示，RM呈下降趋势，尽管该趋势不是线性的，但年- ¹年平均下降了15％，在2019年8月的最近时期约为9.7 10 ⁴ rad m ^-2。不稳定的短期RM变异〜10 ³ rad m ^-2周^-1在MJD 58215–58247之间也观察到。已经提出了将数十年历史的中子星嵌入仍然紧凑的超新星残骸中，或者将其埋藏在巨大黑洞附近的中子星及其吸积环中，以解释高均方根值。我们将观测到的RM与描述超新星残余内FRB的RM演化的理论模型进行比较。FRB 121102的年龄是未知的，我们发现这些模型在2017年首次进行可用RM测量时的源年龄大约为6-17年。我们还对银河系中心磁星PSR J1745的减小RM进行了比较。 −2900。