Fast radio burst source properties from polarization measurements

TL;DR

Polarization measurements of FRBs suggest they originate near the surface of strongly magnetized neutron stars, with polarization angles frozen at a radius where plasma density drops, providing insights into their magnetic environment.

Contribution

This paper introduces a model linking polarization properties of FRBs to their origin near neutron star surfaces, predicting PA variations tied to neutron star rotation.

Findings

FRBs show nearly 100% linear polarization.

PA variations are consistent with a neutron star magnetosphere model.

PA freeze-out occurs where plasma density becomes too low to rotate the electric vector.

Abstract

Recent polarization measurements of fast radio bursts (FRBs) provide new insights on these enigmatic sources. We show that the nearly 100% linear polarization and small variation of the polarization position angles (PAs) of multiple bursts from the same source suggest that the radiation is produced near the surface of a strongly magnetized neutron star. As the emitted radiation travels through the magnetosphere, the electric vector of the X-mode wave adiabatically rotates and stays perpendicular to the local magnetic field direction. The PA freezes at a radius where the plasma density becomes too small to be able to turn the electric vector. At the freeze-out radius, the electric field is perpendicular to the magnetic dipole moment of the neutron star projected in the plane of the sky, independent of the radiation mechanism or the orientation of the magnetic field in the emission region. We discuss a number of predictions of the model. The variation of PAs from repeating FRBs should follow the rotational period of the underlying neutron star (but the burst occurrence may not be periodic). Measuring this period will provide crucial support for the neutron star nature of the progenitors of FRBs. For FRB 121102, the small range of PA variation means that the magnetic inclination angle is less than about 20 degrees and that the observer's line of sight is outside the magnetic inclination cone. Other repeating FRBs may have a different range of PA variation from that of FRB 121102, depending on the magnetic inclination and the observer's viewing angle.