Simulating the Multi-Messenger Emission from Merging Neutron Stars

LIGO探测器最近对两颗中子星碰撞产生的引力波进行了观测，这标志着物理学和天文学都向前迈出了一大步。中子星是非常致密的天体，它们的碰撞为我们提供了一个非凡的实验室来测试引力定律和核物理。它们还为宇宙中观察到的一些最高能量的电磁信号提供动力，并可能是金、铂和许多其他元素的主要生产地。这个项目将使用数值模拟来产生公开可用的、高精度的中子星碰撞发出的引力波的预测。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结果的分析中引入重大误差。该项目还将导致开发新的方法来模拟复杂的物理过程，这些过程决定了由碰撞的中子星提供能量的电磁信号的性质，特别是中微子与中子星材料的相互作用。了解这些相互作用对于分析碰撞后发出的电磁信号，以及确定中子星碰撞在金和其他原子核中的宇宙丰度中所起的作用至关重要。该项目将利用引力波和电磁信号联合观测的力量来研究宇宙。它还资助培养具有高级数值模拟技能的研究生和本科生，这些技能可以转移到学术界或工业界的职业。这个项目将使用广义相对论辐射-流体力学规范代码来模拟中子星双星的后期螺旋和合并。该项目的主要目标是生成一个高精度的数值波形库，涵盖合并中子星的参数空间：每颗恒星的质量和自转，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：它能够有效地在多个轨道上演化中子星双星，以实现足够平滑的状态方程模型。作为这个项目的结果，几十个中子星双星的波形将被公开提供。将特别注意误差估计，以便利使用模拟来校准分析波形模型，并实现模拟的自动化。该项目的次要目标将是开发状态方程的光滑近似，因为SPEC代码在演变光滑状态方程时比目前更常用的不连续近似要高效得多。最后，这个项目将导致最近提出的中微子输运算法的开发和在合并模拟中的使用，该算法结合了目前使用的“两矩”输运公式和更先进的蒙特卡罗输运算法。在这种混合方案中，蒙特卡罗输运的使用消除了使用近似分析公式来封闭辐射输运方程的需要。这一新方案将特别有力地确定中微子在中子星合并后产生的相对论极地外流中的作用，以及它们对物质外流的组成和千新星性质的影响。这一裁决反映了NSF的法定使命，并通过使用基金会的智力优势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.