Polarization Perspectives on Hercules X-1: Further Constraining the Geometry

TL;DR

This study uses IXPE and NuSTAR data to analyze Hercules X-1's polarization and geometry, revealing correlations, constraining magnetic field geometry, and suggesting neutron star precession.

Contribution

It provides new constraints on the magnetic field geometry and neutron star precession in Hercules X-1 through phase-resolved polarization analysis.

Findings

Linear correlation between flux and polarization degree.

Constraints on geometry parameters via phase-resolved modulation fitting.

Evidence for neutron star crust precession.

Abstract

We conduct a comprehensive analysis of the accreting X-ray pulsar, Hercules X-1, utilizing data from IXPE and NuSTAR. IXPE performed five observations of Her X-1, consisting of three in the Main-on state and two in the Short-on state. Our time-resolved analysis uncovers the linear correlations between the flux and polarization degree as well as the pulse fraction and polarization degree. Geometry parameters are rigorously constrained by fitting the phase-resolved modulations of Cyclotron Resonance Scattering Feature and polarization angle with a simple dipole model and Rotating Vector Model respectively, yielding roughly consistent results. The changes of $\chi_{\rm p}$ (the position angle of the pulsar's spin axis on the plane of the sky) between different Main-on observations suggest the possible forced precession of the neutron star crust. Furthermore, a linear association between the energy of Cyclotron Resonance Scattering Feature and polarization angle implies the prevalence of a dominant dipole magnetic field, and their phase-resolved modulations likely arise from viewing angle effects.