Self-modulation of Fast Radio Bursts

TL;DR

This paper explores how self-modulation, a non-linear optical effect, can occur in Fast Radio Bursts as they propagate through their environment, potentially explaining observed temporal and frequency structures.

Contribution

It introduces a model of self-modulation in FRBs, showing how environmental conditions can cause burst fragmentation and frequency modulation, a novel application of non-linear optics to astrophysics.

Findings

Self-modulation occurs within a critical radius depending on plasma density and luminosity.

The burst breaks into pancakes smaller than the Fresnel scale, causing diffraction and interference effects.

Self-modulation can produce the observed temporal and frequency structures in FRBs.

Abstract

Fast Radio Bursts (FRBs) are extreme astrophysical phenomena entering the realm of non-linear optics, a field developed in laser physics. A classical non-linear effect is self-modulation. We examine the propagation of FRBs through the circumburst environment using the idealised setup of a monochromatic linearly-polarised GHz wave propagating through a uniform plasma slab of density $N$ at distance $R$ from the source. We find that self-modulation occurs if the slab is located within a critical radius $R_{\rm crit}\sim 10^{17}(N/10^2{\rm\; cm}^{-3})(L/10^{42}{\rm\; erg\; s}^{-1}){\rm\; cm}$, where $L$ is the isotropic equivalent of the FRB luminosity. Self-modulation breaks the burst into pancakes transverse to the radial direction. When $R\lesssim R_{\rm crit}$, the transverse size of the pancakes is smaller than the Fresnel scale. The pancakes are strongly diffracted as the burst exits the slab, and interference between the pancakes produces a frequency modulation of the observed intensity with a sub-GHz bandwidth. When $R\sim R_{\rm crit}$, the transverse size of the pancakes becomes comparable with the Fresnel scale, and the effect of diffraction is weaker. The observed intensity is modulated on a timescale of ten microseconds, which corresponds to the radial width of the pancakes. Our results suggest that self-modulation may cause the temporal and frequency structure observed in FRBs.