X-ray polarimetry and evolution of isolated neutron stars

偏振测量是研究天体的一个重要渠道，是天体光谱学的补充。X射线偏振研究仍然很少，但情况即将改变。计划在未来十年内建造轨道X射线天文台，配备专为X射线偏振测量而设计的新型仪器（IXPE，NASA; eXTP，中国）。它们将有助于研究相对较热（几个MK的色温）的磁化和潜在磁化中子星（NS）。在这个项目中，我们计划研究几类孤立的（非吸积）NS（INS）的X射线辐射的偏振特性，并观察到X射线热发射：超新星遗迹中的中心致密天体（CCO），X射线INS（XINS）和磁星。一些CCO的光谱与均匀碳气氛的光谱很好地拟合。然而，我们不能完全排除它们拥有热点的可能性，类似于其他显示脉动的CCO。极化测量可以区分均匀的碳和不均匀的氢或氦NS气氛。澄清这个问题是重要的理解NS演化和超致密物质的物理内部的NS。磁星和XINS的X射线偏振研究，具有强磁场，是重要的解决磁场配置的几何简并，并在高度磁化NS表面的物理过程的理解。以前的研究是使用纯氢模型大气或冷凝NS表面模型。我们建议将这些调查扩展到部分电离的氦大气，这是更合理的磁星从理论的角度来看。强磁化中子星具有高度不均匀的表面温度分布，除了光谱研究外，还可以使用偏振研究更有把握地确定。惯性系温度分布是南北地壳和核的磁热演化的结果，并受超致密物质物理的调节。因此，热INS辐射的光谱和偏振研究可以提供超致密物质的物理约束，只有在结合模型的NS的磁热演化。我们将模拟NS的磁和热演化，以构建自洽的模型，为INS的X射线发射的偏振测量的可靠的物理解释。新的光学偏振和X射线光谱观测的几个热发射INS，以及模拟的X射线偏振数据，他们将进行。

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.