Numerical Simulations of Compact Neutron Star Binary Mergers

中子星和黑洞的碰撞是非常罕见的事件，但它们以引力波、红外辐射、X射线和伽马射线的形式释放的巨大能量使它们即使在非常遥远的地方也有可能被探测到。它们的引力波和电磁信号包含关于碰撞物体的性质和未知物理的信息，但为了提取这些信息，天文学家需要可以比较的理论模型。在这个项目中，将对黑洞和中子星相互轨道并最终合并的系统进行计算机模拟。模拟还将探索黑洞-中子星和中子星-中子星合并通常产生的碎片：喷入周围空间的放射性气体，以及围绕黑洞旋转的热磁化核物质。进行这些模拟将涉及对研究生和本科生进行相对论天体物理学中的计算机建模技术的培训。由此产生的引力波和核碎片数据将提供给所有致力于探测和描述这些事件的天文学家。这项研究利用谱爱因斯坦编码(SPEC)的模拟，填补了黑洞-中子星二元合并的磁旋相、盘相和流出相建模中一些最有趣的剩余空白。谱爱因斯坦编码是一种高精度的数值相对论编码，现在包括磁流体动力学、核微物理和中微子冷却。对长激励波的模拟被用来测试和改进用于检测和识别来自这类源的引力波信号的算法。对SPEC处理黑洞视界附近流动的改进使得能够进行更高精度的吸积盘模拟，这些模拟被用于研究盘动力学、伽马射线爆发、能量提取和晚时间(十分之一秒)行为。该项目研究了各种二进制参数的外流。远离合并后黑洞的喷出物的演化通过光滑粒子流体力学和核反应网络代码进行跟踪，使用SPEC数据作为输入，从而能够预测后退、千诺瓦亮度和核合成。

Collisions of neutron stars and black holes are very rare events, but the enormous energies they release in gravitational waves, infrared radiation, X-rays, and gamma-rays make them potentially detectable even when very far away. Their gravitational wave and electromagnetic signals contain information about the properties and unknown physics of the colliding objects, but to extract this information, astronomers need theoretical models to which they can compare. For this project, computer simulations will be carried out of systems with a black hole and neutron star orbiting each other and eventually merging. Simulations will also explore the debris commonly produced by black hole-neutron star and neutron star-neutron star mergers: radioactive gas ejected into the surrounding space and hot, magnetized nuclear matter swirling around a black hole. Carrying out these simulations will involve training both graduate and undergraduate students on techniques of computer modeling in relativistic astrophysics. The resulting gravitational wave and nuclear debris data will be made available to all astronomers working to detect and characterize these events. This research fills some of the most interesting remaining gaps in the modeling of inspiral, disk, and outflow phases of black hole-neutron star binary mergers using simulations with the Spectral Einstein Code (SpEC), a high-accuracy numerical relativity code that now includes magnetohydrodynamics, nuclear microphysics, and neutrino cooling. Simulations of long inspirals are used to test and improve algorithms for detecting and identifying the gravitational wave signals from such sources. Improvements to SpEC's treatment of flows near black hole horizons enable higher-accuracy accretion disk simulations, and these are used to study the disk dynamics, gamma ray burst, energy extraction, and late-time (tenths of a second) behavior. The project studies outflows for a wide range of binary parameters. The evolution of ejecta far from the post-merger black hole is tracked with smoothed-particle hydrodynamics and nuclear reaction network codes using SpEC data as input, enabling predictions of fallback, kilonova brightness, and nucleosynthesis.