Far faster than one would anticipate for a collapsing star, millisecond pulsars spin. Finding a black widow system where the pulsar has vaporized and consumed much of its companion star offers the finest chance to investigate these neutron stars.
Astronomers were only able to weigh the pulsar of one of these companions thanks to the Keck I telescope’s ability to catch its spectrum. It is the heaviest known object and may be getting close to the maximum limit for neutron stars.
The Milky Way galaxy’s fastest spinning neutron star, a compact, collapsed star that spins 707 times per second, has devoured almost all of the stellar companion’s mass, becoming the most massive neutron star ever seen in the process.
This record-breaking neutron star, which weighs 2.35 times as much as the sun, aids astronomers in understanding the peculiar quantum state of matter that exists inside these compact objects, which, if they are much heavier, collapse completely and vanish as black holes.
“We know roughly how matter behaves at nuclear densities, like in the nucleus of a uranium atom,” said Alex Filippenko, Distinguished Professor of Astronomy at the University of California, Berkeley. “A neutron star is like one giant 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 will behave.”
Roger W. Romani, professor of astrophysics at Stanford University, noted that neutron stars are so dense 1 cubic inch weighs over 10 billion tons that their cores are the densest matter in the universe short of black holes, which because they are hidden behind their event horizon are impossible to study. The neutron star, a pulsar designated PSR J0952-0607, is thus the densest object within sight of Earth.
The 10-meter Keck I telescope on Maunakea in Hawai’i, which was only able to record a spectrum of visible light from the furiously blazing companion star, now shrunk to the size of a big gaseous planet, made it possible to measure the neutron star’s mass. The stars are about 3,000 light years from Earth in the direction of the constellation Sextans.
We know roughly how matter behaves at nuclear densities, like in the nucleus of a uranium atom. A neutron star is like one giant 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 will behave.
Professor Alex Filippenko
Discovered in 2017, PSR J0952-0607 is referred to as a “black widow” pulsar an analogy to the tendency of female black widow spiders to consume the much smaller male after mating. Filippenko and Romani have been studying black widow systems for more than a decade, hoping to establish the upper limit on how large neutron stars/pulsars can grow.
“By combining this measurement with those of several other black widows, we show that neutron stars must reach at least this mass, 2.35 plus or minus 0.17 solar masses,” said Romani, who is a professor of physics in Stanford’s School of Humanities and Sciences and member of the Kavli Institute for Particle Astrophysics and Cosmology. “In turn, this provides some of the strongest constraints on the property of matter at several times the density seen in atomic nuclei. Indeed, many otherwise popular models of dense-matter physics are excluded by this result.”
According to the researchers, if 2.35 solar masses is close to the upper limit of neutron stars, the interior is likely to be a soup of neutrons and up and down quarks, the building blocks of regular protons and neutrons, rather than exotic matter like “strange” quarks or kaons, which are particles that contain a strange quark.
“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,” Romani said. “This excludes many proposed states of matter, especially those with exotic interior composition.”
Romani, Filippenko and Stanford graduate student Dinesh Kandel are co-authors of a paper describing the team’s results that has been accepted for publication by The Astrophysical Journal Letters.
How large can they grow?
In general, astronomers concur that when a star with a core mass greater than 1.4 solar masses collapses at the end of its life, it creates a dense, compact object whose interior is under such intense pressure that all atoms are crushed together to create a sea of neutrons and their subnuclear byproducts, quarks.
These neutron stars are born spinning and, despite being too faint to be seen in visible light, reveal themselves as pulsars by flashing Earth with radio waves, X-rays, or even gamma rays as they rotate. This behavior is similar to a lighthouse rotating its beam.
“Ordinary” pulsars spin and flash about once per second, on average, a speed that can easily be explained given the normal rotation of a star before it collapses. It is difficult to explain why some pulsars repeat hundreds or even 1,000 times per second unless stuff has dropped onto the neutron star and spun it up. But for some millisecond pulsars, no companion is visible.
Each single millisecond pulsar may have once had a companion, but it has been stripped away, which is one explanation for their isolation.
“The evolutionary pathway is absolutely fascinating. Double exclamation point,” Filippenko said. “As the companion star evolves and starts becoming a red giant, material spills over to the neutron star, and that spins up the neutron star. By spinning up, it now becomes incredibly energized, and a wind of particles starts coming out from the neutron star. That wind then hits the donor star and starts stripping material off, and over time, the donor star’s mass decreases to that of a planet, and if even more time passes, it disappears altogether. So, that’s how lone millisecond pulsars could be formed. They weren’t all alone to begin with they had to be in a binary pair but they gradually evaporated away their companions, and now they’re solitary.”
The pulsar PSR J0952-0607 and its faint companion star support this origin story for millisecond pulsars.
“These planet-like objects are the dregs of normal stars which have contributed mass and angular momentum, spinning up their pulsar mates to millisecond periods and increasing their mass in the process,” Romani said.
“In a case of cosmic ingratitude, the black widow pulsar, which has devoured a large part of its mate, now heats and evaporates the companion down to planetary masses and perhaps complete annihilation,” said Filippenko.
Spider pulsars include redbacks and tidarrens
One of the few techniques to weigh neutron stars is to find black widow pulsars where the partner is modest but not too small to detect. Similar to how our moon is locked in orbit so that we can only view one side, the neutron star’s mass distorts the companion star in this binary system, which is now only 20 times the mass of Jupiter. This causes it to become tidally locked.
About 6,200 Kelvin, or 10,700 degrees Fahrenheit, are reached on the neutron star-facing side, making it slightly hotter than the sun and just brilliant enough to be visible through a powerful telescope.
During the course of the last four years, Filippenko and Romani turned the Keck I telescope on PSR J0952-0607 six times, each time using the Low-Resolution Imaging Spectrometer to observe at 15-minute intervals to catch the dim companion at certain times in its 6.4-hour orbit around the pulsar. They were able to determine the mass of the neutron star and gauge the orbital velocity of the companion star by comparing the spectra to those of other sun-like stars.
Filippenko and Romani have examined about a dozen black widow systems so far, though only six had companion stars bright enough to let them calculate a mass. All involved neutron stars less massive than the pulsar PSR J0952-060.
They want to learn more about black widow pulsars and their relatives, including redbacks, which are named after the black widow pulsars in Australia and have companions that are closer to one-tenth the mass of the sun, and tidarrens, which the Romani named after a kin of the black widow spider and have companions that are about one-hundredth of a solar mass. The male of this species, Tidarren sisyphoides, is about 1% of the female’s size.
“We can keep looking for black widows and similar neutron stars that skate even closer to the black hole brink. But if we don’t find any, it tightens the argument that 2.3 solar masses is the true limit, beyond which they become black holes,” Filippenko said.
“This is right at the limit of what the Keck telescope can do, so barring fantastic observing conditions, tightening the measurement of PSR J0952-0607 likely awaits the 30-meter telescope era,” added Romani.
