Astronomers have determined the heaviest neutron star ever known, weighing in at 2.35 solar masses, according to a Recent paper Published in The Astrophysical Journal Letters. How did it get so big? Perhaps a mate devours the star—the celestial equivalent of a black widow spider devouring its mate. The work helps set an upper limit on how large neutron stars can get, with implications for our understanding of the quantum state of matter in their cores.
Neutron stars are remnants of supernovae. As Ars Science Editor John Timmer says Wrote last month:
The material that forms neutron stars begins as ionized atoms near the center of a massive star. Once the star’s fusion reactions stop producing enough energy to counteract the gravitational pull, this matter contracts, experiencing greater pressure. The crushing force is enough to obliterate the boundaries between atomic nuclei, creating a giant soup of protons and neutrons. Eventually, even electrons in this region are forced into many protons, converting them into neutrons.
This eventually provides a force to push back against the crushing force of gravity. Quantum mechanics prevents neutrons from occupying the same energy states as nearby ones, and this prevents neutrons from coming any closer and therefore blocking collapse into a black hole. But it is possible that there is an intermediate state between a blob of neutrons and a black hole, where the boundary between neutrons begins to break down, resulting in odd combinations of their constituent quarks.
Short of black holes, the cores of neutron stars are the most dense known objects in the universe, and because they are hidden behind an event horizon, they are difficult to study. “We know roughly how matter behaves at atomic concentrations, such as in the nucleus of a uranium atom,” Alex Filippenko says, an astronomer at the University of California, Berkeley and co-author of the new paper. “A neutron star is like a giant nucleus, but when you have 1.5 solar masses of this material, which is about 500,000 Earth-mass nuclei all stuck together, it’s not at all clear how they behave.”
The neutron star featured in this latest paper is a pulsar, PSR J0952-0607—or J0952 for short—located in the constellation Sexton, 3,200 to 5,700 light-years from Earth. Neutron stars are born spinning, and the spinning magnetic field emits beams of light in the form of radio waves, X-rays, or gamma rays. Astronomers can see pulsars as their beams sweep across Earth. J0952 was Discovered in 2017 Thanks to the Low-Frequency Array (LOFAR) radio telescope, NASA’s Fermi Gamma-ray Space Telescope follows up on data on mysterious gamma-ray sources.
Your average pulsar is spinning at roughly one rotation per second, or 60 per minute. But J0952 is spinning at 42,000 revolutions per minute, making it the second-fastest-known pulsar to date. The current favored hypothesis is that these types of pulsars were once part of binary systems, slowly stripping their companion stars until the latter evaporated. This is why such stars are called black widow pulsars—what Filipenko called A “case of cosmic ingratitude”:
The evolutionary path is absolutely fascinating. Double exclamation point. As the companion star evolves and begins to become a red giant, material is shed into the neutron star and it spins the neutron star. As it spins, it now becomes incredibly powerful and a wind of particles is ejected from the neutron star. That wind then hits the donor star and begins to strip away material, and over time, the donor star’s mass decreases to that of a planet, and if more time passes, it disappears entirely. So, this is how a single millisecond pulsar can be created. They weren’t alone to begin with – they had to be in a binary pair – but they slowly evaporated their mates and now they’re lonely.
This process would explain how J0952 became so massive. And such systems are a boon to scientists like Filippenko and his colleagues who are interested in accurately weighing neutron stars. The trick is to find neutron star binary systems where the companion star is small but not too small to detect. Of the dozen or so black widow pulsars the team studied over the years, only six met that criteria.
J0952’s companion star is 20 times the mass of Jupiter and is tidally locked to the pulsar. The side of J0952 is quite hot, reaching temperatures of 6,200 Kelvin (10,700 °F), making it bright enough to be seen with a large telescope.
Filippenko etc Six observations of J0952 with the 10-meter Keck Telescope in Hawaii have spent the past four years catching the companion star at specific points in its 6.4-hour orbit around the pulsar. They then compared the resulting spectra with spectra of similar Sun-like stars to determine orbital velocities. This, in turn, allowed them to calculate the mass of the pulsar.
Finding more such systems will help put further limits on how large neutron stars can become before collapsing into black holes, as well as help conquer competing theories of the nature of the quark soup at their cores. “We can look for black widows and similar neutron stars that skate closer to the black hole brink.” Filippenko said. “But if we don’t find any, it strengthens the argument that 2.3 solar masses is the real limit, beyond which they become black holes.”