Stars, Black Holes, and Disks: Dense Matter Phenomena in 2D Numerical Relativity

Washington State University, Pullman WA

Investigators

Abstract

The merger of two neutron stars, or a neutron star with a black hole, involves extremes of all the fundamental forces: strong gravity, nuclear matter denser than atomic nuclei, incredibly strong magnetic fields, and huge energies released in the form of radiation of neutrinos and gravitational waves. Such systems give off gravitational waves as the neutron star and its companion spiral together and electromagnetic waves (visible light, infrared, gamma rays, etc) after the merger. Probing the extreme physics involved requires comparing observations with predictions from models. Modeling these events requires computer simulations including general relativity, turbulent magnetized fluid, and radiation. However, the post-merger evolution (which produces important electromagnetic signals and outflows of gas) lasts seconds, while most numerical simulations last less than a tenth of this time and so miss most of it. This project will probe these later times with numerical relativity, following the story of the merger through to completion, when nothing but a stable, quiescent black hole or neutron star remains. The key to evolving to late times is to treat the fluid properties as a combination of average, large-scale components and complicated mulit-scale turbulence. One can take advantage of a rough symmetry in the former about a rotation axis to do less computationally demanding (2D) simulations, so long as the effect of the turbulence on the average flows and magnetic fields are properly incorporated. Part of this project will be to improve models of these effects of turbulence. Another part will be to determine how the properties of matter and spacetime immediately after merger affect the subsequent output of radiation and gas. The PI will train students in STEM areas. The PI proposes to undertake long-time (multi-second) studies of these strong-gravity, dense-matter systems using the Spectral Einstein Code (SpEC), which includes magnetohydrodynamics, nuclear microphysics, neutrino transport, and a turbulence/dynamo model to capture previously omitted qualitative effects. Long-time 2D simulations will extend existing compact binary merger simulations for multiple seconds, yielding accurate predictions about outflows from disk and remnant, delayed remnant collapse, magnetic field evolution, and GRB energetics. The 2D axisymmetric evolution techniques will be imported into the numerical relativity code SpECTRE. 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.