One of the most extreme stars in the Milky Way has just become even more wacky.
Scientists have measured the mass of a neutron star named PSR J0952-0607, and have determined that it is the most massive neutron star ever discovered, clocking in at a whopping 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, making this an excellent laboratory for studying these ultra-dense stars at what we believe is on the verge of collapse in the hope of better understanding the strange quantum state of the matter from which they are made.
"We know roughly how matter behaves at nuclear density, like in the nucleus of a uranium atom," said astrophysicist Alex Filippenko of the University of California, Berkeley.
"A neutron star is like a gigantic nucleus, but when you have one-and-a-half-solar masses of this stuff, which is about 500,000 Earth masses of nuclei all clinging together, it's not at all clear how they'll behave."
Neutron stars are massive stars that collapsed into their cores that were between 8 and 30 times the size of the Sun, before they went supernova and plummeted most of their mass into space.
These are some of the most dense objects in the Universe, with the only thing that isn't a black hole.
Their mass is encased in a sphere about 20 kilometers (12 miles) across; at that 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 maintain the same quantum states, described as degeneracy pressure.
Neutron stars act like massive atomic nuclei in some ways. But what happens at this critical point, when neutrons form unusual structures or blur into a soup of smaller particles, is elusive.
PSR J0952-0607 was already one of the most interesting neutron stars in the Milky Way. A pulsar is a neutron star that is spinning very rapidly, with blasts of radiation emitted from the poles. These beams sweep past the observer (us) in the manner of a cosmic lighthouse.
PSR J0952-0607 is the second-fastest pulsar in the Milky Way, rotating a staggering 707 times per second. (The fastest is only slightly faster, with a rotation rate of 716 times per second.)
It's also known as a "black widow" pulsar. The star is in a close proximity with a binary companion so close that it pulls material from it. This material forms an accretion disk that swirls around and feeds into the neutron star, a bit like water flowing through a drain. The star's spin rate increases.
A Stanford University research group led by astrophysicist Roger Romani aimed to understand better how PSR J0952-0607 fit into the timeline of this process. The binary companion star is tiny, only ten percent of the Sun's mass, and the group then used that data to make a new, precise measurement for the pulsar.
PSR J0952-0607 has slurped up to an entire Sun's worth of matter from its binary companion, according to the researchers.
"This provides some of the most significant constraints on the property of matter at several times the density seen in atomic nuclei." Indeed, many otherwise popular theories of dense-matter physics are excluded by this result.
"A high maximum mass for neutron stars suggests that it is a mixture of nuclei and their dissolved up and down quarks all the way to the core."
The binary also illustrates a mechanism by which isolated pulsars, without binary companions, may have millisecond rotation rates; once the companion is completely eaten, the pulsar (if it is not tipped over the upper mass limit and collapses further into a black hole) will maintain its insanely fast rotation speed for quite some time.
All other millisecond pulsars will be alone.
"Material spills over to the neutron star as it develops and begins to become a red giant," Filippenko said. A wind of particles then strikes the donor star and starts stripping material off, and over time, the donor star's mass reduces to that of a planet.
"So, that's how lone millisecond pulsars could be created." They weren't all alone to begin with, they had to be in a binary pair, but slowly they vanished away their companions, and now they're solitary.
The research has been published in The Astrophysical Journal Letters.