The neutron star "Black Widow" devoured its companion, becoming the heaviest thing found so far

The neutron star “Black Widow” devoured its companion, becoming the heaviest thing found so far

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

NASA’s Goddard Space Flight Center

Astronomers have identified the heaviest neutron star known to date, weighing 2.35 solar masses, according to a research paper recently published in Astrophysical Journal Letters. How did you 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 how massive neutron stars can be, with implications for our understanding of the quantum state of matter in their cores.

Neutron stars are the remnants of supernovae. As Ars Science editor John Timmer wrote last month:

The matter that makes up neutron stars begins as 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 experiences increasing pressures. The crushing force is enough to eliminate the boundaries between atomic nuclei, creating a giant soup of protons and neutrons. In the end, even the electrons in the region are forced to form many protons, converting them into neutrons.

This finally provides a force to compress the crushing force of gravity. Quantum mechanics prevents neutrons from occupying the same energy state in close proximity, and this prevents neutrons from getting too close, and thus prevents collapsing into a black hole. But it is possible that there is an intermediate state between a bubble of neutrons and a black hole, where the boundaries between neutrons begin to collapse, resulting in strange clusters of their constituent quarks.

Because there are no black holes, the nuclei 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 nuclear density, as it does in the nucleus of a uranium atom,” said Alex Filippenko, an astronomer at the University of California, Berkeley and co-author of the new research paper. “A neutron star is like a single giant core, but when you have 1.5 solar masses of that matter, roughly 500,000 Earth masses of cores all clinging to each other, it’s not at all clear how it’s going to behave.”

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 side facing the companion, evaporating it over time.

The neutron star featured in this latest research 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 rotating magnetic field emits beams of light 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 Radio Telescope (LOFAR), following 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 a whopping 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. That’s why these stars are known as Black Widow pulsars – what Filpenko calls “the state of cosmic ingratitude”:

The evolutionary path is quite remarkable. Double exclamation point. As the companion star evolves and begins to transform into a red giant, material seeps into the neutron star, and this orbits the neutron star. By spinning, it is now incredibly energetic, and a wind of particles begins to blow out of the neutron star. Then this wind hits the donor star and begins to strip the material, and over time, the mass of the donor star reduces to the mass of a planet, and if more time passes, it disappears completely. So, this is how millisecond pulsars can form. They were not alone at first—they had to be in a pair—but they gradually evaporated away from their comrades, and are now aloof.

This process explains how J0952 became so heavy. Such systems are a boon to scientists like Filippenko and his colleagues who are keen to weigh neutron stars accurately. 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.

Astronomers have measured the velocity of a faint star (a green circle) that has been stripped of almost all of its mass by an unseen companion, a neutron star and a millisecond pulsar, which they determine is the most massive yet and possibly the upper limit of neutron stars.
Zoom / Astronomers have measured the velocity of a faint star (a green circle) that has been stripped of almost all of its mass by an unseen companion, a neutron star and a millisecond pulsar, which they determine is the most massive yet 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 orbitally locked in orbit with the pulsar. The side facing J0952 is therefore very hot, with temperatures reaching 6,200 K (10,700 degrees Fahrenheit), making it bright enough to be observed by 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 will help put more constraints on the upper limit of how large neutron stars can become before collapsing into black holes, as well as win competing theories about the nature of quark soup in their cores. “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 adds to 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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