Polarized synchrotron emission from the equatorial current sheet in gamma-ray pulsars

TL;DR

This study analyzes polarization properties of gamma-ray pulsars' emission from the equatorial current sheet, revealing mild polarization and characteristic polarization angle swings, which can be tested by future polarimetry observations.

Contribution

It provides the first detailed phase-resolved polarization predictions from particle-in-cell simulations of pulsar magnetospheres, linking polarization features to current sheet emission.

Findings

Emission is mildly polarized with an anticorrelation between flux and polarization degree.

Polarization swings occur due to magnetic polarity changes during pulses.

Model reproduces Crab pulsar's optical polarization pattern with specific geometry.

Abstract

Polarization is a powerful diagnostic tool to constrain the site of the high-energy pulsed emission and particle acceleration in gamma-ray pulsars. Recent particle-in-cell simulations of pulsar magnetosphere suggest that high-energy emission results from particles accelerated in the equatorial current sheet emitting synchrotron radiation. In this study, we re-examine the simulation data to compute the phase-resolved polarization properties. We find that the emission is mildly polarized and that there is an anticorrelation between the flux and the degree of linear polarization (on-pulse: ~15%, off-pulse: ~30%). The decrease of polarization during pulses is mainly attributed to the formation of caustics in the current sheet. Each pulse of light is systematically accompanied by a rapid swing of the polarization angle due to the change of the magnetic polarity when the line of sight passes through the current sheet. The optical polarization pattern observed in the Crab can be well-reproduced for a pulsar inclination angle ~60 degrees and an observer viewing angle ~130 degrees. The predicted high-energy polarization is a robust feature of the current sheet emitting scenario which can be tested by future X-ray and gamma-ray polarimetry instruments.