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

We conduct a comprehensive analysis of the accreting X-ray pulsar, HerculesX-1, utilizing data from IXPE and NuSTAR. IXPE performed five observations ofHer X-1, consisting of three in the Main-on state and two in the Short-onstate. Our time-resolved analysis uncovers the linear correlations between theflux and polarization degree as well as the pulse fraction and polarizationdegree. Geometry parameters are rigorously constrained by fitting thephase-resolved modulations of Cyclotron Resonance Scattering Feature andpolarization angle with a simple dipole model and Rotating Vector Modelrespectively, yielding roughly consistent results. The changes of χ_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 precessionof the neutron star crust. Furthermore, a linear association between the energyof Cyclotron Resonance Scattering Feature and polarization angle implies theprevalence of a dominant dipole magnetic field, and their phase-resolvedmodulations likely arise from viewing angle effects.