X-ray polarization signals from the surface layers of highly magnetized neutron stars

Pulsed soft X-ray emission from select young pulsars and the entire population of magnetars is expected to be highly polarized due to the anisotropizing influence of the strong magnetic fields present. This paper details new Monte Carlo simulations devised to explore magnetized radiative transfer of such polarized X rays in ionized atmospheres of neutron stars. We compute transport due to magnetic Thomson scattering sampling different magnetic colatitudes. Our results provide intensity and Stokes parameter angular distributions, indicating strong spectral and polarization signatures from the persistent surface emission. Profound differences in the signals between magnetic polar and equatorial locales exist, suggesting fruitful geometrical diagnostics enabled by sensitive phase-resolved polarimetry. The simulations capture the wide disparity in cross sections between polarization modes at frequencies well below the cyclotron resonance. These computations are of significant interest for future X-ray polarimeters include IXPE. Although the present focus is on neutron star atmospheres, the transport simulation technique can be extended to treat hard X-rays in bursts from magnetar magnetospheres.