Merging neutron stars in sharper focus with connecting software

20 December 2023

An international team of scientists from universities including Utrecht University and Nikhef have developed new computer programs that can simultaneously analyze signals from different observatories of merging neutron stars.

In an article published this week in the journal Nature Communications, the researchers show that it is possible to use gravitational waves, optical signals and radio and X-ray signals in a combined analysis.

Ejected material from a simulated merger of two neutron stars, red light elements, blue heavy elements. IMAGE Ivan Markin (U.Potsdam)

This allows better conclusions to be drawn about the colliding neutron stars and the specific events during the collision. It also allows adding new observations and experiments in future analyses.

Now neutron star mergers are usually analyzed on a signal-by-signal basis, and the analyses are combined after the fact. That leads to uncertain assumptions and systematic errors, said first author Peter Pang of the study. Pang is affiliated with the GRASP Institute at Utrecht University and Nikhef.

Gravitational waves

Neutron stars are super-compact stars left over when a massive star explodes in a supernova at the end of its existence. Like black holes, for example, neutron stars can orbit each other in pairs. In that lightning-fast motion, gravitational waves are emitted, causing vibrations of space and time.

The collisions and mergers make it possible to study matter under the most extreme conditions in the universe. GRASP in Utrecht does the same through participation in the ALICE experiment at CERN, where heavy atoms are shot at each other in the LHC accelerator and so-called quark-gluon plasmas can be studied.

August 2017

The new method was first applied to the analysis of observations in different wavelengths of a collision of two neutron stars in August 2017. Gravitational waves from that were captured by the LIGO and Virgo detectors in the US and Italy at the time. For now, the 2017 collision is the only multi-messenger event from a binary neutron star merger that has been extensively observed.

The collision created new chemical elements like gold, with radioactive products driving up the temperature, whose thermal radiation was seen for weeks with telescopes. A gamma ray flash also occurred, blowing material outward, causing radio and X-ray radiation in the surrounding material for months.

Halving uncertainty

An analytical combination of the events by the various observatories previously led to estimates for the size of the neutron stars involved, which had to be somewhere around 11 to 12 kilometers in diameter with an uncertainty of about 800 meters. The new method roughly halves the uncertainty in that outcome to 400 meters, Pang said.