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

When the 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 source. These two properties can theoretically cause a highly magnified Fast Radio Burst (FRB) to 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 spectra just before its extinction on September 29, 2021. 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 and width of the one-dimension Gaussian lens and its distance to the source. We reformulate them as a combined parameter P_0 ∝ ( a/AU )√(kpc/D_LSpc cm^-3/N_0). The frequency spectra can give an accurate estimation of P_0 corresponding to ( a/AU )√(kpc/D_LSpc cm^-3/N_0)≈ 28.118, while the time of arrival only give a relatively loose constraint on a^2/D_LS. Comparing with the observation dynamic spectra, we suggest that for a plasma lens in host galaxy, e.g., D_LS≈ 1kpc, the width of lens can not be larger than 40AU. At last, we estimate the relative transverse motion velocity between the lens and source, v≈98(a/AU)km/s.