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出的巨大能量使它们即使在很远的地方也有可能被探测到。他们的引力波和电磁信号包含了关于碰撞物体的特性和未知物理的信息，但是为了提取这些信息，天文学家需要理论模型来进行比较。在这个项目中，将对黑洞和中子星相互绕转并最终合并的系统进行计算机模拟。模拟还将探索通常由黑洞-中子星和中子星-中子星合并产生的碎片：放射性气体被喷射到周围空间，热的、磁化的核物质在黑洞周围旋转。进行这些模拟将包括训练研究生和本科生在相对论天体物理学中的计算机建模技术。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。这项研究利用光谱爱因斯坦代码（SpEC）的模拟，填补了黑洞-中子星双子星合并的吸气、盘和流出阶段建模中一些最有趣的空白。光谱爱因斯坦代码是一种高精度的数值相对论代码，现在包括磁流体力学、核微物理和中微子冷却。长吸气的模拟被用来测试和改进检测和识别引力波信号的算法。SpEC改进了黑洞视界附近流动的处理方法，实现了更高精度的吸积盘模拟，并用于研究吸积盘动力学、伽马射线爆发、能量提取和后期（十分之一秒）行为。该项目研究了大范围二元参数的流出量。用光滑粒子流体动力学和核反应网络代码跟踪远离合并后黑洞的抛射物的演化，使用SpEC数据作为输入，可以预测回退、千新星亮度和核合成。

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.