Plasma lensing interpretation of FRB 20201124A bursts at the end of September 2021

When radio photons propagate through a non-uniform electron density volume, the plasma lensing effect can induce an extreme magnification to the observed flux at certain frequencies. Because the plasma lens acts as a diverging lens, it can extremely suppress the observed flux when aligned with the source. These two properties can theoretically cause a highly magnified fast radio burst (FRB) to become faint or even disappear for a period of time. In this paper, we interpret that the significant increase in burst counts followed by a sudden quenching in FRB 20201124A in September 2021 can be attributed to plasma lensing. Based on the one-dimensional Gaussian lens model, we search for double main-peak structures in the spectra just before its extinction on 2021 September 29. After the de-dispersion and de-scintillation procedures, we find eight bursts with double main-peaks at stable positions. There are three parameters in our modelling, the height N0, width a of the lens and its distance DLS to the source. We reformulate them as a combined parameter P₀ ∝ (a/au)√kpcDLS pccm⁻³N₀. The frequency spectra can give an accurate estimation of P0 corresponding to (a/au)√kpcDLS pccm⁻³N₀ ≈ 28.118, while the time of arrival only give a relatively loose constraint on a2/DLS. Comparing with the observation dynamic spectra, we suggest that for a plasma lens in host galaxy, e.g. DLS ≈ 1 kpc, the width of lens can not be larger than 40 au. At last, we estimate the relative transverse motion velocity between the lens and source, v≈ 98(a/au)km\,s⁻¹.