The research team has modeled the various signatures of a kilonova explosion simultaneously for the first time

Numerical simulation of the resulting ejecta material of two merging neutron stars. Red colors refer to ejected material with a high fraction of neutrons which usually appear redder than blue material with a higher fraction of protons. Credit: I. Markin (University of Potsdam)

Neutron stars are the end products of massive stars and accumulate a large part of the original mass of the star in a super-dense star with a diameter of only about ten kilometers. On August 17, 2017, researchers observed several signatures of an explosive merger of two orbiting neutron stars for the first time: gravitational waves and large bursts of radiation, including gamma-ray bursts .

An international research team has developed a method to simultaneously model these visible signals of a kilonova. This will enable them to accurately describe what exactly happens during a merger, how nuclear matter behaves under extreme conditions, and why the gold on Earth turns out to be such events.

Using a new software tool, a team involving the Max Planck Institute for Gravitational Physics and the University of Potsdam jointly interpreted different types of astrophysical data from a kilonova.

In addition, data from radio and X-ray observations of other neutron stars, nuclear physics calculations, and even data from heavy ion collision experiments in earthbound accelerators can be used. Until now, the different data sources were analyzed separately, and the data were interpreted using different physical models in some cases.

“By analyzing the data in parallel and simultaneously, we obtained more accurate results,” said Peter TH Pang, a scientist at Utrecht University.

“Our new method will help analyze the properties of matter at extreme densities. It will also allow us to better understand the expansion of the universe and how much heavy elements are formed during mergers of neutron stars,” explains Tim Dietrich, professor at the University of Potsdam and head of the Max Planck Fellow group at the Max Planck Institute for Gravitational Physics.

The research team has modeled the various signatures of a kilonova explosion simultaneously for the first time

Overview of EOS constraints from different information channels. We show a set of possible EOSs (blue lines) constrained to 1.5nsat in Quantum Monte Carlo calculations using chiral EFT interactions and extended to higher densities using the speed of sound model. Different regions of the EOS can then be constrained using different astrophysical messengers, shown in rectangles: GWs from inspirals of NS mergers, data from radio and X-ray pulsars, and EM signals that associated with NS mergers. Note that the bounds are not rigid but depend on the EOS and properties of the studied system. Credit: Communication in Nature (2023). DOI: 10.1038/s41467-023-43932-6

Extreme conditions in a cosmic laboratory

A neutron star is a superdense astrophysical object formed at the end of a massive star’s life in a supernova explosion. Like other compact objects, some neutron stars orbit each other in binary systems. They lose energy by constantly emitting gravitational wavestiny ripples in the fabric of space-time and eventually collide.

Such combinations allow researchers to study physical principles under the most extreme conditions of the universe. For example, the conditions of these high-energy collisions lead to the formation of heavy elements such as gold. Indeed, merging neutron stars are exceptional objects for studying the properties of matter at densities far beyond those found in atomic nuclei.

The new method was used in the first and only multi-messenger observation of binary neutron star mergers to date. In this event, discovered on August 17, 2017, the last few thousand orbits of stars around each other distort space-time enough to generate gravitational waves, which are detected by terrestrial gravitational-wave observatories Advanced LIGO and Advanced Virgo. As the two stars merge, the newly formed heavy elements are ejected.

Some of these elements decay radioactively, causing the temperature to rise. Because of this thermal radiation, an optical, infrared, and ultraviolet signal was detected up to two weeks after the collision. Gamma-ray bursts, also caused by neutron star mergers, release more material. The reaction of neutron star matter with the surrounding medium produces X-ray and radio emissions that can be monitored on time scales from days to years.






The simulation of the neutron star coalescence GW170817. Credit: Max Planck Institute for Gravitational Physics

More accurate results for future detections

The gravitational-wave detectors are currently in their fourth observing run. The next detection of a neutron star merger could come any day, and researchers are eagerly waiting to use the tool they’ve developed.

The work is published in the journal Communication in Nature.

More information:
Peter TH Pang et al, An updated nuclear physics and multi-messenger astrophysics framework for binary neutron star mergers, Communication in Nature (2023). DOI: 10.1038/s41467-023-43932-6

Provided by the Max Planck Society

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