Astronomers have identified the heaviest neutron star known to date, weighing 2.35 solar masses. final paper It was published in Astrophysical Journal Letters. How did it get so big? Most likely, by swallowing a companion star – the celestial equivalent of a black widow spider eating its mate. With implications for our understanding of the quantum state of matter in their cores, the work helps to set an upper bound on how large neutron stars can be.
Neutron stars are remnants of supernovae. Ars Science Editor as John Timmer wrote last month:
The matter that makes up neutron stars starts out as ionized atoms near the core of a massive star. When the star’s fusion reactions stop producing enough energy to counteract the gravitational force, that matter contracts and is subjected to increasingly greater pressures. The crushing force is enough to dissolve the boundaries between atomic nuclei, creating a giant soup of protons and neutrons. Eventually, even the electrons in the region are forced into most of the protons and turn them into neutrons.
This finally provides a force to push back against the overwhelming force of gravity. Quantum mechanics prevents neutrons from occupying the same energy state at close range, and this prevents the neutrons from getting any closer, thus preventing it from collapsing into a black hole. But it is possible that there is an intermediate state between a block of neutrons and a black hole, where the boundaries between neutrons begin to break down, resulting in strange combinations of constituent quarks.
Other than black holes, the nuclei of neutron stars are the densest known objects in the Universe and are difficult to study because they are hidden behind an event horizon. “We know roughly how matter behaves at nuclear densities, as 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 similar to a giant core, but when you have 1.5 solar masses of this material, about 500,000 Earth-mass cores all stuck together, it’s not clear how they will behave.”
The neutron star featured in this latest article is a pulsar, PSR J0952-0607, or J0952 for short, located 3,200 to 5,700 light-years from Earth in the constellation Sextans. Neutron stars are born by spinning, and the rotating magnetic field emits beams of light in the form of radio waves, X-rays, or gamma rays. Astronomers can detect pulsars when their rays sweep the Earth. J0952 (previous value) discovered in 2017 Through the Low Frequency Array (LOFAR) radio telescope, it is tracking data on mysterious gamma-ray sources collected by NASA’s Fermi Gamma-ray Space Telescope.
Your average pulsar rotates at about one rotation per second, or 60 rotations per minute. But J0952 spins at 42,000 revolutions per minute, making it the second fastest pulsar ever known. The current preferred hypothesis is that such pulsars were once part of binary systems and gradually eroded until their companion stars evaporated and disappeared. This is why such stars are called black widow pulsars. Filippenko calls and “a case of cosmic ingratitude”:
The evolutionary path is absolutely fascinating. Double exclamation mark. As the companion star evolves and becomes a red giant, material overflows into the neutron star, which spins the neutron star. Spinning, it is now incredibly energized and a wind of particles begins to emanate from the neutron star. This wind then hits the donor star and begins to rob the material, and over time the mass of the donor star decreases to the mass of a planet, and as more time passes, it disappears altogether. So, millisecond stand-alone pulsars can be created this way. At first they were not all alone – they were supposed to be in a double pair – but gradually they evaporated from their friends and are now alone.
This process will explain how J0952 got so heavy. And such systems are a boon for scientists like Filippenko and their colleagues who want to weigh neutron stars precisely. The trick is to find neutron star binary systems where the companion star is small but not too small to be detected. Of the dozen or so black widow pulsars the team has studied over the years, only six met this criterion.
J0952’s companion star is 20 times the mass of Jupiter and is tidally locked in orbit with the pulsar. The side facing J0952 is therefore quite hot, reaching a temperature of 6,200 Kelvin (10,700° F), making it bright enough to be detected with a large telescope.
Filipenko et al. has spent the last four years making six observations of J0952 with the 10-metre Keck telescope in Hawaii, capturing 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 similar Sun-like stars to determine orbital velocity. This allowed them to calculate the mass of the pulsar.
Finding even more such systems would help place more constraints on the upper bound on how large neutron stars can be before collapsing into black holes, while also debunking rival theories about the nature of quark soup in their cores. We can continue to look for black widows and similar neutron stars even closer to the black hole’s threshold. said Filippenko. “But if we don’t find anything, it tightens up the argument that 2.3 solar masses is the true limit, and beyond that it turns into black holes.”
DOI: Astrophysical Journal Letters, 2022. 10.3847/2041-8213/ac8007 (About DOIs).
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