Astronomers have identified the heaviest neutron star known to date. final paper Published in Astrophysical Journal Letters. How did it get so big? Most likely by eating a companion star – the celestial equivalent of a black widow spider eating its own mate. The work helps put an upper limit on how massive neutron stars can get, affecting our understanding of the quantum state of matter in their cores.
Neutron stars are remnants of supernovae. Like Ars Science Editor John Timmer wrote last month:
The matter that forms neutron stars begins as ionized atoms near the core of a large star. After the star’s fusion reactions stop producing enough energy to counteract gravity, this matter is compressed and subjected to ever greater pressures. The crushing force is enough to break 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, turning them into neutrons.
This ultimately provides a force to push back against the crushing force of gravity. Quantum mechanics prevents neutrons from occupying the same energy state in close proximity, which prevents neutrons from getting closer and thus preventing them from collapsing into a black hole. But perhaps there is an intermediate state between a neutron blob and a black hole, where the boundaries between neutrons begin to break down, resulting in strange combinations of the quarks that make them up.
Because there are no 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 roughly how matter behaves at a nuclear density like that in the nucleus of a uranium atom.” Alex Filippenko said, an astronomer at the University of California, Berkeley, and co-author of the new paper. “A neutron star looks like one giant nucleus, but it’s not at all clear how the matter will behave when it has 1.5 solar masses, or about 500,000 Earth nuclei stuck together.”
The neutron star featured in this latest article is the pulsar PSR J0952-0607, or J0952 for short, located 3,200-5,700 light-years from Earth in the constellation Sextants. Neutron stars are born spinning, and the spinning magnetic field emits light beams in the form of radio waves, X-rays, or gamma rays. Astronomers can see pulsars when their rays sweep across the Earth. It was J0952 It was discovered in 2017 Thanks to the Low-Frequency Array (LOFAR) radio telescope, it tracks data on mysterious gamma-ray sources collected by NASA’s Fermi Gamma-ray Space Telescope.
Your average pulsar rotates at about one revolution per second, or 60 revolutions per minute. But J0952 spins at 42,000 revolutions per minute, making it the second-fastest pulsar ever. The current preferred hypothesis is that such pulsars were once part of binary systems and gradually destroyed their companion stars until the latter evaporated. Therefore, such stars are known as black widow pulsars Filippenko calls “in cosmic ingratitude”:
The path of evolution is absolutely fascinating. Double exclamation mark. As the companion star evolves and begins to become a red giant, material is dumped onto the neutron star, which spins the neutron star. As it spins, it is now incredibly energized, and a wind of particles begins to shoot out of the neutron star. That wind then hits the donor star and begins stripping material, and over time the mass of the donor star decreases to the mass of the planet, and if more time passes, it disappears altogether. This is how single millisecond pulsars can form. They weren’t alone to begin with – they were supposed to be in a double pair – but gradually they drifted away from their companions and are now alone.
This process will explain how J0952 is accreting. And such systems are a boon to scientists like Filippenko and his colleagues who want to measure the mass of neutron stars with precision. The trick is to find neutron star binary systems where the companion star is small but not too small to detect. Of the dozen or so black widow pulsars the team studied over the years, only six met these criteria.
J0952’s companion star is 20 times the mass of Jupiter and orbits with the pulsar. Thus, the side facing J0952 is quite hot, reaching a temperature of 6,200 Kelvin (10,700 °F), making it bright enough to see with a large telescope.
Filipenko and b. Over the past four years, it has conducted six observations of J0952 with the 10-meter Keck telescope in Hawaii to capture the companion star at certain 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, made it possible to calculate the mass of the pulsar.
Finding more such systems would help put further limits on how large neutron stars can be before collapsing into black holes, as well as dispel competing theories about the nature of the quark soup they contain. “We can continue to look for black widows and similar neutron stars 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.”