The “black widow” neutron star devoured its partner to become the heaviest found so far

Astronomers have determined the heaviest neutron star known to date, weighing 2.35 solar masses, according to a recent article published in the Astrophysical Journal Letters. How did it get so big? Most likely devouring a companion star, the celestial equivalent of a black widow spider devouring her 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 the remains of supernovae. Like Ars Science editor John Timmer wrote last month:

The matter that makes up neutron stars begins as ionized atoms near the core of a massive star. Once the star’s fusion reactions stop producing enough energy to counteract the pull of gravity, this matter contracts and experiences ever-increasing pressures. The crushing force is enough to remove the boundaries between atomic nuclei, creating a giant soup of protons and neutrons. Eventually even the electrons in the region are forced into many of the protons, turning them into neutrons.

This ultimately provides a force to push against the crushing power of gravity. Quantum mechanics prevents neutrons from occupying the same energy state in close proximity, and this prevents neutrons from getting closer together and blocks collapsing into a black hole. But it is possible that there is an intermediate state between a droplet of neutrons and a black hole, one in which the boundaries between neutrons begin to break down, resulting in strange combinations of their constituent quarks.

Apart from black holes, the cores of neutron stars are the densest known objects in the Universe and, because they are hidden behind an event horizon, they are difficult to study. “We know more or less how matter behaves at nuclear densities, like in the nucleus of a uranium atom.” alex filipenko said, an astronomer at the University of California, Berkeley and a co-author of the new paper. “A neutron star is like a giant core, but when you have 1.5 solar masses of this stuff, which is about 500,000 Earth masses of cores bonded together, it’s not entirely clear how they’ll behave.”

The neutron star featured in this latest article is a pulsar, PSR J0952-0607, or J0952 for short, located in the constellation Sextans between 3,200 and 5,700 light-years distant from Earth. Neutron stars are born by spinning, and the rotating magnetic field emits beams of light in the form of radio waves, X-rays, or gamma rays. Astronomers can detect pulsars when their beams sweep across Earth. J0952 was discovered in 2017 thanks to the Low-Frequency Array Radio Telescope (LOFAR), tracking data on mysterious gamma-ray sources collected by NASA’s Fermi Gamma-ray Space Telescope.

Your average pulsar spins at about one rotation per second, or 60 per minute. But J0952 is spinning at a whopping 42,000 revolutions per minute, making it the second-fastest known pulsar yet. The current favorite hypothesis is that these kinds of pulsars were once part of binary systems, gradually stripping away their companion stars until the latter evaporated. That is why these stars are known as black widow pulsars, what Filippenko calls a “case of cosmic ingratitude”:

The evolutionary path is absolutely fascinating. Double exclamation mark. As the companion star evolves and begins to become a red giant, material spills into the neutron star, causing it to spin. As it spins, it now becomes incredibly energized, and a wind of particles begins to pour out of the neutron star. Then that wind hits the donor star and starts shedding material, and over time, the donor star’s mass decreases to that of a planet, and if more time passes, it disappears altogether. So, this is how lone millisecond pulsars could form. To begin with, they were not alone, they had to be in a binary pair, but gradually they evaporated their partners, and now they are alone.

This process would explain how J0952 became so heavy. And such systems are a boon to scientists like Filippenko and his colleagues interested in accurately weighing neutron stars. The trick is to find binary neutron star systems in which the companion star is small but not too small to detect. Of the dozen black widow pulsars the team has studied over the years, only six met that criteria.

J0952’s companion star is 20 times the mass of Jupiter and is tidally locked into orbit with the pulsar. The side facing J0952 is therefore quite hot, reaching temperatures of 6,200 Kelvin (10,700 °F), making it bright enough to be seen with a large telescope.

filippenko et al. spent the past four years making six observations of J0952 with the Keck 10-meter telescope in Hawaii to pick up the companion star at specific points in its 6.4-hour orbit around the pulsar. They then compared the resulting spectra with the spectra of stars similar to the Sun to determine the orbital velocity. This, in turn, allowed them to calculate the mass of the pulsar.

Finding even more such systems would help place further constraints on the upper bound on how large neutron stars can get before collapsing into black holes, as well as filter out competing theories about the nature of quark soup in their cores. “We can continue to look for black widows and similar neutron stars that come even closer to the edge of the black hole.” Philippenko said. “But if we don’t find any, it strengthens the argument that 2.3 solar masses is the true limit, beyond which they become black holes.”

DOI: Astrophysical Journal Letters, 2022. 10.3847/2041-8213/ac8007 (About DOIs).