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

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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 three repeaters now show clear secular DM-decrease episodes, FRB 20190520B, FRB 20220529A, and FRB 20121102 (the last with a two-stage trend: early mild rise and late decline), supporting a dense, dynamically evolving local environment. We adopt a forward-modeling approach and use time-dependent 1D supernova remnant (SNR) simulations for a young magnetar embedded in SN ejecta, combining single-star (SS) and binary-stripped (BS) progenitors with hydrodynamic and nonequilibrium ionization calculations to follow shock structure, ionization, and electron density. The shocked region contributes only limited DM (≲10 pc cm −3 ), while the dominant time-varying component is the unshocked ejecta, whose early behavior follows DM ∝ t − α with α ≃ 1.8–1.9. Although shocked-region DM is small, shock-amplified magnetic fields can still generate substantial rotation measure (RM); in our shock-only RM framework, only the 11 M ⊙ SS model reproduces the FRB 20121102 RM evolution. BS progenitors generally yield smaller DM than SS models at fixed M ZAMS , with composition-dependent mean molecular weights introducing nonmonotonic mass trends. Matching the observed d DM/ d t of FRB 20190520B, FRB 20220529A, and the late-stage slope of FRB 20121102, we infer local SNR DM contributions of ∼10 to a few × 10 2 pc cm −3 . We also find that GHz escape is allowed in most models, with τ ff = 1 typically reached by t esc ≲ 70 yr; for weakly ionized ejecta, the source can be nearly transparent from very early times. These results support a young core-collapse supernova/SNR origin for a substantial fraction of DM source and highlight that physically consistent local-environment modeling is essential for robust FRB cosmological DM inferences.
Funding Information
  • MEXT ∣ Japan Society for the Promotion of Science (JP22K21349)

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