Quasi-steady emission from repeating fast radio bursts can be explained by magnetar wind nebulae

Among more than 1000 known fast radio bursts (FRBs), only five sources – FRBs 20121102A, 20190520B, 20201124A, 20240114A, and 20190417A –have confirmed associations with persistent radio sources. The observed quasi-steady emission is consistent with synchrotron radiation from a composite of magnetar wind nebula and supernova (SN) ejecta. Using a phenomenological model that incorporates simplified treatments of the nebular dynamics and particle acceleration, we compute the synchrotron flux by solving kinetic equations for energized electrons, accounting for electromagnetic cascades of electron positron pairs interacting with nebular photons. Within the framework of our model, the rotation-powered scenario requires a young neutron star (NS) with age t_age ≈ 20yr , dipolar magnetic field B_dip ≈ (3–5) × 10¹² G and initial spin period P_i ≈ 1 . 5–3ms in an ultra-stripped SN progenitor to account for emissions from FRBs 20121102A and 20190520B. In contrast, FRB 20201124A requires t_age ≈ 10yr , B_dip ≈ 5 . 5 × 10¹³ G, and P_i ≈ 10ms in a conventional core-collapse SN progenitor. For the magnetar- flare-powered model, NS aged tage ≈ 25 / 40yr in a USSN progenitor and tage ≈ 12 . 5yr in a CCSN progenitor explains theobserved flux for FRB 20121102A/20190520B and FRB 20201124A, respectively. Finally, we estimate a minimum NS age t_{age , min} ∼ 1–3yr based on the near-source plasma contribution to observed DM, and tage , min ∼ 6 . 5–10yr from the absence of radio signal attenuation.