Polarisation of Accreting X-ray Pulsars. II. Hercules X-1

TL;DR

This paper introduces a new model for polarised emission from accreting X-ray pulsars, specifically applied to Hercules X-1, accounting for complex physics and predicting high, phase- and energy-dependent polarisation fractions.

Contribution

The study develops a novel emission model that independently predicts polarisation parameters and incorporates accretion column structure, general relativity, and quantum electrodynamics effects.

Findings

Model describes observed pulse shape and cyclotron line modulation.

Predicts high polarisation fraction between 60-80% in 1-10 keV range.

Polarisation properties vary with phase and energy, aiding geometry discrimination.

Abstract

We employ our new model for the polarised emission of accreting X-ray pulsars to describe the emission from the luminous X-ray pulsar Hercules X-1. In contrast with previous works, our model predicts the polarisation parameters independently of spectral formation, and considers the structure and dynamics of the accretion column, as well as the additional effects on propagation due to general relativity and quantum electrodynamics. We find that our model can describe the observed pulse fraction and the pulse shape of the main peak, as well as the modulation of the cyclotron line with phase. We pick two geometries, assuming a single accretion column or two columns at the magnetic poles, that can describe current observations of pulse shape and cyclotron modulation with phase. Both models predict a high polarisation fraction, between 60 and 80\% in the $1-10$ keV range, that is phase- and energy-dependent, and that peaks at the same phase as the intensity. The phase and energy dependence of the polarisation fraction and of the polarisation angle can help discern between the different geometries.