One of the most extreme stars in the Milky Way just got even more absurd.
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, with a mass 2.35 times the mass of the Sun.
If true, this is very close to the theorized upper mass limit of about 2.3 solar masses for neutron stars, representing an excellent laboratory for studying these ultradense stars on the brink of collapse , hoping to better understand the strange quantum state of the 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 core, but when you have a solar mass and a half of this material, which is about 500,000 Earth masses of cores all put together, it’s not at all clear how they’re going to behave.”
Neutron stars are the collapsed cores of massive stars that were 8 to 30 times the mass of the Sun before they went supernova and ejected most of their mass into space.
These nuclei, which tend to be around 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 sphere only about 20 kilometers (12 miles) across; at this density, protons and electrons can combine into neutrons. 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 that tipping point, where the neutrons form exotic structures or blur 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 what’s known as a pulsar: a rapidly spinning neutron star with beams of radiation emitting from the poles. As the star rotates, these poles pass in front of the observer (us) like a cosmic beacon so that the star appears to beat.
These stars can be incredibly fast, their rotation speed on millisecond scales. PSR J0952-0607 is the second fastest pulsar in the Milky Way, rotating at a mind-boggling 707 times per second. (The fastest is only slightly faster, with a rotation speed of 716 times per second).
It is also what is known as 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 spins and feeds into the neutron star, a bit like water swirling around a drain. The accretion disk’s angular momentum is transferred to the star, causing its rotation speed to increase.
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 small, less than 10 percent 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 new and precise measurement of the pulsar.
Their calculations returned a result of 2.35 times the mass of the Sun, with 0.17 solar masses. Assuming that the initial mass of a standard neutron star is about 1.4 times the mass of the Sun, this means that PSR J0952-0607 has absorbed all of the Sun’s worth of matter from its binary companion. This, the team says, is very important information to have about neutron stars.
“This provides some of the strongest constraints on the property of matter at several times the density observed in atomic nuclei. In fact, many popular models of dense matter physics are ruled out by this result,” Romani explained.
“A high maximum mass for neutron stars suggests that they are a mixture of nuclei and their up- and down-quarks dissolved into the core. This rules out many proposed states of matter, especially those with exotic interior compositions.”
The binary also shows a mechanism by which isolated pulsars, without binary companions, can have rotation speeds of milliseconds. J0952-0607’s companion is almost gone; once completely devoured, the pulsar (if it doesn’t tip over the upper mass limit and sink further into a black hole) will maintain its incredibly fast rotation rate for quite some time.
And it will be alone, like those other isolated millisecond pulsars.
“As the companion star evolves and starts to become a red giant, material is poured into the neutron star, and this causes the neutron star to spin. As it spins, it is now incredibly energized , and a wind of particles starts coming out of the neutron. Then that wind hits the donor star and starts ejecting material, and over time the mass of the donor star decreases to that of a planet, and if it still more time passes, it disappears altogether,” Filippenko said.
“This is how solitary millisecond pulsars could form. They weren’t alone to begin with, they had to be in a binary pair, but gradually their companions evaporated and now they’re alone.”
The research has been published in The Astrophysical Journal Letters.