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This neutron star is the most massive observed to date

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[EN VIDÉO] A “black widow” pulsar devours its companion
When talking about spiders, black widow spiders are the ones that devour their mates after mating. And astronomers have observed similar behavior in the sky. When a pulsar and a low mass star form a binary system. Faced with the radiation emitted by the pulsar, the star has little chance of surviving for long. (in English) © NASA Goddard

She is part of “black widows”, those dead stars that have devoured most of their companion star. A nickname that refers to these spiders which devour the male after thecoupling. Named PSR J0952-0607, she was first spotted in 2017 as a potential pulsar millisecond, i.e. having a rotation period between 1 and 10 milliseconds. And for good reason: it spins at the prodigious speed of 707 turns on itself per second, which makes it, according to a study published in the journal The Astrophysical Journal Letters and freely accessible, “the fastest spinning pulsar on the disk of the Milky Way » ! Several observation campaigns followed following these first measurements which made it possible to study the properties of this astonishing star.

It spins at the prodigious speed of 707 turns on itself per second

In practice, black widow spiders correspond to neutron stars which spin at astronomical speeds, allowing them to attract enough matter of their companion binary so that only a small fraction of mass solar. Only two dozen are currently known, as they are particularly hard to detect.

“As the companion star evolves and begins to become a red giantthe material spreads over theneutron star, and that makes the neutron star spin. As she spins, she becomes incredibly energized, and a wind of particles starts coming out of the neutron. This wind then hits the donor star and begins to remove material, and over time the mass of the donor star decreases to that of a planet and, if even more time passes, it completely disappears. Black Widows weren’t lonely to begin with — they must have been in a binary pair — but they gradually evaporated their companions, and now they’re lonely.” explains Alex Filippenko, co-author of the study and professor emeritus of astronomy at the University of California at Berkeley.

Its companion star has shrunk to the size of a giant planet

Thus, characterizing PSR J0952-0607 was not an easy task. As the researchers explain, if it could be detected thanks to the pulsed signal emitted by its rapid rotation, this signal is not however sufficient to characterize it. It is necessary to be able to observe its companion in light visible. But that’s the problem: she was almost completely devoured! Its mass has gone down to 20 times the mass of Jupiterbarely 2% of the mass of the Sun !

“These planet-like objects are the dregs of normal stars that contributed mass and cinematic momentspinning their pulsar companion at times of milliseconds and increasing their mass in the process”said Roger W. Romani, first author of the study and astronomer at Stanford University. It then undergoes a locking of tide : as its part closest to the black widow is much more attracted than the farthest part, it rotates synchronous and performs at the same time a turn on itself and a turn around the other star in 6.4 hours. It is this phenomenon that occurs in the case of the Pluto-Charon system, but also the Earth-Moon system!

And it is ultimately this locking that allowed its observation. Because the temperature of its face, on the neutron star side, climbs to 6,200 Kelvinslightly more than the temperature of surface of the Sunjust allowing it to be observed with great telescope. Thus, it is Keck I, a telescope of the eponymous observatory located on the island of Hawaii and possessing a mirror primary 10 meters in diameter, which was turned towards PSR J0952-0607, located at about 3,000 light years in the direction of constellation from Sextant. A total of six 15-minute observations have taken place over the past four years, in order to trap the system in a particular position of theorbit making the brightness acceptable for measurements.

The most massive neutron star observed to date

After these observations, the researchers compared the spectrum obtained with that of known stars in order to deduce the speed orbital of the star, to finally be able to calculate the mass of the neutron star which accompanies it. They found a mass of 2.35 ± 0.17 solar masses, making it theneutron star the most massive ever observed! Above this mass, these stars at extreme density could well turn into a black hole. “We can continue to search for black widows and similar neutron stars skating even closer to the edge of the black hole. 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.”says A.Filippenko.

But even for such a mass, researchers wonder about the nature of the star. It corresponds to a heart ofmassive star which contracted on itself, while the outer layers of this same star were blown away by a violent explosion called supernova. The heart has contracted so much that it is no longer a question of atoms this time, but of the particles which compose them, hence the term neutron star. The density averages 1017 kg/m3which is the same as that of the atomic nucleus! “We know roughly how matter behaves at nuclear densities, such as in the nucleus of a uranium atom.comments Alex Filippenko. A neutron star is like a giant nucleus, but when you have a solar mass and a half of that stuff, or about 500,000 Earth masses of nuclei all hooked together, there’s no telling how they’re going to behave. »

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