This award funds research into mergers of binary neutron stars and of binaries consisting of a neutron star and a black hole. These phenomena are among the primary sources of gravitational radiation for ground-based detectors such as the Laser Interferometer Gravitational Wave Observatory (LIGO), and they may explain the observed short-duration gamma ray bursts (GRBs). Computer models are needed both to calculate the expected gravitational waveforms for these mergers and also to test their viability as sources of GRBs. This project will carry out large-scale, detailed computer simulations of black hole-neutron star and neutron star-neutron star mergers, focusing on three main issues. First, it will study the evolution of the nuclear matter debris left around the post-merger black hole, especially the effects of magnetic fields and neutrino radiation that are expected to be critical in delivering energy to a GRB. Second, it will explore the variety of black hole-neutron star binaries by modeling currently unstudied extremes of black hole mass and spin. Third, it will characterize the effects of neutron star physics on the gravitational waveforms.
Accurate gravitational waveform predictions will be crucial for detecting binary merger signals in LIGO data and for extracting information about the neutron star structure. Such information might be able to constrain unknown properties of nuclear matter. Numerical techniques developed for this project will be applicable to other problems in astrophysical fluid dynamics. Most of the funding from this award is used for student support. The PI directs both graduate and undergraduate students. These students receive valuable training in computer programming, numerical problem solving, and hydrodynamics, gaining important skills applicable to a broad range of technical fields. Results from this project will be used in communicating the excitement of astrophysics to the wider public.
The collision of a neutron star with a black hole creates a flash of electromagnetic radiation perhaps visible billions of light years away, but only when the black hole has a fairly low mass or high spin. This project investigated high spin, moderate mass cases (which had previously gotten less attention) using computer models. To accurately capture strong gravity effects, these models used full general relativity. This also allowed us to compute the gravitational waves, ripples of spacetime curvature, produced as the black hole and neutron star swirl around each other prior to merger. These gravitational wave predictions have been passed on to other scientists who will use them to learn how to detect such signals in real detector data. We used a nuclear physics-based model of neutron star matter. A mergers cause most of the matter to fall into the black hole, but some of it remains outside orbiting the hole for a fraction of a second. The merger heats this matter to tens of billions of degrees, at which point it starts radiating enormous amounts of energy, primarily in the form of nearly-indetectable particles called neutrinos. Our simulations include the effects of neutrino emisson in cooling the matter and in changing its composition from neutrons to protons and back. We find that, for black hole masses in the expected range, bright merger signals occur only for high black hole spins and low neutron star masses. We find that the main effect of neutrino cooling is to make the cloud of nuclear debris produced by the merger smaller. Lastly, we observe that these collisions eject a lot of matter into space. As the escaped matter expands and cools, it becomes regular matter. These ejection events may, in fact, be a major source of some of the universe's heavy elements. The subsequent evolution of the matter left in orbit around the black hole depends on forces created by magnetic fields. We have augmented our computer code to include magnetic fields and tested its ability to simulate flows around black holes. We first applied this code to study the effects of magnetic fields in neutron stars with very high but nonuniform spin (that is, the neutron star material shears against itself). Neutron stars are expected to be born with strong, shearing spins of this sort, and binary neutron star mergers will also produce it. These stars are often unstable, meaning that distortions in the star are expected to grow and give off gravitational waves, but it has been suggested that sufficiently strong magnetic fields might stabilize them. In fact, our simulations show magnetic fields triggering additional instabilities, and such rapidly spinning neutron stars remain a promising source of gravitational waves. Funds from this grant were mainly used to support two PhD students carry out their dissertation work.
