On frequency dependence of pulsar linear polarization

TL;DR

This paper uses simulations to explore how pulsar linear polarization varies with frequency, revealing that emission height, mode separation, and refraction effects influence polarization characteristics across frequencies.

Contribution

It introduces a simulation-based model explaining the frequency dependence of pulsar linear polarization through emission and propagation effects.

Findings

Low frequency emission shows higher linear polarization due to mode separation at higher altitudes.

Higher frequencies exhibit more depolarization as emission originates from lower regions with overlapping modes.

Refraction effects can decrease polarization and profile width when emission occurs at the same height across frequencies.

Abstract

Frequency dependence of pulsar linear polarization is investigated by simulations of emission and propagation processes. Linearly polarized waves are generated through curvature radiation by relativistic particles streaming along curved magnetic field lines, which have ordinary mode (O-mode) and extra-ordinary mode (X-mode) components. As emitted waves propagate outwards, two mode components are separated due to re- fraction of the O mode, and their polarization states are also modified. According to the radius to frequency mapping, low frequency emission is generated from higher magnetosphere, where significant rotation effect leads the X and O modes to be sepa- rated. Hence, the low frequency radiation has a large fraction of linear polarization. As the frequency increases, emission is generated from lower heights, where the rotation effect becomes weaker and the distribution regions of two modes are more overlapped. Hence, more significant depolarization appears for emission at higher frequencies. In addition, refraction effect of the O mode becomes serious in very deep magnetosphere, which bends the O mode emission towards outer parts of a pulsar beam and also causes the separation of mode distribution regions and hence the fractional linear polariza- tion increasing with frequency. If emission of different frequencies is generated from a region of the same height, serious O mode refraction can result in the decrease of both profile width and fractional linear polarization. The observed frequency dependence of linear polarization for some pulsars can be naturally explained within the scope of our scenario.