Simulating the Multi-Messenger Emission from Merging Neutron Stars

LIGO探测器最近观测到两颗中子星碰撞产生的引力波，这在物理学和天文学上都是一个巨大的进步。中子星是非常紧凑的物体，它们的碰撞为我们提供了一个非凡的实验室来测试万有引力和核物理定律。它们还为宇宙中观测到的一些最具能量的电磁信号提供动力，并且可能是金、铂和许多其他元素的主要产地。该项目将使用数值模拟对中子星碰撞发出的引力波进行公开、高精度的预测。有了这些结果，引力波信号的模型可以达到足够的精度，以避免在即将到来的LIGO结果分析中引入重大误差。该项目还将开发新的方法来模拟复杂的物理过程，这些物理过程决定了中子星碰撞产生的电磁信号的特性，特别是中微子与中子星物质的相互作用。了解这些相互作用对于分析碰撞后发射的电磁信号，以及确定中子星碰撞在宇宙中金和其他原子核富集中的作用至关重要。该项目将利用联合观测引力波和电磁信号的力量来研究宇宙。它还资助了研究生和本科生的培训，这些学生在数值模拟方面具有高级技能，可转移到学术界或工业界的职业生涯中。该项目将使用广义相对论辐射-流体力学SpEC代码来模拟中子星双星的后期激励和合并。该项目的主要目标是生成一个涵盖合并中子星参数空间的高精度数值波形库：每颗恒星的质量和自旋，以及核物质的状态方程。该项目将利用SpEC代码的主要优势之一：它能够在许多轨道上有效地演化中子星双星，以获得足够光滑的状态方程模型。作为这个项目的结果，中子星双星的几十种波形将被公开。将特别注意误差估计，以便于使用模拟来校准分析波形模型，并注意模拟的自动化。该项目的第二个目标是开发状态方程的光滑近似，因为SpEC代码在演化光滑状态方程时比目前更常用的不连续近似要有效得多。最后，该项目将导致最近提出的中微子传输算法的开发和合并模拟的使用，该算法将目前使用的“两矩”传输形式与更先进的蒙特卡罗传输算法相结合。在这种混合方案中，蒙特卡罗输运的使用消除了使用近似解析公式来关闭辐射输运方程的需要。这个新方案将特别有力地确定中微子在中子星合并后产生相对论性极性流出物中的作用，以及它们对物质流出物组成和千新星性质的影响。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The recent observation by the LIGO detectors of gravitational waves produced in the collision of two neutron stars represents a giant step forward in both physics and astronomy. Neutron stars are extremely compact objects, and their collision provides us with an extraordinary laboratory to test the laws of gravity and nuclear physics. They also power some of the most energetic electromagnetic signals observed in the Universe, and may be the main production site of gold, platinum, and many other elements. This project will use numerical simulations to produce publicly available, high-accuracy predictions for the gravitational waves emitted by colliding neutron stars. With these results, models of the gravitational wave signal can be brought to an accuracy sufficient to avoid the introduction of significant errors in the analysis of upcoming LIGO results. This project will also lead to the development of novel methods to simulate the complex physical processes that determine the properties of the electromagnetic signals powered by colliding neutron stars, particularly the interaction of neutrinos with neutron star material. Understanding these interactions is critical to analyzing electromagnetic signals emitted after the collision, and to determine the role of neutron star collisions in the enrichment of the Universe in gold and other atomic nuclei. This project will leverage the power of joint observations of gravitational waves and electromagnetic signals to study the Universe. It also funds the training of graduate and undergraduate students with advanced skills in numerical modeling transferable to careers in either academia or industry.This project will use the general relativistic radiation-hydrodynamics SpEC code to simulate the late inspiral and merger of neutron star binaries. The main goal of the project is to generate a library of high-accuracy numerical waveforms covering the parameter space of merging neutron stars: the mass and spin of each star, and the equation of state of nuclear matter. The project will leverage one of the main strengths of the SpEC code: its ability to efficiently evolve neutron star binaries over many orbits, for sufficiently smooth equation of state models. As a result of this project, dozens of waveforms for neutron star binaries will be made publicly available. Particular attention will be paid to error estimates, in order to facilitate the use of the simulations for the calibration of analytical waveform models, and to the automation of the simulations. A secondary objective of this project will be the development of smooth approximations to the equation of state, as the SpEC code is significantly more efficient when evolving smooth equations of state than for the discontinuous approximations that are more commonly used today. Finally, this project will lead to the development and use in merger simulations of a recently proposed algorithm for neutrino transport, which combines the currently used "two-moment" transport formalism with a more advanced Monte-Carlo transport algorithm. In this hybrid scheme, the use of Monte-Carlo transport removes the need to use approximate analytical prescriptions to close the equations of radiation transport. This new scheme will be particularly powerful to determine the role of neutrinos in the production of relativistic polar outflows after a neutron star merger, and their impact on the composition of matter outflows and the properties of kilonovae.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.