Neutron stars are extremely compact remnants of large ordinary stars that have undergone a supernova explosion and a subsequent gravitational collapse. In their incredibly dense cores, the density of matter exceeds even that of atomic nuclei, which may be enough to produce an entirely new phase of matter. In this quark matter, even neutrons and protons no longer exist, but are replaced by their constituents, quarks and gluons, which have been liberated form their colour confinement.
In a Nature Physics article from 2020, a predominantly Finnish research team – consisting of Eemeli Annala and Aleksi Vuorinen from the University of Helsinki, Tyler Gorda from the Technical University of Darmstadt, Aleksi Kurkela from Stavanger University, and Joonas Nättilä from Columbia University – presented first-ever evidence for the presence of quark matter inside the cores of massive neutron stars. The finding was based on modelling the equation of state of neutron-star matter, i.e. the relationship between its pressure and energy density.
Now the same group, strengthened by the Greek researchers Evangelia Katerini (University of Thessaloniki) and Vasileios Paschalidis (University of Arizona), has found further evidence supporting their earlier result. When new observational data on neutron stars was added to the analysis, a large fraction of those equations of state that were consistent with even the most massive stable stars being built from nuclear matter alone were ruled out. The study utilized two novel types of observations: the radius measurement of a single massive neutron star, published by the NICER collaboration in the spring of 2021 and the observation of an electromagnetic signal related to the GW170817 gravitational wave event, observed in August 2017.
About 1.7 seconds after the LIGO observatory measured a gravitational wave signal in the famous GW170817 binary neutron star merger event, the FERMI gamma ray satellite orbiting Earth observed a burst of gamma rays from roughly the same location in the sky. This observation has been commonly interpreted to signal the gravitational collapse of the collision product into a black hole, which allows deducing an upper bound for the mass of a rapidly rotating neutron star using the approximately known masses of the stars involved in the merger.
While it is straightforward to include the radius measurement in the analysis performed in the 2020 Nature Physics article, which was based on generating millions of equations of state and the corresponding non-rotating neutron-star configurations, incorporating the black hole formation hypothesis turned out more involved. This in particular required building rotating neutron star solutions from each of the millions of equations of state analysed, which was now achieved in collaboration with Katerini and Paschalidis.
The results of the new analysis turned out very interesting. All equations of state ruled out by the new observations featured very large speeds of sound, which is known to be a prerequisite for all neutron stars to be built from nuclear matter. Furthermore, the uncertainties in the neutron-star-matter equation of state were dramatically reduced, and the researchers were able to make very precise predictions for many measurable properties of neutron stars. For example, if a so-called hypermassive neutron star appeared as an intermediate state in the GW170817 merger, we now know that the mass of a non-rotating neutron star cannot exceed 2.19 solar masses.
The new analysis that includes data from neutron-star radius measurements as well as the electromagnetic counterpart of the gravitational-wave event GW170817, the first recorded binary neutron star merger was published in the journal Physical Review X on 25 March 2022.