What Happens to the Remains of Neutron Star Mergers?

Simulations of massive neutron star merger remnants reveal their structure and early evolution as they cool down by emitting neutrinos.

False color plot showing the density of the mass in the equatorial (bottom) and meridional, or “southern” (top) planes of a neutron star merger remnant about 100 milliseconds after the merger. — Image courtesy of David Radice

The Science

In the aftermath of a collision of neutron stars, a new celestial object called a remnant emerges, shrouded in mystery. Scientists are still unraveling its secrets, including whether it collapses into a black hole and how quickly this might happen. Through advanced supercomputer simulations, scientists have delved into the internal structure of these remnants and explored their cooling process, primarily through neutrino emissions. These findings reveal a central object surrounded by a rapidly rotating ring of hot matter. If these remnants avoid collapse, scientists expect that they release the majority of their internal energy within seconds of when they form.

The Impact

By observing when neutron stars merge in space, scientists gain insights into how nuclear matter behaves under extreme conditions that cannot be replicated on Earth. Nuclear matter is a hypothetical substance made up of protons and neutrons held together by the strong force. Of particular interest to scientists is whether the pressure from the strong force can stop black holes from forming. In this study, scientists focused on what happens after neutron stars merge but don't become black holes. The research explored neutron stars’ early evolution, just moments after they are created. This research is a starting point for identifying the astronomical signals that could help answer questions about neutron stars and black hole formation.

Summary

Scientists at Pennsylvania State University have used supercomputer simulations with general-relativistic neutrino-radiation hydrodynamics to understand the internal structure of neutron star merger remnants. They also studied how the remnant cools down by emitting neutrinos. This work used the computational resources available through the Department of Energy’s National Energy Research Scientific Computing Center, the Leibniz Supercomputing Centre in (Germany), and the Institute for Computational and Data Science at the Pennsylvania State University.

The research found that neutron star merger remnants consist of a central object endowed with most of the mass of the system, surrounded by a ring of hot matter in fast rotation that contains a small fraction of the mass but a large fraction of the angular momentum. Unlike most stars, the inner remnant has a higher temperature on its surface than in its core, so convective plumes are not expected to form as the remnant cools down by emitting neutrinos.

Contact

David Radice
The Pennsylvania State University
david.radice@psu.edu

Funding

This research was primarily funded by the Department of Energy Office of Science, Nuclear Physics program. Additional funding was provided by the National Science Foundation and the European Union.