Origin of Strong Linear Polarization from Fast Radio Bursts

TL;DR

This paper investigates the origin of the strong linear polarization in FRB 121102 by studying high-energy particle orbits in a dipole magnetic field, proposing that relativistic electrons in stable quasi-periodic orbits produce the observed polarization.

Contribution

It introduces a novel analysis of high-energy particle orbits in a dipole field, linking stable quasi-periodic orbits to the polarization of fast radio bursts.

Findings

High-energy particles can have stable quasi-periodic orbits in a dipole magnetic field.

Electrons in these orbits oscillate around the equatorial plane, potentially causing polarized emission.

Relativistic electrons in stable orbits may explain the polarization observed in FRB 121102.

Abstract

The detection of almost 100% linearly polarized emission from the fast radio burst source FRB 121102 implies coherent emission of relativistic electrons moving in perpendicular to the ambient magnetic field. The origin of such a particle distribution is very intriguing. Given that FRB 121102 is likely driven by a neutron star, we explored orbits of charged particles trapped in a dipole magnetic field (the St\"ormer problem). Most previous studies focused on particles with relatively low energies so that the guiding center approximation may be applied. High energy particles usually have chaotic orbits except those on a periodic orbit or near stable periodic orbits. Via evaluation of the maximum Lyapunov exponent of orbits of particles launched from the equatorial plane with an axial velocity (the angular velocity sets the length and energy scales of the system), we found prominent regions of quasi-periodic orbits around stable periodic orbits in the equatorial plane at high-energies. Particles in these orbits oscillate around the equatorial plane and their radial distance from the dipole can vary by a factor of/,2. Relativistic electrons in such orbits may be responsible for the almost 100% polarized emission from FRB 121102.