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

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 ${\rm DM}\propto t^{-α}$ with $α\simeq1.8$--$1.9$. 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 $τ_{\rm ff}=1$ 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.

Figures (13)

• Figure 1: Radius evolution for the $11\,M_\odot$ (top) and $30\,M_\odot$ (bottom) models. Dashed: single-star; solid: binary-stripped. Blue, green, and red curves indicate the $R_r$, $R_c$, and $R_b$, respectively.
• Figure 2: Comparison of the time evolution for $11\,M_\odot$ and $30\,M_\odot$ progenitor models in the SS and BS channels. (a) DM. (b) Volume-averaged electron density $\langle n_e \rangle$. (c) Effective thickness of the shocked ionized region, $\Delta R_{\rm sh}$. (d) Peak (maximum) SNR expansion velocity. (e) Cumulative swept-up (shocked) shell mass. Solid curves correspond to the fiducial unshocked-ionization case $\chi_{e,\mathrm{unsh}}=1$ (HH in Table \ref{['tab:toccur_tesc']}).
• Figure 3: Time evolution of optical depths and mass-weighted averages in the shocked region for the $11\,M_\odot$ (left) and $30\,M_\odot$ (right) progenitor models. Each panel shows $\tau_{\rm es}$ and $\tau_{\rm ff}$, together with $\langle T_e\rangle$, $\langle Z\rangle$, and $\langle g_{\rm ff}\rangle$, comparing the SS and BS channels. Throughout this work, the Gaunt factor is evaluated at $\nu=10^{9}\,{\rm Hz}$, representative of the GHz band relevant for FRBs.
• Figure 4: Time evolution of the characteristic quantities for the full-region DM calculation, analogous to Fig. \ref{['fig:dm_mass_model_comparison']}. The results correspond to the fiducial HH case with $\chi_{e,\mathrm{unej}}=\chi_{e,\mathrm{ISM}}=0.1$. Here we adopt a dynamical path length $\Delta R_{\rm dyn}=\Delta R_{\rm ej}+\Delta R_{\rm sh,CSM}$. In the third panel, $R_{\rm CSM,max}$ is a constant maximum integration radius for the ISM/CSM contribution.
• Figure 5: Decomposition of the total SNR DM into individual components in the full-ionized-region calculation. The time-scaling shown for the unshocked-ejecta component is obtained from our fit to the unshocked-ejecta evolution.