X-ray Polarimetry of X-ray Pulsars

TL;DR

This study used IXPE observations to analyze X-ray polarization in pulsars, confirming theoretical predictions and revealing insights into their magnetic geometry and scattering processes.

Contribution

First polarization measurements of X-ray pulsars with IXPE, confirming models and uncovering new scattering phenomena and neutron star precession.

Findings

Polarization degree varies with pulse phase.

Polarization angle follows the rotating vector model.

Detected unpulsed polarized emission from scattering.

Abstract

Radiation from X-ray pulsars (XRPs) was expected to be strongly linearly polarized owing to a large difference in their ordinary and extraordinary mode opacities. The launch of IXPE allowed us to check this prediction. IXPE observed a dozen X-ray pulsars, discovering pulse-phase dependent variation of the polarization degree (PD) and polarization angle (PA). Although the PD showed rather erratic profiles resembling flux pulse dependence, the PA in most cases showed smooth variations consistent with the rotating vector model (RVM), which can be interpreted as a combined effect of vacuum birefringence and dipole magnetic field structure at a polarization-limiting (adiabatic) radius. Application of the RVM allowed us to determine XRP geometry and to confirm the free precession of the NS in Her X-1. Deviations from RVM in two bright transients led to the discovery of an unpulsed polarized emission likely produced by scattering off the accretion disk wind.