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G.G. Pavlov (The Pennsylvania State University), V.E. Zavlin (Max-Planck-Institut f\"ur extraterrestrische Physik)
Since the opacity of a magnetized plasma depends on polarization of radiation, the radiation emergent from atmospheres of neutron stars (NSs) with strong magnetic fields is strongly polarized. We calculated the Stokes parameters of radiation emitted from hydrogen atmospheres of NSs with dipole magnetic fields, with allowance for the General Relativity (GR) effects. The polarization spectrum depends on effective temperature and magnetic field. For NSs with temperatures ~106~K and magnetic fields ~ 1012~G, the degree of linear polarization reaches peak values of 20-30% at energies 0.2-0.3~keV. Both the degree of polarization and its position angle oscillate with the NS rotation period. Since the shape of the polarization pulse profiles depends on orientations of the NS rotational and magnetic axes, observations of the pulsed polarization of the thermal X-ray radiation from isolated NSs enable one to infer these orientations. Moreover, since the polarization is substantially modified by the GR effects, the X-ray polarimetry provides a new method for evaluating the mass-to-radius ratio of NSs, which is particularly important for elucidating the properties of the superdense matter in the NS interiors.
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