Polarization Images of Neutron Stars Illuminated by a Thin Accretion Disk: A Comparison with Black Holes
We investigate linear polarization images of static, spherically symmetric neutron stars illuminated by a geometrically and optically thin accretion disk. Neutron star equilibrium configurations are constructed with a polytropic equation of state, and the null geodesic equations and the parallel transport equation for the linear polarization vector are solved in the geometric optics approximation. The numerical results show that the total polarized intensity is generally positively correlated with the total intensity and reaches its maximum near the neutron star surface. As the observer inclination increases, the symmetry of the polarization images is progressively broken. In addition, the magnetic field configuration mainly affects the direction of the polarization vectors, while its influence on the overall polarized intensity distribution is comparatively limited. To further characterize the spatial structure of the polarization direction, we introduce the net electric vector position angle $\chi_{\mathrm{net}}$ and the second azimuthal Fourier mode $\angle\beta_2$. A comparison with polarization images of a Schwarzschild black hole reveals clear differences between the two types of compact objects in the locations of strongly polarized regions and the size of the central region without a polarization signal. These results show that linear polarization images provide information beyond total intensity images for distinguishing neutron stars from black holes.