Merging Black Holes and Neutron Stars

Black holes and neutron stars both provide us with remarkable environments to test the laws of Physics in extreme environments that cannot be reproduced in laboratory on Earth. They are the most compact macroscopic objects that we have been able to observe in the Universe so far, and are thus surrounded by extremely strong gravitational fields -- or, in the language of Einstein's theory of general relativity, strong spacetime curvature. As a result, black holes and neutron stars are invaluable tools to test our understanding of gravity and of the structure of spacetime. Their scientific values does not however stop there. While the interior of black holes remains a mystery that we cannot probe experimentally, the size and internal structure of neutron stars can provide us with a wealth of information about the properties of matter in very dense environments. By studying neutron stars millions of light years away, we can in fact improve our understanding of nuclear physics, and of interactions between densely packed neutrons and protons!

The first detection of gravitational waves from the collision of two black holes in 2015, and the subsequent observation of two colliding neutron stars in 2017, opened up an entirely new way for us to observe the Universe in general, and compact objects in particular. Mergers of black holes and neutron stars are the loudest systems observable by ground-based gravitational wave detectors such as Advanced LIGO (US), Advanced VIRGO (EU) and Kagra (Japan). However, gravitational waves from merging binaries are too faint to be detected without the use of ``matched filtering'' techniques, in which an accurate template is used to extract the signal from the detector's noise. Whether to detect binaries or to determine the parameters of the merging objects, the LIGO/VIRGO detectors rely on the existence of accurate models for the gravitational wave signal. For the last 10-50 orbits before the merger, we do not have analytical solutions for the evolution of these binaries, and 3D numerical simulations with general relativistic codes are required to test and calibrate gravitational wave templates.

By observing the population of merging black holes and neutron stars, we can obtain invaluable information about the properties of compact objects, the evolution of massive stars in binaries, and the result of core-collapse supernovae. For black hole-neutron star (BHNS) and binary neutron star (BNS) mergers, the gravitational wave signal also carries information about the unknown properties of the neutron star. In fact, 20-50 detections of BNS mergers may be sufficient to measure the size of neutron stars with enough precision to meaningfully constrain nuclear physics... as long as the signal is modeled with enough accuracy. Neutron star mergers can also emit much more than gravitational waves. Short gamma-ray bursts are powered by the black hole-accretion disk system formed as the result of at least some BHNS and BNS mergers, while optical/infrared transients are powered by r-process nucleosynthesis in the matter ejected by the merger. The products of that nucleosynthesis include heavy elements such as gold and uranium, whose origin remains uncertain today. The first detection of a BNS merger (GW170817) confirmed most of these theoretical observations: the gravitational wave signal was followed by a gamma-ray burst, and an optical/infrared transient consistent with the ejection of a few percents of a solar mass of material. Since then, at least one more BNS merger (GW190425) and two BHNS mergers (GW200105, GW200115) have been detected, although without observed electromagnetic counterpart to the gravitational wave signal.

Detecting electromagnetic counterparts can teach us about the origin of gamma-ray bursts, the localization and environment of the mergers, the properties of the merging binaries, and the origin of heavy elements. However, in order to understand these electromagnetic signals, and connect them to the properties of the source, numerical simulations are required. These simulations need to take into account a wide range of complex physical effects: general relativity, the properties of neutron stars, neutrino transport, strong magnetic fields, and nuclear reactions all play an important role in these extreme astrophysical events. Developing numerical methods for the study of these systems, and performing simulations to predict the properties of the gravitational wave and electromagnetic signals powered by mergers is the main are of research in my group at the University of New Hampshire. We mostly work with the Spectral Einstein Code (SpEC), developed by the SxS Collaboration.

Featured result:

Monte-Carlo neutrino transport in neutron star merger simulations: We evolved for the first time the collision of two neutron star using Monte-Carlo neutrino transport. Neutrinos play a crucial role in the evolution of neutron star mergers, and neutrino-matter interactions are in particular the main determinant of the composition of the matter ejected during merger, of the color and duration of post-merger optical/infrared signals, and of the outcome of nucleosynthesis in the matter outflows. Previous simulations relied on approximate neutrino transport scheme, creating uncertainty as to the robustness of our predictions for the properties of matter outflows. Our Monte-Carlo schemes allows us to better take into account neutrino-matter interactions and study their effect on the composition of the remnant, and to estimate errors in previous neutrino transport schemes. It will also in the future allow for detailed studies of the role of neutrinos in neutron star mergers.

Current and Past Group Members

Undergraduate Students:

Izzak Boucher

Alana Gudinas

Diana Horangic

Max Miller

Ronny Nguyen

Teresita Ramirez (REU)

Michael Rose

Graduate Students:

Sasha Chernoglazov

Amelia Henkel

Phillip Kovarick

Alex Knight

Tia Martineau

Postdoctoral Fellow:

Christian Krueger