Physicists set limits on the size of neutron stars

What is the size of a neutron star? Previous estimates ranged from eight to 16 kilometers. The astrophysicists of Goethe University Frankfurt and ISAF have now successfully determined the size of neutron stars within 1.5 km using an elaborate statistical approach supported by data from gravitational wave measurement. . The researchers' report appears in the current issue of Letters of physical examination.

Neutron stars are the densest objects in the universe, with a mass greater than that of our compacted sun in a relatively small sphere whose diameter is comparable to that of the city of Frankfurt. This is actually just a rough estimate, though. For more than 40 years, determining the size of neutron stars has been a holy grail in nuclear physics whose solution would provide important insights into the fundamental behavior of matter at nuclear densities.

The data from gravitational wave detection from the fusion of neutron stars (GW170817) is an important contribution to solving this puzzle. At the end of 2017, Professor Luciano Rezzolla, Institute of Theoretical Physics at Goethe University Frankfurt and FIAS, together with his students Elias Most and Lukas Weih, have already used this data to answer a long-standing question about maximum mass. neutron stars. support before collapsing to a black hole – a result that has also been confirmed by various other groups around the world. As a result of this first important result, the same team, with the help of Professor Juergen Schaffner-Bielich, worked to establish stricter constraints on the size of neutron stars.

The crux of the problem is that the state equation that describes the matter inside neutron stars is not known. Physicists decided to follow another path: they chose statistical methods to determine the size of neutron stars within narrow limits. In order to set new limits, they calculated more than two billion theoretical models of neutron stars by solving Einstein's equations describing the equilibrium of these relativistic stars and combined this large set of data with constraints from GW170817 gravitational wave detection.

"Such an approach is not unusual in theoretical physics," adds Rezzolla, adding, "By exploring the results for all possible parameter values, we can actually reduce our uncertainties." As a result, researchers were able to determine the radius of a typical neutron star in a range of only 1.5 km: it is between 12 and 13.5 kilometers, a result that can be refined by future detections gravitational waves.

"However, there is a twist to all this because neutron stars can have double solutions," says Schaffner-Bielich. It is indeed possible that at ultra-high densities, the material changes radically in its properties and undergoes a "phase transition". This is similar to what happens to the water when it freezes and goes from a liquid state to a solid state. In the case of neutron stars, this transition is supposed to transform ordinary matter into "quark matter" producing stars that will have exactly the same mass as their "twin star", but which will be much smaller and therefore more compact.

Although there is no definitive proof of their existence, these are plausible solutions and the Frankfurt researchers took this possibility into account, despite the additional complications that twin stars imply. This effort finally paid off because their calculations revealed an unexpected result: the twin stars are statistically rare and can not be very deformed during the fusion of two of these stars. This is an important discovery because it now allows scientists to potentially exclude the existence of these very compact objects. Future observations of gravitational waves will therefore reveal whether or not neutron stars have exotic twins.

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More information:
Elias R. Most et al., New stresses on radii and deformations due to the tide of neutron stars of GW170817, Letters of physical examination (2018). DOI: 10.1103 / PhysRevLett.120.261103