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The neutron star “Black Widow” gobbled up its partner and became the heaviest yet found

The neutron star "Black Widow" gobbled up its partner and became the heaviest yet found
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A spinning neutron star periodically sweeps its radio (green) and gamma rays (magenta) past Earth.  A black widow pulsar heats the far side of its stellar partner to temperatures twice that of the Sun's surface and slowly vaporizes.
Enlarge / A spinning neutron star periodically sweeps its radio (green) and gamma rays (magenta) past Earth. A black widow pulsar heats the far side of its stellar partner to temperatures twice that of the Sun’s surface and slowly vaporizes.

NASA’s Goddard Space Flight Center

According to a, astronomers have identified the heaviest neutron star known to date, weighing 2.35 solar masses newer paper published in the Astrophysical Journal Letters. how did it get so big Most likely by devouring a companion star – the celestial equivalent of a black widow spider devouring its mate. The work is helping place an upper limit on how large neutron stars can get, which has implications for our understanding of the quantum state of matter at their cores.

Neutron stars are the 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 and experiences increasing pressure. The refractive power is enough to break down the boundaries between atomic nuclei, creating a huge soup of protons and neutrons. Eventually even the electrons in the region will be forced into many of the protons, turning them into neutrons.

This eventually provides a force to push back against the crushing force of gravity. Quantum mechanics prevents neutrons in close proximity from assuming the same energy state, and this prevents the neutrons from getting closer, thus blocking collapse into a black hole. But it’s possible that there is an intermediate state between a neutron clump and a black hole, a state in which the boundaries between neutrons begin to break down, resulting in strange combinations of their quarks.

Aside 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 density, 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 article. “A neutron star is like a giant core, but when you have 1.5 solar masses of this material, which is about 500,000 Earth masses of cores all stuck together, it’s not at all clear how they’re going to behave.”

This animation shows a Black Widow pulsar along with its small stellar companion. Strong radiation and the pulsar’s “wind” — an outflow of high-energy particles — intensely heat the far side of the companion and vaporize it over time.

The neutron star featured in this latest article is a pulsar, PSR J0952-0607—or J0952 for short—in the constellation of Sextant, located between 3,200 and 5,700 light-years from Earth. Neutron stars are born rotating, and the rotating magnetic field emits light beams in the form of radio waves, X-rays, or gamma rays. Astronomers can detect pulsars as their beams sweep across the Earth. J0952 was Discovered in 2017 thanks to the Low-Frequency Array (LOFAR) radio telescope, which is following up data on mysterious gamma-ray sources collected by NASA’s Fermi Gamma-ray Space Telescope.

Your average pulsar spins at about one revolution per second, or 60 per minute. But J0952 spins at a whopping 42,000 rpm, making it the second fastest known pulsar to date. The currently favored hypothesis is that these types of pulsars were once part of binary star systems and gradually stripped away from their companion stars until the latter evaporated. That’s why such stars are known as black widow pulsars – what Filippenko calls a “case of cosmic ingratitude”:

The evolutionary path is absolutely fascinating. Double exclamation point. As the companion star evolves and begins to become a red giant, material spills over to the neutron star, spinning the neutron star up. It is now incredibly energized by spinning up, and a wind of particles begins to come out of the neutron star. This wind then hits the donor star and begins to shed material, and over time the donor star’s mass decreases to that of a planet, and as more time passes, it disappears entirely. This is how individual millisecond pulsars could be formed. They weren’t entirely alone at first—they had to be in a binary pair—but they’ve gradually drifted away from their mates and are now loners.

This process would explain why 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 binaries where the companion star is small but not too small to detect. Of the dozen or so Black Widow pulsars the team has studied over the years, only six met these criteria.

Astronomers have measured the speed of a faint star (green circle) stripped of almost all of its mass by an unseen companion, a neutron star and millisecond pulsar, which they determine to be the most massive yet found and perhaps the upper limit for neutron stars to have .
Enlarge / Astronomers have measured the speed of a faint star (green circle) stripped of almost all of its mass by an unseen companion, a neutron star and millisecond pulsar, which they determine to be the most massive yet found and perhaps the upper limit for neutron stars to have .

WM Keck Observatory, Roger W. Romani, Alex Filippenko

J0952’s companion star has 20 times the mass of Jupiter and is tidally linked to the pulsar in orbit. The side facing J0952 is therefore quite hot, reaching temperatures of 6,200 Kelvin (10,700 °F), making it bright enough to be spotted with a large telescope.

Fillipenko et al. spent the last four years making six observations of J0952 with the 10-meter Keck telescope in Hawaii to capture 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 similar 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 further constrain the upper limit on how large neutron stars can grow before collapsing into black holes and disprove competing theories about the nature of the curd soup at their cores. “We can continue to search for black widows and similar neutron stars slipping even closer to the edge of the black hole,” said Filippenko. “But if we don’t find any, it reinforces 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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