Astronomers have identified the heaviest neutron star known to date, weighing 2.35 solar masses, according to recent paper published in Astrophysical Journal Letters. How did it get so big? Most likely by engulfing a companion star – the celestial equivalent of a black widow spider devouring its mate. The work helps establish an upper limit on how large neutron stars can get, with implications for our understanding of the quantum state of matter in their cores.
Neutron stars are remnants of supernovae. As Ars Science editor John Timmer wrote last month:
The matter that forms neutron stars begins as ionized atoms near the core of a massive star. Once the star’s fusion reactions stop producing enough energy to counteract the pull of gravity, this matter contracts under increasing pressure. The crushing force is enough to eliminate the boundaries between atomic nuclei, creating a giant soup of protons and neutrons. Eventually, even the electrons in the region are pushed into many of the protons, turning them into neutrons.
This finally provides a repulsive force against the crushing force of gravity. Quantum mechanics prevents neutrons from occupying the same energy state in close proximity, and this prevents neutrons from getting closer and thus blocking the collapse into a black hole. But it is possible that there is an intermediate state between a blob of neutrons and a black hole, where the boundaries between neutrons begin to break down, resulting in strange combinations of their constituent quarks.
With the exception of black holes, the cores of neutron stars are the densest known objects in the universe, and because they are hidden behind the event horizon, they are difficult to study. “We know approximately how matter behaves at nuclear densities, such as in the nucleus of a uranium atom,” said Alex Filipenko, an astronomer at the University of California, Berkeley, and co-author of the new paper. “A neutron star is like one giant core, but when you have 1.5 solar masses of this thing, which is about 500,000 Earth masses of cores all stuck together, it’s not at all clear how they’re going to behave.”
The neutron star featured in this latest article is a pulsar, PSR J0952-0607, or J0952 for short, located in the constellation Sextans between 3,200 and 5,700 light-years from Earth. Neutron stars are born spinning, and the spinning magnetic field emits beams of light in the form of radio waves, X-rays, or gamma rays. Astronomers can spot pulsars when their rays pass through Earth. J0952 was opened in 2017 thanks to the LOFAR (Low-Frequency Array) radio telescope, following data on mysterious gamma-ray sources collected by NASA’s Fermi Gamma-ray Space Telescope.
Your average pulsar rotates at approximately one rotation per second, or 60 per minute. But J0952 spins at a whopping 42,000 revolutions per minute, making it the second fastest known pulsar to date. The current preferred hypothesis is that these types of pulsars were once part of binary systems, gradually removing their companion stars until the latter evaporated. That’s why such stars are known as black widow pulsars—what Filipenko calls “a case of cosmic ingratitude”:
The evolution path is absolutely fascinating. Double exclamation mark. As the companion star evolves and begins to become a red giant, material spills toward the neutron star and this spins the neutron star. As it spins up, it becomes incredibly energized, and a wind of particles begins to emerge from the neutron star. This wind then hits the donor star and starts ejecting material, and over time the mass of the donor star decreases to that of a planet, and if more time passes, it disappears completely. This is how solitary millisecond pulsars can form. At first they were not alone—they were supposed to be in a binary pair—but gradually they evaporated from their companions and are now solitary.
This process would explain how J0952 became so massive. And such systems are a boon to scientists like Filipenko and his colleagues who want to precisely weigh neutron stars. The trick is to find binary neutron star systems in which the companion star is small, but not too small to be detected. Of the dozen black widow pulsars the team has studied over the years, only six meet these criteria.
J0952’s companion star is 20 times the mass of Jupiter and locked in orbit with the pulsar. The side facing J0952 is therefore quite hot, reaching temperatures of 6,200 Kelvin (10,700° F), making it bright enough to be seen with a large telescope.
Filipenko et al. spent the past four years making six observations of J0952 with the Keck 10-meter telescope in Hawaii to catch the companion star at specific points in its 6.4-hour orbit around the pulsar. They then compared the resulting spectra with the spectra of Sun-like stars to determine the orbital velocity. This, in turn, allowed them to calculate the mass of the pulsar.
Finding even more such systems would help put further limits on the upper limit of how big neutron stars can get before collapsing into black holes, as well as dispel competing theories about the nature of the quark soup in their cores. “We can continue to look for black widows and similar neutron stars that move even closer to the edge of the black hole.” Filipenko said. “But if we don’t find any, it strengthens the argument that 2.3 solar masses is the true limit beyond which they become black holes.”