Flux-resolved Spectropolarimetric Evolution of the X-Ray Pulsar Hercules X-1 Using IXPE

Abstract

We conduct a spectropolarimetric study of the accreting X-ray pulsar Hercules X-1 using observations with the Imaging X-ray Polarimetry Explorer (IXPE). IXPE monitored the source in three different epochs, sampling two “Main-on” and one “Short-on” state of the well-known super-orbital period of the source. We find that the 2–7 keV polarization fraction increases significantly from ∼7% to 9% in the Main-on state to ∼15%–19% in the Short-on state, while the polarization angle remains more or less constant or changes slightly, ∼47°–59°, in all three epochs. The polarization degree and polarization angle are consistent with being energy independent for all three epochs. We propose that in the Short-on state, when the neutron star is partially blocked by the disk warp, the increase in the polarization fraction can be explained as a result of the preferential obstruction of one of the magnetic poles of the neutron star.