X-ray polarimetry and evolution of isolated neutron stars

旋光法是研究天体的一个重要渠道，是对其光谱的补充。X射线偏振研究仍然很少，但这种情况即将改变。计划在未来十年在轨道X射线观测站上安装新的仪器，用于X射线偏振测量(IXPE，NASA；eXTP，中国)。它们将有助于研究相对较热(几个MK的色温)磁化和潜在磁化的中子星(NSS)。在这个项目中，我们计划研究几类有X射线热发射的孤立(非吸积)NSS(INSS)的偏振特性：超新星遗迹中的中心致密天体(CCO)、X射线INSS(XINS)和磁星。一些CCOs的光谱与均匀碳气氛的光谱很好地吻合。然而，人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。澄清这个问题对于理解NS的演化和NSS内部的超稠密物质物理是很重要的。对于强磁场的磁星和XINS的X射线极化研究，对于解决磁场位形的几何简并问题和理解高磁化NS表面的物理过程具有重要意义。以前的研究是使用纯氢模型大气或凝聚的NS表面模型进行的。我们建议将这些研究扩展到部分电离的氦大气，从理论上看，这对磁星来说是更合理的。强磁化中子星的表面温度分布高度不均匀，除了光谱研究外，还可以用偏振研究更确定地确定这一点。INS的温度分布是南北向地壳和核的磁热演化的结果，这种演化受超稠密物质物理的控制。因此，对INS热辐射的光谱和偏振研究只有与NSS的磁热演化模型相结合才能提供对超密度物质物理的约束。我们将模拟NS的磁场和热演化来构建自洽模型，以便可靠地物理解释INSS的X射线发射的偏振测量。将对几种热发射INS进行新的光学偏振和X射线光谱观测，并对它们的X射线偏振数据进行模拟。

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.