Merging neutron stars can now be studied more accurately

A new method to study the signals associated with merging neutron stars can help researchers to collect data through multiple channels in parallel. The method was developed by an international team of scientists, including the Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University, and Nikhef. By analysing a neutron star merger’s emitted gravitational waves, the kilonova, and the afterglow of the gamma-ray burst simultaneously, it was possible to model and interpret most of the observable signals from the merger for the first time. The research results were published in Nature Communications today.

Extreme conditions in a cosmic laboratory

A neutron star is a super dense astrophysical object formed at the end of a massive star life in a supernova explosion. Some neutron stars orbit each other in binary systems. They lose energy through the constant emission of gravitational waves – tiny ripples in the fabric of space-time – and eventually collide. These high-energy collisions lead to the formation of heavy elements, such as gold, and merging neutron stars therefore allow researchers to study physical principles under the most extreme conditions in the universe.

Various signals

So far, only one multi-messenger observation of a binary neutron star has been recorded. In such a multi-messenger observation, various astronomical signals are measured, such as electromagnetic radiation, gravitational waves and cosmic radiation, all of which provide different information about their source.

In this event, discovered on August 17, 2017, the last few thousand orbits around each other had warped space-time enough to create gravitational waves, which were detected by the gravitational-wave observatories LIGO and Virgo in the US and Italy. As the two stars merged, newly formed heavy elements were ejected and X-rays and radio emissions could be observed on time scales ranging from days to years. Some of these elements decayed radioactively, causing the temperature to rise. Triggered by this thermal radiation, an electromagnetic signal in the optical, infrared, and ultraviolet was detected up to two weeks after the collision. A gamma-ray burst, also caused by the neutron star merger, ejected additional material.

More accurate results

The researchers developed a code structure that allows them to analyse and interpret astrophysical data from different sources simultaneously. Additionally, the new tool helps to incorporate additional information from radio and X-ray observations of neutron stars (e.g., from NASA’s NICER telescope), from nuclear physics calculations, and even from heavy-ion collision experiments at accelerators on Earth. "Our new method will help to analyse the properties of matter at extreme densities and allows us to better understand the expansion of the universe. It also helps to learn more about heavy elements that are formed during neutron star mergers,” explains Tim Dietrich, leader of a Max Planck Fellow group at the Max Planck Institute for Gravitational Physics.

Future detections

The newly developed technique forms the basis for analysing future events. "We can now get more precise results by analysing coherently and simultaneously, going beyond the usual step-by-step combination process that we have done before," says Peter T. H. Pang of GRASP (Utrecht University and Nikhef), who is the lead developer of the code and first author of the paper.

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