Astronomers have determined the heaviest neutron star known to date, weighing 2.35 solar masses, according to a recent paper published in Astrophysical Journal Letters. How did he get so big? Most likely by devouring a companion star – the celestial equivalent of a black widow spider devouring its companion. The work helps set an upper limit on the size of neutron stars, with implications for our understanding of the quantum state of matter at their core.
Neutron stars are the remnants of supernovae. As Ars Science editor John Timmer wrote last month:
The matter that forms neutron stars begins with ionized atoms near the core of a massive star. Once the star’s fusion reactions stop producing enough energy to counter the pull of gravity, this material contracts, coming 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 region’s electrons are forced into many protons, converting them into neutrons.
This finally provides a force to repel the overwhelming power of gravity. Quantum mechanics prevents neutrons from occupying the same energy state nearby, which prevents neutrons from getting closer and thus blocks the collapse into a black hole. But it’s possible that there is an intermediate state between a blob of neutrons and a black hole, a state where the boundaries between neutrons begin to break down, resulting in strange combinations of their constituent quarks.
Apart from black holes, the cores of neutron stars are the densest known objects in the Universe, and because they are hidden behind an event horizon, they are difficult to study. “We know roughly how matter behaves at nuclear densities, like in the nucleus of a uranium atom,” said Alex Filippenko, an astronomer at the University of California, Berkeley and co-author of the new paper. “A neutron star is like a giant nucleus, but when you have 1.5 solar masses of this stuff, or about 500,000 Earth masses of nuclei all hooked together, there’s no telling how they’ll behave.”
The neutron star featured in this final 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 by rotating and the rotating magnetic field emits beams of light in the form of radio waves, X-rays or gamma rays. Astronomers can spot pulsars as their beams sweep across Earth. J0952 was discovered in 2017 using the Low-Frequency Array Radio Telescope (LOFAR), following data on mysterious gamma-ray sources collected by NASA’s Fermi Gamma-ray Space Telescope.
Your average pulsar spins at about one rotation per second, or 60 per minute. But J0952 spins at 42,000 rpm, making it the second-fastest pulsar known to date. The currently favored hypothesis is that these types of pulsars were once part of binary systems, gradually stripping their companion stars until the latter evaporated. This is why these stars are known as Black Widow pulsars – what Filippenko calls a “case of cosmic ingratitude”:
The evolutionary path is absolutely fascinating. Double exclamation mark. As the companion star evolves and begins to become a red giant, matter spills over the neutron star, and this spins the neutron star. As it spins, it now becomes incredibly energized and a wind of particles begins to shoot out of the neutron star. This wind then hits the donor star and begins to remove material, and over time the mass of the donor star decreases to that of a planet, and if even more time passes, it completely disappears. This is how solitary millisecond pulsars could form. They weren’t alone at first – they must have been in a binary pair – but they gradually faded away from their companions, and now they’re lonely.
This process would explain how J0952 became so heavy. And such systems are a boon to scientists like Filippenko and his colleagues who want to accurately weigh neutron stars. The trick is to find neutron star binary systems in which the companion star is small but not too small to detect. Of the dozen Black Widow pulsars the team has studied over the years, only six met these criteria.
J0952’s companion star has 20 times the mass of Jupiter and is 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 spot with a large telescope.
Fillipenko et al. has 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 to 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 constraints on the upper limit of neutron star size before collapsing into black holes, as well as eliminate competing theories about the nature of quark soup. to their heart. “We can continue to search for black widows and similar neutron stars skating even closer to the edge of the black hole,” Filippenko 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.”
DOI: Astrophysical Journal Letters, 2022. 10.3847/2041-8213/ac8007 (About DOIs).
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