This ‘Black Widow’ pulsar is the most massive neutron star to date

One of the most extreme stars in the Milky Way is getting even more excited.

Scientists have measured the mass of a neutron star called PSR J0952-0607, and found it to be the most massive neutron star discovered so far, at 2.35 times the mass of the Sun.

If true, this is very close to the theoretical maximum mass of about 2.3 solar masses for neutron stars, which is an excellent laboratory for studying these ultra-dense stars in what we think are on the verge of collapse, in the hope of a better understanding. The state of an odd quantity of the material it is made of.

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

“A neutron star is like one giant core, but when you have one and a half solar masses of that matter, which is about 500,000 Earth masses of cores all clinging to each other, it’s not at all clear how they’ll behave.”

Neutron stars are the collapsed cores of massive stars that had a mass between 8 and 30 times the mass of the Sun, before the supernova exploded and blew most of their mass into space.

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

Its mass is packed into a ball 20 kilometers (12 miles) or so wide; At this density, protons and electrons can combine to form neutrons. The only thing stopping this ball of neutrons from collapsing into a black hole is the force it would take for it to occupy the same quantum states, which is described as the degeneracy pressure.

In some ways, this means that neutron stars behave like massive atomic nuclei. But it’s hard to say what happens at this tipping point, as neutrons form strange structures or transform into a jumble of smaller particles.

PSR J0952-0607 was indeed one of the most exciting neutron stars in the Milky Way. It’s what’s known as a pulsar – a neutron star that rotates very quickly, with jets of radiation releasing from the poles. As the star rotates, these poles pass through the observer (us) in the manner of a cosmic beacon so that the star appears to be pulsing.

These stars can be insanely fast, spinning on millisecond scales. PSR J0952-0607 is the second-fastest pulsar in the Milky Way, spinning at an astonishing speed of 707 times per second. (The fastest is slightly faster, with a spin rate of 716 times per second.)

It’s also what’s known as a “black widow” pulsar. The star is in a close orbit with a binary companion — so close that the massive gravitational field pulls material out of the companion star. This material forms an accretion disk that orbits and feeds on the neutron star, like water circling a drain. Angular momentum is transferred from the accretion disk to the star, increasing its rate of rotation.

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

Their calculations returned a result of 2.35 times the mass of the Sun, which is 0.17 solar masses. Assuming a standard neutron star starting with a mass of about 1.4 times the mass of the Sun, this means that PSR J0952-0607 could inflame up to the entire Sun’s value of matter from its binary companion. The team says this is really important information about neutron stars.

“This provides some of the strongest constraints on the property of matter at many times the density seen in atomic nuclei. In fact, many common models of dense matter physics have been ruled out by this result,” Romani explained.

“The high maximum mass of neutron stars suggests that they are a mixture of cores and quarks melting up and down to the core. This excludes many of the proposed states of matter, especially those with a strange internal configuration.”

The binary system also shows a mechanism by which isolated pulsars, without binary companions, can have millisecond rotation rates. Companion J0952-0607 is nearly gone; Once fully devoured, the pulsar (if it does not tilt above the upper limit of mass and collapses further into the black hole) will maintain its insanely fast rotational speed for some time.

And she’d be on her own, just like all the other solitary millisecond pulsars.

“As the companion star evolves and begins to transform into a red giant, the material is seeping into the neutron star, and that’s orbiting the neutron star. Through spinning, it’s now incredibly energetic, and a wind of particles starts to come out of the neutron star. Then that wind hits the star. The donor star begins to strip matter, and with time, the mass of the donor star decreases to the mass of a planet, and if more time passes, it completely disappears.”

“So, this is how millisecond pulsars can be formed. They weren’t alone at first—they had to be in a binary pair—but they gradually evaporated away from their companions, and are now isolated.”

The search was published in Astrophysical Journal Letters.

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