Isolated Neutron Stars: The Impact of Magnetic Field Decay

中子星是自然界在极端条件下的实验室：高中心密度探测处于理论上难以处理的区域的粒子物理，而极端磁场需要包括量子电动力学。二十多年来，天文学家一直试图利用对中子星表面的X射线观测来测量它们的半径和冷却速度，因为这些可以强烈地限制恒星的内部结构和性质。然而，绝大多数已知的中子星对于这些测量并不理想，因为它们的表面被其磁场的发射(在旋转驱动的脉冲星的情况下)或被轨道上运行并落到其表面上的物质(在吸积驱动的X射线双星的情况下)所遮盖。孤立中子星(INSS)是通过软X射线发射发现的年轻(1Myr)、附近(1kpc)冷却的中子星的一个特殊样本。它们与大多数脉冲星的不同之处在于，它们的周期很长(S 3章)，没有射电辐射，X射线辐射似乎直接来自它们的表面。它们之所以令人感兴趣，既是因为它们的表观丰度(与正常脉冲星相当)，也是因为它们有望从热辐射中推断出中子星的参数。然而，目前还没有出现对热发射的一致解释，困难主要与它们复杂的、高度磁化的大气有关。此外，最近的证据表明，我们现在测量的磁场实际上是复杂的、非线性的磁场衰减的结果。这种行为长期以来一直被怀疑存在于中子星中，但以前从未在这个水平上观察到过，这带来了一些挑战和机遇。这个项目将解决三个广泛的问题：(1)惯性导航系统大气的组成是什么，我们如何测量它们的半径？(2)从X射线调查中推断出的磁场衰变对中子星的数量有多重要，它们的性质与整个中子星的数量相比如何？(3)我们能否限制表面磁场(影响X射线发射)，而不依赖于偶极场(影响时间)？观测将包括对INSS和具有类似特性的射电脉冲星的相关射电、光学和X射线观测。特别是，对射电脉冲星的研究允许来自射电天体测量和偏振测量的详细限制，这些限制对于INSS来说是不可用的，但将唯一地帮助我们了解它们的热X射线发射。这项工作将为解决中子星天体物理学中的一些悬而未决的问题提供重要线索。只有获得一些不同来源(每个来源都有自己的特点和系统不确定性)的一致结果，我们才能有强大的天体物理学结果，才能向更广泛的物理学界提供信息。PI还将继续与天文俱乐部合作，拓宽主题范围，并让学生参与分析最近的结果；招募本科生参加夏季项目，重点放在特定的天体或数据集上，他们将参与端到端的研究过程；通过接触天文学入门课程和其他可能的感兴趣的群体，扩大对这些机会的认识和参与；并协助开发威斯康星大学曼弗雷德·奥尔森天文馆的展览，该天文馆每年有近10,000名游客。所有这些努力都是一个更大的计划的一部分，该计划通过课程开发、推广和指导来自代表性不足人口的学生来增加天文学在威斯康星大学的存在。国际和平协会还将与高中生和普通公众开展外联活动。

Neutron stars are Nature's laboratory for extreme conditions: the high central density probes particle physics in a theoretically intractable regime, and the extreme magnetic fields require the inclusion of quantum electrodynamics. For over two decades, astronomers have attempted to use X-ray observations of neutron star surfaces to measure their radii and cooling rates, as these can strongly constrain the interior structure and properties of the stars. However, the vast majority of the known neutron stars are not ideal for these measurements, as their surfaces are obscured by emission from their magnetic fields (in the cases of rotation-powered pulsars) or by matter orbiting and falling onto their surfaces (in the cases of accretion-powered X-ray binaries). The Isolated Neutron Stars (INSs) are a particular sample of young ( 1 Myr), nearby ( 1 kpc) cooling neutron stars discovered via their soft X-ray emission. They differ from most pulsars in that they have long periods ( 3 s), no radio emission, and X-ray emission that appear to come directly from their surfaces. They are interesting both because of their apparent abundance (comparable to normal pulsars), and because of the promise of inferring neutron star parameters from their thermal emission. However, no consistent interpretation of the thermal emission has emerged, with the difficulty mostly related to their complicated, highly-magnetized atmospheres. Moreover, recent evidence suggests that the magnetic fields we measure now are in fact the results of complicated, non-linear magnetic field decay. Such behavior has been long suspected in neutron stars but never previously observed at this level, and it presents a number of challenges and opportunities. This project will address three broad questions: (1) What is the composition of INS atmospheres, and how can we measure their radii? (2) How important is magnetic field decay to the population of neutron stars inferred from X-ray surveys, and how do their properties compare with those of the population as a whole? (3) Can we constrain the surface magnetic fields (which affect X-ray emission), independent of the dipole fields (which affect timing)? The observations will comprise related radio, optical, and X-ray observations of the INSs and radio pulsars with similar properties. In particular, studies of radio pulsars allow detailed constraints from radio astrometry and polarimetry that are not available for the INSs but will uniquely help us understand their thermal X-ray emission. This work will provide vital clues to a number of outstanding questions in neutron star astrophysics. Only by obtaining consistent results for a number of disparate sources (each with its own peculiarities and systematic uncertainties) will we have robust astrophysical results that can inform the broader physics community.The PI will also continue work with the astronomy club, broadening the range of topics and engaging students in analysis of recent results; recruit undergraduate students for summer projects focusing on specific objects or data-sets, where they will participate in the end-to-end research process; broaden the awareness of and participation in these opportunities through outreach to introductory astronomy classes and other likely interested groups; and assist with the development of shows at the Manfred Olson Planetarium at UWM, which has nearly 10,000 visitors a year. All of these efforts are part of a larger plan to increase the presence of astronomy at UWM through curriculum development, outreach, and mentoring of students from under-represented populations. The PI will also engage in outreach activities with high school students and the general public.