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

TL;DR

This paper explores how magnetized inhomogeneous environments near repeating FRB sources influence their polarization, scattering, and persistent radio emissions, proposing theoretical models that align with recent observations and suggest a common origin for scattering and RM variations.

Contribution

It provides theoretical predictions on the relationships among temporal scattering, depolarization, and persistent radio sources in magnetized plasma environments near FRBs, aligning with recent observational correlations.

Findings

The RM scatter and temporal scattering are likely from the same magnetized plasma region.

Predicted RM scatter-timing relation varies between 0.54 and 0.83 depending on scenarios.

A positive correlation exists between PRS luminosity and plasma screen RM, consistent with observations.

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 ($\sigma_{\rm RM}$) and the temporal scattering time ($\tau_{\rm s}$), $\sigma_{\rm RM}\propto\tau_{\rm s}^{1.0\pm0.2}$, both of which can be well described by multi-path 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 $\sigma_{\rm RM}$. In this work, we focus on some theoretical predictions on the relations among temporal scattering, depolarization by RM scatter, and PRS contributed by the magnetized plasma environment close to a repeating 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 $\sigma_{\rm RM}$-$\tau_{\rm s}$ relation is $\sigma_{\rm RM}\propto\tau_{\rm s}^{(0.54-0.83)}$ for different astrophysical scenarios. We further make a general discussion on PRS that does not depend on specific astrophysical scenarios. We 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.