The Role of Plasma Lensing in Fast Radio Bursts

Abstract Growing evidence indicates that some fast radio bursts (FRBs) reside in dense, magnetoionic environments where extrinsic propagation effects can substantially reshape the observed signal. Within a 1D Gaussian plasma-lens framework, we show that small, monotonic variations in the incidence angle of the FRB wavefront naturally generate both downward and upward sub-burst frequency drifts. We further demonstrate that distinct lensed paths that probe different rotation measures (RMs) can produce orthogonal polarization-angle (PA) jumps at gigahertz frequencies. In this picture, a ∼90° PA transition requires only a modest RM contrast of order a few ×10 rad m −2 between the multiple images. The chromatic activity of FRB 20180916B—earlier and narrower activity windows at higher frequencies—can be explained as preferential magnification near the outer caustic. Finally, the intrinsic resolution of a plasma lens provides an upper limit on the transverse emission size: lenses located close to the source yield magnetospheric-scale constraints and offer a practical means of discriminating between inner- and outer-magnetospheric emission scenarios. These results suggest that plasma lensing could account for multiple complex observational features of FRBs and may play a nonnegligible role in modulating their observable properties.