The Role of Plasma Lensing in Fast Radio Bursts

TL;DR

This paper explores how plasma lensing in dense environments can explain various complex observational features of fast radio bursts, including frequency drifts, polarization jumps, and activity window variations.

Contribution

It introduces a 1D Gaussian plasma-lens model to explain FRB phenomena and provides new insights into the role of plasma lensing in shaping FRB signals.

Findings

Plasma lensing can produce both upward and downward frequency drifts in FRBs.

Distinct lensed paths with different RMs can cause orthogonal polarization-angle jumps.

The model constrains the emission region size, distinguishing between magnetospheric scenarios.

Abstract

Growing evidence indicates that some fast radio bursts (FRBs) reside in dense, magneto-ionic 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 $\sim 90^\circ$ PA transition requires only a modest RM contrast of order a few $\times10~\rm{rad~m^{-2}}$ 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 non-negligible role in modulating their observable properties.