Astronomers have determined the most massive neutron star known to date, weighing 2.35 solar masses, according to a recent newspaper published in the Astrophysical Journal Letters. How did it get so big? Most likely by devouring a companion star – the celestial equivalent of a black widow spider devouring its mate. The work helps establish an upper bound for how big neutron stars can get, with implications for our understanding of the quantum state of matter in their cores.
Neutron stars are the remnants of supernovae. As 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 increasing pressure. The crushing force is enough to eliminate 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, converting them into neutrons.
This eventually creates a force to push back against the crushing force of gravity. Quantum mechanics prevents neutrons from getting close to each other in the same energy state, and this prevents the neutrons from getting closer, thus blocking the collapse in a black hole. But it’s possible that there’s an intermediate state between a blob of neutrons and a black hole, a state where 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 roughly how matter behaves at nuclear densities, such as in the nucleus of a uranium atom,” said Alex Filippenko, an astronomer at the University of California, Berkeley and co-author of the new paper. “A neutron star is like one giant core, but if you have 1.5 solar masses of this stuff, which amounts to about 500,000 Earth masses of cores all stuck together, it’s not at all clear how they’re going to behave.”
The neutron star featured in this latest article is a pulsar, PSR J0952-0607 — or J0952 for short — located in the constellation Sextans, 3,200 to 5,700 light-years from Earth. Neutron stars are born spinning and the rotating magnetic field emits rays of light in the form of radio waves, X-rays or gamma rays. Astronomers can see pulsars as their beams sweep across the Earth. J0952 was discovered in 2017 thanks to the Low-Frequency Array (LOFAR) radio telescope, which tracks data on mysterious gamma-ray sources collected by NASA’s Fermi Gamma-ray Space Telescope.
Your average pulsar spins at about one revolution per second, or 60 per minute. But J0952 spins at a whopping 42,000 revolutions per minute, making it the second fastest known pulsar to date. The current preferred hypothesis is that these types of pulsars were once part of binary systems, gradually breaking apart their companion stars until the latter evaporated. That’s why such 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 overflows into the neutron star, causing the neutron star to spin. By spinning, it now becomes incredibly energetic and a wind of particles begins to come out of the neutron star. That wind then hits the donor star and begins to strip off material, and over time, the donor star’s mass decreases to that of a planet, and as more time passes, it disappears altogether. So that’s how lone millisecond pulsars can be formed. To begin with, they were not all alone – they had to form a binary pair – but gradually their companions evaporated and now they are lonely.
This process would explain how J0952 became so heavy. And such systems are a boon to scientists like Filippenko and his colleagues who want to accurately weigh neutron stars. The trick is to find binary systems of neutron stars in which the companion star is small, but not too small to detect. Of the roughly 12 black widow pulsars the team has studied over the years, only six met those criteria.
The companion star of J0952 is 20 times the mass of Jupiter and is tidally locked in orbit with the pulsar. Thus, 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.
Philipenko et al. has made six observations of J0952 in the past four years with the 10-meter Keck telescope in Hawaii to capture 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 similar Sun-like stars to determine the orbital speed. This, in turn, allowed them to calculate the pulsar’s mass.
Finding more such systems would help further narrow the upper bound on how big neutron stars can get before collapsing into black holes, and solve competing theories about the nature of the quark soup in their cores. “We can keep looking for black widows and similar neutron stars that skate even closer to the edge of the black hole,” Filippenko said. “But if we don’t find one, it sharpens the argument that 2.3 solar masses is the real limit, beyond which they become black holes.”