One of the most extreme stars in the Milky Way just got even weirder. Scientists have measured the mass of a neutron star called PSR J0952-0607 and found it to be the most massive neutron star yet discovered, with a mass 2.35 times that of the Sun.

If true, this is very close to the theoretical upper mass limit of about 2.3 solar masses for neutron stars, providing an excellent laboratory for studying these ultra-dense stars on what we believe are on the brink of collapse, in the hope to better understand the strange quantum state of the matter they are made of.

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

The most massive neutron star in the Universe

“A neutron star is like a giant nucleus, but when you have a solar mass and a half of this matter, which is about 500,000 Earth masses of nuclei all stuck together, it’s not at all clear how they’re going to behave.”

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

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

Their mass is packed into a sphere with a diameter of only 20 kilometers. At this density, protons and electrons can combine into neutrons.

The second fastest pulsar in the Milky Way

The only thing keeping this ball of neutrons from collapsing into a black hole is the force it would take for them to occupy the same quantum states, described as degeneracy pressure.

In a sense, this means that neutron stars behave like massive atomic nuclei. But what happens at this point is hard to say.

PSR J0952-0607 was already one of the most interesting neutron stars in the Milky Way. It’s what’s called a pulsar – a neutron star that rotates very quickly, with jets of radiation emitted from the poles. As the star rotates, these poles pass the observer in the manner of a cosmic beacon, so the star appears to pulsate.

These stars can be extremely fast, with their rotation rate being on the order of milliseconds. PSR J0952-0607 is the second fastest pulsar in the Milky Way, rotating at an astounding 707 times per second. The fastest is only slightly faster, with a rotation rate of 716 times per second.

An important clue about neutron stars

It is also what is called a “black widow” pulsar. The star is in close orbit with a binary companion – so close that its immense gravitational field pulls material away from the companion star. This material forms an accretion disk that swirls around the neutron star and feeds on it, much like water swirls around a drain.

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

Their calculations yielded 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 absorbed an amount of matter equivalent to that of an entire Sun from its binary companion. According to the team, this is very important information about neutron stars.

J0952-0607’s companion is almost gone

“A high mass for neutron stars suggests that it is a mixture of nuclei. This rules out many 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 spin rates of milliseconds. J0952-0607’s companion is almost gone. Once it is fully consumed, the pulsar will retain its fast rotation speed for quite a long time. And it will be alone, just like all the other isolated millisecond pulsars, he writes ScienceAlert.

“As the companion star evolves and begins to become a red giant, material pours into the neutron star, and this causes the neutron star to spin. As it spins, it becomes incredibly energized, and a wind of particles begins to emerge from the neutron star. This wind then hits the star and begins to remove material, and over time, its mass drops to that of a planet, and if even more time passes, it disappears altogether,” the team explains.

“So this is how millisecond solitary pulsars could form. At first they weren’t alone – they had to be in a binary pair – but gradually their companions evaporated, and now they’re solitary.”

The research was published in The Astrophysical Journal Letters.

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