Temporal Scattering, Depolarization, and Persistent Radio Emission from Magnetized Inhomogeneous Environments Near Repeating Fast Radio Burst Sources

Abstract

Some repeating fast radio burst (FRB) sources exhibit complex polarization behaviors, including frequency-dependent depolarization, variation of rotation measure (RM), and oscillating spectral structures of polarized components. Very recently, Feng et al. (2022) reported that active repeaters exhibit conspicuous frequency-dependent depolarization and a strong correlation between RM scatter (σ RM ) and the temporal scattering time (τ s ), σ RM ∝ τ 1.0±0.2 s , both of which can be well described by multipath propagation through a magnetized inhomogeneous plasma screen. This observation strongly suggests that the temporal scattering and RM scatter originate from the same region. Besides, a particular finding of note in Feng et al. ( 2022) is that the FRBs with compact persistent radio sources (PRS) tend to have extreme σ RM . In this work, we analyze the temporal scattering, RM scatter and the PRS emission contributed by the magnetized inhomogeneous plasma screen near an FRB source. The behaviors of the RM scatter imply that the magnetized plasma environment is consistent with a supernova remnant or a pulsar wind nebula, and the predicted σ RM -τ s relation is σ RM ∝ τ (0.54−0.83) s for different astrophysical scenarios. We also show that the specific luminosity of a PRS should have a positive correlation with the RM contributed by the plasma screen. This is consistent with the observations of FRB 121102 and FRB 190520B.

Magnetic Field Reversal around an Active Fast Radio Burst

“…The localization of FRB 20190520B constrains the PRS and FRB source to be within ∼1 kpc 2 , and the scattering time scale observed can be well interpreted by a more compact configuration at ≲100 pc 22 . This is consistent with the FRB propagating in a plasma screen, like an SNR 23 . A young SNR would imply that its DM and the maximum absolute value of the RM will decrease with evolution on longer time scales 24,25 .…”

“…Next, we consider that the RM originates from an expanding SNR along the line of sight of the FRB source. It is noteworthy that some active repeating FRBs (including FRB 20190520B) exhibit conspicuous frequency-dependent linear polarization fraction that can be well described by RM scatter 10 , and the relation between RM scatter and temporal scattering for various repeaters suggests that both of them are due to multi-path progagation through a magnetized inhomogeneous plasma screen 23 . Due to σ RM ≪ |RM|, the RM reversal must be mainly caused by the large-scale magnetic field, otherwise, the random small-scale magnetic field would cause σ RM ∼ RM, which is inconsistent with the observation of FRB 20190520B.…”