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Science

This Record-Breaking ‘Black Widow’ Pulsar Is The Most Massive Neutron Star Yet

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One of the most extreme stars in the Milky Way just got even more wack.

Scientists have measured the mass of a neutron star named PSR J0952-0607, and found that it’s the most massive neutron star discovered yet, clocking in at a whopping 2.35 times the mass of the Sun.

 

If true, this is very close to the theorized upper mass limit of around 2.3 solar masses for neutron stars, representing an excellent laboratory for studying these ultra-dense stars at what we think is the brink of collapse, in the hope of better understanding the weird quantum state of the matter of which they are made.

“We know roughly how matter behaves at nuclear densities, like in the nucleus of a uranium atom,” said astrophysicist Alex Filippenko of 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.”

Neutron stars are the collapsed cores of massive stars that were between around 8 and 30 times the mass of the Sun, before they went supernova and blew most of their mass off into space.

These cores, tending to be around 1.5 times the mass of the Sun, are among the densest objects in the Universe; the only thing denser is a black hole.

 

Their mass is packed into a sphere just 20 kilometers (12 miles) or so 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 occupy the same quantum states, described as degeneracy pressure.

In some ways this means neutron stars behave like massive atomic nuclei. But what happens at this tipping point, where neutrons form exotic structures or blur into a soup of smaller particles, is hard to say.

PSR J0952-0607 was already one of the most interesting neutron stars in the Milky Way. It’s what is known as a pulsar – a neutron star that is spinning very fast, with jets of radiation emitting from the poles. As the star spins, these poles sweep past the observer (us) in the manner of a cosmic lighthouse so that the star appears to pulse.

These stars can be insanely fast, their rotation rate on millisecond scales. PSR J0952-0607 is the second-fastest pulsar in the Milky Way, rotating a mind-blowing 707 times per second. (The fastest is only slightly faster, with a rotation rate of 716 times per second.)

 

It’s also what is known as a “black widow” pulsar. The star is in a close orbit with a binary companion – so close that its immense gravitational field pulls material from the companion star. This material forms an accretion disk that whirls around and feeds into the neutron star, a bit like water swirling around a drain. Angular momentum from the accretion disk is transferred to the star, causing its spin rate to increase.

A team led by astrophysicist Roger Romani of Stanford University wanted to understand better how PSR J0952-0607 fit into the timeline of this process. The binary companion star is tiny, less than 10 percent of the mass of the Sun. The research team made careful studies of the system and its orbit and used that information to obtain a new, precise measurement for the pulsar.

Their calculations returned a result of 2.35 times the mass of the Sun, give or take 0.17 solar masses. Assuming a standard neutron star starting mass of around 1.4 times the mass of the Sun, that means that PSR J0952-0607 has slurped up to an entire Sun’s worth of matter from its binary companion. This, the team says, is really important information to have about neutron stars.

“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,” Romani explained.

“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. This excludes many proposed states of matter, especially those with exotic interior composition.”

The binary also shows a mechanism whereby isolated pulsars, without binary companions, can have millisecond rotation rates. J0952-0607’s companion is almost gone; once it’s entirely devoured, the pulsar (if it’s not tipped over the upper mass limit and collapses further into a black hole) will retain its insanely fast rotation speed for quite some time.

And it will be alone, just like those all the other isolated millisecond pulsars. 

“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,” Filippenko said.

“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 research has been published in The Astrophysical Journal Letters.

 

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Gluttonous cosmic ‘black widow’ is heaviest-known neutron star

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By Will Dunham

WASHINGTON (Reuters) – Astronomers have observed the most massive known example of an object called a neutron star, one classified as a “black widow” that got particularly hefty by gobbling up most of the mass of a stellar companion trapped in an unhappy cosmic marriage.

The researchers said the neutron star, wildly spinning at 707 times per second, has a mass about 2.35 times greater than that of our sun, putting it perhaps at the maximum possible for such objects before they would collapse to form a black hole.

A neutron star is the compact collapsed core of a massive star that exploded as a supernova at the end of its life cycle. The one described by the researchers is a highly magnetized type of neutron star called a pulsar that unleashes beams of electromagnetic radiation from its poles. As it spins, these beams appear from the perspective of an observer on Earth to pulse – akin to a lighthouse’s rotating light.

Only one other neutron star is known to spin more quickly than this one.

“The heavier the neutron star, the denser the material in its core,” said Roger Romani, director of Stanford University’s Center for Space Science and Astrophysics and a co-author of the research published this week in the Astrophysical Journal Letters.

“So as the heaviest neutron star known, this object presents the densest material in the observable universe. If it was any heavier it should collapse to a black hole, and then the stuff inside would be behind the event horizon, forever sealed off from any observation,” Romani added.

A black hole’s event horizon is the point of no return beyond which anything including light gets sucked in irretrievably.

“Since we don’t yet know how matter works at these densities, the existence of this neutron star is an important probe of these physical extremes,” Romani said.

The neutron star, residing in our Milky Way galaxy in the direction of the constellation Sextans and formally named PSR J0952-0607, is located roughly 20,000 light years from Earth, Romani said. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km). The researchers studied it using the Keck I telescope in Hawaii.

Stars that are about eight or more times the sun’s mass transform hydrogen into heavier elements through thermonuclear fusion in their cores. When they build up about 1.4 times the mass of our sun in iron, that core collapses into a neutron star having a diameter only about the size of a city, with the rest blown off in the supernova explosion.

Its matter is so compact that an amount about the size of a sugar cube would outweigh Mount Everest.

This neutron star inhabits what is called a binary system, in an orbit with another star. The neutron star is a kind dubbed a “black widow,” named in honor of female black widow spiders that eat their male partners after mating.

It apparently was born with the usual mass of a neutron star, about 1.4 times that of our sun, but its gravitational pull poached material from its companion star, enabling it to grow to a mass seemingly at the uppermost limit before physics would dictate a collapse into a black hole, the densest of all known objects.

Its companion star has been stripped almost bare, losing perhaps 98% of its mass to the black widow, leaving it at about 20 times the mass of our solar system’s largest planet Jupiter – a far cry for its original size.

“It has swallowed nearly a full sun’s worth of mass without yet becoming a black hole. So it should be just on the edge of black hole collapse,” Romani said.

(Reporting by Will Dunham; Editing by Lisa Shumaker)

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Science

Heaviest neutron star results after devouring companion star

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Called a neutron star, the dense, collapsed remnants of a massive star weighs more than twice the mass of our sun, making it the heaviest neutron star known to date. The object spins 707 times per second, which also makes it one of the fastest-spinning neutron stars in the Milky Way.

The neutron star is known as a black widow because, much like these arachnids known for female spiders that consume much smaller male partners after mating, the star has shredded and devoured almost the entire mass of its companion star.

This stellar feast has allowed the black widow to become the heaviest neutron star observed so far.

Astronomers were able to weigh the star, called PSR J0952-0607, by using the sensitive Keck telescope at the W. M. Keck Observatory on Maunakea in Hawaii.

The observatory’s Low Resolution Imaging Spectrometer recorded visible light from the shredded companion star, which glowed due to its high heat.

The companion star is now about the size of a large gaseous planet, or 20 times the mass of Jupiter. The side of the companion star that faces the neutron star is heated to 10,700 degrees Fahrenheit (5,927 degrees Celsius) — hot and bright enough to be seen by a telescope.

Neutron star cores are the densest matter in the universe, outside of black holes, and 1 cubic inch (16.4 cubic centimeters) of a neutron star weighs more than 10 billion tons, according to study author Roger W. Romani, a professor of physics at Stanford University in California.

This particular neutron star is the densest object within sight of Earth, according to the researchers.

“We know roughly how matter behaves at nuclear densities, like in the nucleus of a uranium atom,” said study coauthor Alex Filippenko in a statement. Filippenko holds dual titles of professor of astronomy and distinguished professor of physical sciences 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.”

A neutron star like PSR J0952-0607 is called a pulsar because as it spins, the object acts like a cosmic lighthouse, regularly beaming out light through radio waves, X-rays or gamma rays.

Normal pulsars spin and flash about once a second, but this one is pulsing hundreds of times per second. This is because the neutron star becomes more energized as it strips material away from the companion star.

“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,” Filippenko said.

Astronomers first discovered the neutron star in 2017, and Filippenko and Romani have studied similar black widow systems for more than a decade. They have been trying to understand how large neutron stars can become. If neutron stars become too heavy, they collapse and become black holes.

The PSR J0952-0607 star is 2.35 times the mass of the sun, which is now considered to be the upper limit for a neutron star, the researchers said.

“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.

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Science

Heaviest Neutron Star Ever Discovered Is a “Black Widow” Devouring Its Mate

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A spinning neutron star periodically swings its radio (green) and gamma-ray (magenta) beams past Earth in this artist’s concept of a black widow pulsar. The pulsar heats the facing side of its stellar partner to temperatures twice as hot as the sun’s surface and slowly evaporates it. Credit: NASA’s Goddard Space Flight Center/Cruz deWilde

Observations of faint, planet-size star help weigh its millisecond pulsar companion.

A dense, collapsed star has shredded and consumed nearly the entire mass of its stellar companion and, in the process, grown into the heaviest

“We know roughly how matter behaves at nuclear densities, like in the nucleus of a uranium

Astronomers measured the velocity of a faint star (green circle) that has been stripped of nearly its entire mass by an invisible companion, a neutron star and millisecond pulsar that they determined to be the most massive yet found and perhaps the upper limit for neutron stars. The objects are in the constellation Sextans. Credit: W. M. Keck Observatory, Roger W. Romani, Alex Filippenko

The extreme sensitivity of the 10-meter Keck I telescope on Maunakea in Hawai’i was what made it possible to measure of the neutron star’s mass. It recorded a spectrum of visible light from the hotly glowing companion star, which is now reduced to the size of a large gaseous planet. Located in the direction of the constellation Sextans, the stars are about 3,000 light-years from Earth.

Discovered in 2017, PSR J0952-0607 is referred to as a “black widow” pulsar. Their name is an analogy to the tendency of female black widow spiders to consume the much smaller male after mating. Hoping to establish the upper limit on how large neutron stars/pulsars can grow, Filippenko and Romani have been studying black widow systems for more than a decade.

“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.”

If 2.35 solar masses is close to the upper limit of neutron stars, the astronomers say, then the interior is likely to be a soup of neutrons as well as up and down quarks — the constituents of normal protons and neutrons — but not exotic matter, such as “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 were published today (July 26, 2022) in The Astrophysical Journal Letters.

How large can they grow?

Astrophysicists generally agree that when a star with a core larger than about 1.4 solar masses collapses at the end of its life, it forms a dense, compact object with an interior under such high pressure that all atoms are smashed together to form a sea of neutrons and their subnuclear constituents, quarks. These neutron stars are born spinning, and though too dim to be seen in visible light, reveal themselves as pulsars, emitting beams of light — radio waves, X-rays or even gamma rays — that flash Earth as they spin, much like the rotating beam of a lighthouse.

“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. But some pulsars repeat hundreds or up to 1,000 times per second, which is hard to explain unless matter has fallen onto the neutron star and spun it up. But for some millisecond pulsars, no companion is visible.

One possible explanation for isolated millisecond pulsars is that each did once have a companion, but it stripped it down to nothing.

“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

Finding black widow pulsars in which the companion is small, but not too small to detect, is one of few ways to weigh neutron stars. In the case of this binary system, the companion star — now only 20 times the mass of

“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.

Reference: “PSR J0952-0607: The Fastest and Heaviest Known Galactic Neutron Star” by Roger W. Romani, D. Kandel, Alexei V. Filippenko, Thomas G. Brink and WeiKang Zheng, 26 July 2022, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ac8007

Other co-authors of the ApJ Letters paper are UC Berkeley researchers Thomas Brink and WeiKang Zheng. The work was supported by the National Aeronautics and Space Administration (80NSSC17K0024, 80NSSC17K0502), the Christopher R. Redlich Fund, the TABASGO Foundation, and UC Berkeley’s Miller Institute for Basic Research in Science.



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