The most massive neutron star has a big surprise in store
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Between 20 and 30 kilometers. This is the diameter of PSR J0740+6620, the most massive neutron star known. This result was announced on April 17, 2021 following observations by the Nicer mission, a NASA telescope installed on the International Space Station (ISS). A surprising observation, many theoretical models predicting a smaller size for this cosmic object.
Neutron star Interior Composition Explorer, the full name of Nicer, is a project launched in 2017 by NASA with a specific goal: to observe neutron stars, the remnants of massive stars composed mainly, as the name suggests, of neutrons. Analysis of the data produced by this instrument regularly leads to announcements of results, such as the one on April 17, 2021, by two teams, one from the Anton Pannekoek Institute of Astronomy in Amsterdam (Netherlands) and the other from the Department of Astronomy at the University of Maryland (USA). These scientists focused on PSR J0740+6620, a particularly massive neutron star in our galaxy, located 4,600 light-years from Earth, and determined its diameter: about 24.8 kilometers for the European team, and 27.4 kilometers for the American.
Pulsars, objects that rotate very quickly on themselves
These results are consistent with each other when measurement and calculation uncertainties are taken into account. And above all, their production by two independent teams promises a certain robustness of the results. The researchers exploit the detection by Nicer of X-rays emitted by hot spots on the surface of these stars: the neutron stars observed here are pulsars, objects that rotate very quickly on themselves. Thus, the emission of X-rays depends on the face presented by the star to the telescope, and thus on its frequency and speed of rotation. These two quantities are related to the diameter, which can be calculated, making X-rays a relevant tool to determine the characteristics of the star.
"Quantum objects of macroscopic size"
Why study these distant stars? With a mass that can be twice that of the Sun, concentrated in a sphere with a diameter more than 50,000 times smaller, neutron stars have extraordinary characteristics that give them an extraordinary density. Hence the particular interest of physicists. The behavior of the particles that make up their core is at the limit of our knowledge in physics. "We consider them to be quantum objects of macroscopic size, a sort of super atomic nucleus, with a radius 1019 times larger!" describes Jérôme Margueron, director of research at the Institut de physique des deux infinis in Lyon.
The matter at the heart of these objects could well have an exotic behavior: "At very high pressure, neutrons and protons could get so close that we would see their constituents - the quarks - interacting directly," says the physicist. This makes neutron stars the objects of choice for testing models of particle interactions under extreme conditions. Theories, by modeling the forces that structure these stars, predict their properties; being able to observe them directly allows us to validate or not these predictions.
The data recorded by Nicer goes against many models
In the case of PSR J0740+6620, the data recorded by Nicer goes against many models: "This result indicates that the radius of a 2.1 solar mass neutron star like J0740 is comparable to that of a much lighter neutron star, J0030, with a measured radius of about 12 kilometers," says Jérôme Margueron. Many models predict a reduction in the radius of the star as its mass increases. It is now the turn of the theorists to work on their models, taking into account these new results and waiting for the new data that Nicer will produce in the years to come.