X-ray polarimetry and evolution of isolated neutron stars

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Polarimetry is an important channel of study of astronomical objects, complementary to their spectroscopy. X-ray polarization studies are still rare, but the situation is about to change. Orbital X-ray observatories with new instruments onboard, designed for X-ray polarization measurements, are planned for the next decade (IXPE, NASA; eXTP, China). They will be useful for studies of relatively hot (color temperatures of several MK) magnetized and potentially magnetized neutron stars (NSs). In this project, we plan to investigate the polarization properties of X-ray radiation of several classes of isolated (non-accreting) NSs (INSs) with observed X-ray thermal emission: central compact objects (CCOs) in supernova remnants, X-ray INSs (XINSs), and magnetars. Spectra of some CCOs are well fitted by the spectra of uniform carbon atmospheres. However, one cannot completely exclude the possibility that they possess hot spots, similar to the other CCOs that show pulsations. Polarization measurements can distinguish between uniform carbon and non-uniform hydrogen or helium NS atmospheres. Clarification of this issue is important for understanding NS evolution and the physics of superdense matter inside the NSs. X-ray polarization studies of magnetars and XINSs, which possess strong magnetic fields, are important for resolving geometrical degeneracy of the magnetic field configuration, and for understanding of the physical processes at the highly magnetized NS surface. Previous investigations were performed using pure hydrogen model atmospheres or condensed NS surface models. We suggest to extend these investigations to partially ionized helium atmospheres, which are more plausible for magnetars from theoretical point of view. Strongly magnetized neutron stars have highly non-uniform surface temperature distributions, which can be more confidently determined using polarimetric studies in addition to spectroscopic ones. The INS temperature distribution results from magneto-thermal evolution of the NS crust and core, which is regulated by the physics of superdense matter. Therefore, spectroscopic and polarimetric studies of thermal INS radiation can provide constraints on the physics of superdense matter only in combination with models of the magneto-thermal evolution of the NSs. We will simulate NS magnetic and thermal evolution to construct self-consistent models for reliable physical interpretation of the polarization measurements of X-ray emission of the INSs. New optical polarization and X-ray spectral observations of several thermally emitting INSs, as well as simulations of X-ray polarization data for them will be performed.