Neutron stars are formed when the core of a massive star collapses. They are small but extremely dense. But how massive can such a star remnant become? Astronomers have now found that a particularly fast-rotating pulsar around 3,000 light-years away could be very close to the upper limit of what is possible. According to their measurement data, this pulsar has a mass of 2.35 solar masses. This makes it the heaviest and densest neutron star in the Milky Way to date. If this pulsar weren’t rotating extremely fast, it would probably already have collapsed into a black hole. The neutron star could owe its large mass to its companion star, from which it has subtracted matter.
Neutron stars are only 10 to 15 kilometers across, but can weigh one to two solar masses. Thus belong to the densest objects in the cosmos. The matter within them is so compressed that a teaspoonful of it would weigh billions of tons. The pressure inside causes even atoms to disintegrate and electrons to fuse with protons. This creates neutrons, so that these stellar remnants consist only of neutrons and their building blocks, the quarks. But how heavy can a neutron star get? It is clear that there must be an upper mass limit from which such a celestial body collapses into a black hole. In 2018, astrophysicists were able to determine an upper limit of 2.16 solar masses for non-rotating neutron stars using the equations of state that describe the interdependence of pressure, volume and temperature in these objects. However, it is still unclear where this limit lies in the case of rapidly rotating neutron stars, for example in the form of millisecond pulsars. This is because the forces generated by the rotation of these stellar remnants counteract the gravitation and thereby stabilize it.
Fast rotating heavyweight
Now the fastest known neutron star in the Milky Way is providing new information on this question. Discovered in 2017, the pulsar PSR J0952-0607 rotates on its axis at a rate of 42,000 times per minute, emitting strong radio and gamma-ray emissions. It is also a “black widow”: It is orbited by a companion star with a mass of only around 20 Jupiters, which it has almost completely eroded and destroyed over time. To determine the mass of this pulsar, Stanford University’s Roger Romani and his colleagues have now repeatedly targeted the unlikely pair using the 10-meter telescope at Keck Observatory in Hawaii. Using a high-resolution spectrograph, they observed tiny fluctuations in the light spectrum of the companion star. These told them how this star moves, but also what gravitational influence the neutron star has on it. From this they were able to determine the mass of the pulsar.
The measurements showed that the pulsar PSR J0952-0607 has a mass of around 2.35 solar masses. It is thus the heaviest known neutron star in the Milky Way – and is probably close to the absolute upper limit for neutron stars, as the astronomers report. “We will look for other black widows and similar neutron stars moving even closer to the black hole boundary,” says Romani. “But if we don’t find any, that strengthens the argument that a good 2.3 solar masses is the absolute upper limit.” PSR J0952-0607 likely owes its unusually large mass to the fact that it’s a black widow: over time, it has enormous amounts of material from its companion star and only then became a heavyweight. “From our data, we conclude that PSR J0952-0607 gained at least 0.5 solar masses, but more likely 1 solar mass, through accretion,” the astronomers report.
Notes on the inner workings
Determining the mass of this pulsar also allows conclusions to be drawn about what it looks like inside neutron stars. Because the matter in this remnant of the star is so strongly compressed that it is probably also the densest visible celestial object – only black holes are even denser. According to current models, the pressure inside the neutron stars is so high that the neutrons partially decay into their basic components, the up and down quarks. Together these particles form an exotic, superfluid liquid of neutrons and quarks. So far, however, it is unclear whether even more exotic particles might exist in the center of the stellar remnants, including strange quarks or the kaons composed of them. Because their formation depends on the conditions inside, determining the upper mass limit for neutron stars can reveal whether these particles can exist there.
According to Romani and his colleagues, the mass of the pulsar they have now determined speaks against such exotic mixtures. “A high maximum mass for neutron stars indicates that the mixture of neutrons and up and down quarks reaches the center,” explains Romani. “That rules out many of the more exotic states of matter proposed in some models.”
Source: Roger Romani (Stanford University) et al., The Astrophysical Journal Letters, in press; arXiv:2207.05124