This record-breaking ‘Black Widow’ pulsar is the most massive neutron star yet

One of the most extreme stars in the Milky Way just got even crazier.

Scientists measured the mass of a neutron star named PSR J0952-0607 and found it to be the most massive neutron star discovered to date, reaching 2.35 times the mass of the Sun.

If true, this is very close to the theoretical upper mass limit of about 2.3 solar masses for neutron stars, which represents an excellent laboratory to study these ultra-dense stars at what we think are at edge of collapse, hoping to better understand the strange quantum state of matter they are made of.

“We know roughly how matter behaves at nuclear densities, like in the nucleus of a uranium atom,” said astrophysicist Alex Filippenko of the University of California, Berkeley.

“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’ll behave.”

Neutron stars are the collapsed cores of massive stars that had between 8 and 30 times the mass of the Sun, before they went supernova and carried most of their mass out into space.

These nuclei, which tend to be about 1.5 times the mass of the Sun, are among the densest objects in the Universe; the only denser thing is a black hole.

Their mass is packed into a sphere about 20 kilometers (12 miles) in diameter; at this density, protons and electrons can combine into neutrons. The only thing that keeps this ball of neutrons from collapsing into a black hole is the force it would take for them to occupy the same quantum states, called degeneracy pressure.

In some ways this means that neutron stars behave like massive atomic nuclei. But what happens at this tipping point, where neutrons form exotic structures or coalesce into a soup of smaller particles, is hard to say.

PSR J0952-0607 was already one of the most interesting neutron stars in the Milky Way. It’s called a pulsar – a very fast spinning neutron star, with jets of radiation emitted from the poles. As the star rotates, these poles pass in front of the observer (us) like a cosmic beacon, so the star appears to be pulsating.

These stars can be incredibly fast, their rate of rotation on the scale of milliseconds. PSR J0952-0607 is the second fastest pulsar in the Milky Way, spinning at a breathtaking 707 times per second. (The fastest is only slightly faster, with a spin rate of 716 times per second.)

This is also called a “black widow” pulsar. The star is in a close orbit with a binary companion – so close that its immense gravitational field pulls material from the companion star. This material forms an accretion disk that swirls and powers the neutron star, much like water swirling around a drain. The angular momentum of the accretion disk is transferred to the star, causing its rotational speed to increase.

A team led by Stanford University astrophysicist Roger Romani wanted to better understand how PSR J0952-0607 fit into the timeline of this process. The binary companion star is tiny, less than 10% the mass of the Sun. The research team made careful studies of the system and its orbit and used this information to get a new, accurate measurement of the pulsar.

Their calculations gave a result of 2.35 times the mass of the Sun, plus or minus 0.17 solar masses. Assuming a standard neutron star starting mass of about 1.4 times the mass of the Sun, this means that PSR J0952-0607 has sucked up a Sun’s worth of matter from its binary companion. . According to the team, this is really important information to have about neutron stars.

“This provides some of the strongest constraints on the property of matter at many times the density seen in atomic nuclei. Indeed, many otherwise popular models of dense matter physics are precluded by this result,” Romani explained. .

“A high peak mass for neutron stars suggests that they are a mixture of nuclei and their rising and falling quarks dissolved down to the nucleus. This rules out many proposed states of matter, especially those with an exotic interior composition.”

The binary also shows a mechanism by which isolated pulsars, without binary companions, can have rotation speeds in milliseconds. J0952-0607’s companion is almost gone; once it is fully devoured, the pulsar (if not tipped over the upper mass limit and further collapses into a black hole) will retain its incredibly fast spin rate for some time .

And it will be alone, like all the other isolated millisecond pulsars.

“As the companion star evolves and begins to become a red giant, matter spills over the neutron star, and this spins the neutron star. As it spins, it now becomes incredibly energized, and a wind of particles starts coming out of the neutron This wind then hits the donor star and starts removing material, and over time the mass of the donor star decreases to that of a planet, and if even more time passes, it disappears completely,” Filippenko said.

“So that’s how solitary millisecond pulsars were able to form. They weren’t all alone to begin with – they must have been in a binary pair – but they gradually evaporated their companions, and now they’re solitary.”

The research has been published in Letters from the Astrophysical Journal.

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