Probing the Dispersion and Rotation Measure Contributions from Supernova Remnants in Fast Radio Burst Source Environments with 1D SNR Simulation

TL;DR

This paper uses 1D supernova remnant simulations to analyze how young SNRs contribute to dispersion measure and rotation measure in FRB environments, revealing the importance of local ionized ejecta and shock effects.

Contribution

It introduces a forward-modeling approach with time-dependent 1D SNR simulations to quantify local environment contributions to FRB dispersion and rotation measures, incorporating detailed shock and ionization physics.

Findings

Shocked regions contribute limited DM (<10 pc cm^{-3})

Unshocked ejecta dominate the DM evolution, following t^{-1.8} to t^{-1.9}

Shock-amplified magnetic fields can produce significant RM in young SNRs

Abstract

Fast radio bursts (FRBs) provide a sensitive probe of ionized baryons through their dispersion measure (DM). In addition to slowly evolving cosmological terms, at least two repeaters now show clear secular DM-decrease episodes: FRB~20190520B and FRB~20121102 , supporting a dense, dynamically evolving local environment. We adopt a \emph{forward-modeling} approach and use time-dependent 1D SNR simulations for a young magnetar embedded in SN ejecta, combining single-star and binary-stripped progenitors with HD+NEI calculations to follow shock structure, ionization, and electron density. The shocked region contributes only limited DM ($\lesssim10\,{\rm pc\,cm^{-3}}$), while the dominant time-varying component is the unshocked ejecta, whose early behavior follows ${\rm DM}\propto t^{-\alpha}$ with $\alpha\simeq1.8$--$1.9$. Although shocked-region DM is small, shock-amplified magnetic fields can still generate substantial RM; in our shock-only RM framework, only the $11\,M_\odot$ SS model reproduces the FRB~20121102 RM evolution. Binary-stripped progenitors generally yield smaller DM than single-star models at fixed $M_{\rm ZAMS}$, with composition-dependent mean molecular weights introducing non-monotonic mass trends. Matching the observed ${\rm dDM}/{\rm d}t$ of FRB~20190520B (and the late-stage slope of FRB~20121102), we infer local SNR DM contributions of tens to hundreds ${\rm pc\,cm^{-3}}$. We also find GHz escape is allowed in most models, with $\tau_{\rm ff}=1$ typically reached by $t_{\rm esc}\lesssim70$ yr; for weakly ionized ejecta, the source can be nearly transparent from very early times. These results support a young CCSN/SNR origin for a substantial fraction of ${\rm DM}_{\rm source}$ and highlight that physically consistent local-environment modeling is essential for robust FRB cosmological DM inferences.