The Role of Plasma Lensing in Fast Radio Bursts

TL;DR

This paper investigates how propagation through magneto-ionic environments can imprint complex FRB signatures, arguing that a 1D Gaussian plasma lens can reproduce key phenomena. It develops the lensing formalism with the mapping $y = x - \\frac{1}{P_0^2 \\nu^2} \\nabla_x n_e(x)$ and the magnification $\\mu = |J|^{-1}$, showing caustics arise when $J(x)=0$ and that multiple images carry different rotation measures. The authors demonstrate that small changes in the wavefront incidence angle produce both downward and upward sub-burst frequency drifts via shifts in the effective source coordinate $\\Delta y$, that orthogonal PA jumps can occur when lensed images with distinct RM overlap, and that a slowly rotating beam crossing an asymmetric lens yields a chromatic active window consistent with FRB 20180916B. They further show that near-source plasma lenses can constrain emission-region sizes down to magnetospheric scales, enabling discrimination between inner and outer magnetospheric emission scenarios, as illustrated by FRB 20121102A. Overall, plasma lensing emerges as a plausible, unifying framework for several complex FRB observables and offers practical diagnostics for probing FRB environments and emission regions.

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.

Figures (7)

• Figure 1: The flux magnification map for the 1D Gaussian plasma lens with the color bar scaled logarithmically. The white line represents the caustic curve. $y = 0$ represents the center of the Gaussian distribution of electron column density. So the regions where $y>0$ and $y<0$ are symmetric. For simplicity, only the region where $y>0$ is shown in the figure.
• Figure 2: Panel (a): Schematic diagram illustrating the geometric setup of the plasma lensing model. The coordinate system is centered on the peak of the lens's plasma density distribution. $Y$ and $X$ denote the physical coordinates on the source plane and lens plane, respectively. $\beta$ and $\theta$ represent the angular positions of the source (unlensed ray) and the image (lensed ray). $\alpha$ are the deflection angles of the incident rays. The dashed lines indicate the unlensed lines of sight, while the solid lines trace the lensed ray paths at two epochs, $t_1$ and $t_2$. This time interval corresponds to a small change in the beam incidence angle $\delta \theta$, resulting in an effective source displacement of $\Delta Y$. Panel (b): A schematic diagram illustrating how two incident rays, with different values of $y$, are magnified by the lens across different frequency ranges due to the plasma lensing effect. The blue vertical line represents the first observed emission, while the orange dashed and solid vertical lines represent the second observed emission, corresponding to either a positive or negative value of $\Delta y$. The gray dotted lines represent the observing frequency band pass.
• Figure 3: Simulated dynamic spectra of four sub-bursts modeled with multi-spot emission and 1D Gaussian plasma lensing. Each panel shows the waterfall (center) with the time-averaged profile (top) and the band-averaged spectrum (right). Frequencies span 1000–1500 MHz with 256 channels. Time spans $-15$ to $+15$ ms with 0.05 ms sampling. Intrinsic radiation is modeled as a Gaussian profile with FWHM of 0.35 GHz, centered at $\nu_0=1.25$ GHz. Lens parameters are defined with a transverse lens scale $a = 7.2 \times 10^{10}$cm, $\rm{DM_l}=1~pc~cm^{-3}$, and $D_{ds} = 6$ AU. Noise is added to the signal after summing the contributions of the individual FRBs. The signal-to-noise ratio is set to 10, and Gaussian noise is added to the simulated signal to match this signal-to-noise ratio. Panel (a) and (b) use $y_0=\{1.9, 2.0, 2.1, 2.2\}$; panel (c) and (d) use $y_0=\{2.2, 2.1, 2.0, 1.9\}$. The time separations between the four sub-bursts are $\Delta t=5\rm{ms}$ for panel (a) and (c), and $\Delta t=1\rm{ms}$ for panel (b) and panel (d).
• Figure 4: Time-domain polarization evolution of a simulated FRB burst lensed by a near-source 1D Gaussian plasma lens for two different lens configurations. The parameters are set to $\alpha_0=2.3, y=2.15$, $\alpha_0=2.0, y=2.20$ and $\rm{RM_0}=50~rad~m^{-2}$ for panel (a) and panel (b), respectively. In each column, the top panel shows the band-averaged intensity of the individual lensed images and the total intensity, the middle panel shows the band-averaged PA, and the bottom panel shows the fractional linear $L/I$. The observing band is 1.15–1.35 GHz with 0.5 MHz channels, and a transverse RM gradient is chosen such that the two brightest images differ by $\Delta \rm{RM}\simeq20~rad~m^{-2}$.
• Figure 5: Schematic illustration of a near-source 1D Gaussian plasma lens. A sweeping emission beam (caused by slow rotation or precession) intercepts a plasma lens with a transverse Gaussian electron-density profile. Only during part of the sweep phases, the rays pass through the caustic segment of the lens and are refracted toward the observer; at other phases, the rays miss the line of sight. In this configuration, the combination of the geometric sweep and the limited observable caustic region of plasma lens naturally gives rise to a frequency-dependent active window.