The neutron star "Black Widow" devoured its companion, becoming the heaviest star ever found

The neutron star “Black Widow” devoured its companion, becoming the heaviest star ever found

Zoom / A rotating neutron star periodically oscillates radio (green) and gamma (purple) rays in front of the Earth. The Black Widow pulsar heats the opposite side of its partner star to temperatures twice the surface temperature of the Sun and slowly evaporates.

NASA Goddard Space Flight Center

Astronomers have identified the heaviest neutron star known to date, at 2.35 solar masses, according to a recent publication published in 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 companion. The work helps put an upper bound 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 science editor for Ars John Timmer wrote last month:

The matter that makes up neutron stars begins with ionized atoms near the core of a massive star. Once a star’s fusion reactions stop producing enough energy to counteract the gravitational pull, this material contracts, and is subjected to increasing pressure. The crushing force is enough to remove the boundaries between the atomic nuclei, creating a giant soup of protons and neutrons. Eventually, even the region’s electrons are forced to form many protons, turning them into neutrons.

This finally provides strength to repel the crushing force of gravity. Quantum mechanics prevents neutrons from occupying the same near-energy state, which prevents neutrons from getting close and thus preventing collapse into a black hole. But it is possible that there is an intermediate state between a mass of neutrons and a black hole, a state in which the boundaries between neutrons begin to collapse, resulting in strange clusters of their constituent quarks.

Aside from black holes, the nuclei of neutron stars are the densest 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 nuclear densities, as it does in the nucleus of a uranium atom,” says Alex Filippenko, an astronomer at the University of California, Berkeley and co-author of the new paper. “A neutron star is like a giant core, but when you have 1.5 solar masses of that matter, or about 500,000 Earth masses of cores all bound together, there’s no telling how it behaves.”

This animation shows a black widow’s pulsar with her young stellar companion. The strong radiation and “wind” of the pulsar – a stream of high-energy particles – strongly heat the opposite side of its companion, evaporating it over time.

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 as their beams sweep across the Earth. J0952 was discovered in 2017 thanks to the Low Frequency Array Radio Telescope (LOFAR), tracking data on mysterious gamma ray sources collected by NASA’s Fermi Gamma Ray Space Telescope.

The pulsar rotates at a rate of about one rotation per second, or 60 per minute. But J0952 is spinning at 42,000 revolutions per minute, making it the second-fastest pulsar known to date. The currently preferred 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. Filippenko calls a “state of cosmic ingratitude”:

The evolutionary path is quite remarkable. Double exclamation point. When the companion star evolves and begins to transform into a red giant, the material seeps over the neutron star, and this spins the neutron star. As it spins, it’s now incredibly energetic and a wind of particles starts blasting off the neutron star. Then this wind hits the donor star and begins removing the material, and over time the mass of the donor star decreases to the mass of the planet, and if more time passes, it disappears completely. This is how single millisecond pulsars can form. They were not alone at first—they must have been in a pair—but gradually faded away from their companions, and they are now lonely.

This process explains how J0952 became so heavy. Such systems are a boon for scientists like Filippenko and his colleagues who want to accurately measure the weight of neutron stars. The trick is to find binary systems of neutron stars in which the companion star is small but not too small to be detected. Of the dozens of Black Widow pulsars the team has studied over the years, only six met these criteria.

Zoom / Astronomers have measured the speed of a faint star (a green circle) that has been stripped of nearly all of its mass by an unseen companion, a neutron star and a millisecond pulsar that they determine is the most massive star ever discovered and possibly the upper limit of neutron stars.

WM Keck Observatory, Roger W. Romani, Alex Filippenko

J0952’s companion star is 20 times the mass of Jupiter and is trapped in orbit with the pulsar. So the side facing J0952 is very hot, with temperatures reaching 6,200 K (10,700 degrees Fahrenheit), making it bright enough to detect it with a large telescope.

Filpenko et al. He’s spent the past four years making six observations of J0952 using the 10-meter Keck 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 more of these systems would help put more constraints on the upper limit of a neutron star’s size before it collapses into black holes, as well as eliminate competing theories about the nature of quark soup. “We can continue to search for black widows and similar neutron stars skating near 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, after which they become black holes.”

DOI: Astrophysical Journal Letters, 2022. 10.3847/2041-8213/ac8007 (about DOIs).

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