Differentiating Neutron Star Models by X-Ray Polarimetry

The nature of pulsars is still unknown because of the non-perturbative effects of the fundamental strong interaction, so various models of pulsar inner structures are suggested, either for conventional neutron stars or quark stars. Additionally, a quark-cluster matter state is conjectured for cold matter at supranuclear density, and as a result pulsars can be quark-cluster stars. Besides understanding the different manifestations, the most important issue is to find an effective way to observationally differentiate these models. X-ray polarimetry plays an important role here. The thermal x-ray polarization of quark/quark-cluster stars is focused on, and while the thermal x-ray linear polarization percentage is typically higher than ～10% in normal neutron star models, the percentage of quark/quark-cluster stars is almost zero. This could then be an effective method to identify quark/quark-cluster stars by soft x-ray polarimetry. We are therefore expecting to detect thermal x-ray polarization in the coming decades.