On the magnetoionic environments of fast radio bursts

Abstract

Observations of the Faraday rotation measure, combined with the dispersion measure, can be used to infer the magnetoionic environment of a radio source. We investigate the magnetoionic environments of FRBs by deriving their estimated average magnetic field strengths along the line of sight $⟨B_{\parallel }⟩$ in their host galaxies and comparing them with those of Galactic pulsars and magnetars. We find that for those FRBs with RM measurements, the mean $⟨B_{\parallel }⟩$ is $1.77_{-1.48}^{+9.01}\mu \mathrm{G}$ and $1.74_{-1.55}^{+14.82}\mu \mathrm{G}$ using two different methods, which is slightly larger but not inconsistent with the distribution of Galactic pulsars, $1.00_{-0.60}^{+1.51}\mu \mathrm{G}$. Only six Galactic magnetars have estimated $⟨B_{\parallel }⟩$. Excluding PSR J1745--2900 that has an anomalously high value due to its proximity with the Galactic Centre, the other three sources have a mean value of $1.70\mu \mathrm{G}$, which is statistically consistent with the $⟨B_{\parallel }⟩$ distributions of both Galactic pulsars and FRBs. There is no apparent trend of evolution of magnetar $⟨B_{\parallel }⟩$ as a function of age or surface magnetic field strength. Galactic pulsars and magnetars close to the Galactic Centre have relatively larger $⟨B_{\parallel }⟩$ values than other pulsars/magnetars. We discuss the implications of these results for the magnetoionic environments of FRB 121102 within the context of magnetar model and the model invoking a supermassive black hole, and for the origin of FRBs in general.

8 pages, 3 figures, MNRAS accepted