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New Neutron Camera has a Shutter Speed of One Trillionth of a Second – PetaPixel
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These Massive Neutron Stars Existed For Less Than The Blink of an Eye : ScienceAlert
Not much can be accomplished in a few hundred milliseconds. Yet for the neutron stars seen in the glints of two gamma-ray bursts, it’s more than enough time to teach us a thing or two about life, death, and the birth of black holes.
Sifting through an archive of high-energy flashes in the night sky, astronomers recently uncovered patterns in the oscillations of light left by two different sets of colliding stars, indicating a pause on their journey from super-dense object to infinite pit of darkness.
That pause – somewhere between 10 and 300 milliseconds – technically equates to two newly formed, mega-sized neutron stars, which researchers suspect were each spinning fast enough to briefly hold off their inevitable fates as black holes.
“We know that short GRBs form when orbiting neutron stars crash together, and we know they eventually collapse into a black hole, but the precise sequence of events is not well understood,” says Cole Miller, an astronomer at the University of Maryland, College Park (UMCP) in the US.
“We found these gamma-ray patterns in two bursts observed by Compton in the early 1990s.”
For nearly 30 years, the Compton Gamma Ray Observatory circled Earth and collected the shine of X-rays and gamma rays that spilled from distant cataclysmic events. That archive of high energy photons contains a trove of data on things like colliding neutron stars, which release powerful pulses of radiation known as gamma-ray bursts.
Neutron stars are true beasts of the cosmos. They pack double the mass of our Sun inside a volume of space roughly the size of a small city. Not only does this do weird things to matter, forcing electrons into protons to turn them into a heavy dusting of neutrons, it can generate magnetic fields unlike anything else in the Universe.
Spun into high rotation, these fields can accelerate particles to ridiculously high velocities, forming polar jets that appear to ‘pulse’ like supercharged lighthouses.
Neutron stars are formed as more ordinary stars (around 8 to 30 times the mass of our Sun) burn off the last of their fuel, leaving a core of around 1.1 to 2.3 solar masses, too cold to resist the squeeze of its own gravity.
Add a little more mass – such as by cramming two neutron stars together – and not even the lackluster jiggling of its own quantum fields can resist gravity’s urge to crush the living physics out of the dead star. From a dense blob of particles we get, well, whatever the unspeakable horror is that happens to be the heart of a black hole.
The basic theory on the process is pretty clear, setting general limits on just how heavy a neutron star can be before it collapses. For cold, non-rotating balls of matter, this upper boundary is just under three solar masses, but that also implies complications that just might make the journey from neutron star to black hole less than straightforward.
For example, earlier last year physicists announced the observation of a burst of gamma-rays dubbed GRB 180618A, detected back in 2018. In the afterglow of the burst they detected the signature of a magnetically-charged neutron star called a magnetar, one with a mass close to that of the two colliding stars.
Barely a day later this heavyweight neutron star was no more, no doubt succumbing to its extraordinary mass and transforming into something not even light can escape from.
How it managed to resist gravity for as long as it did is a mystery, though its magnetic fields may have played a role.
These two new discoveries could also provide a few clues.
The more accurate term for the pattern observed in the gamma-ray bursts recorded by Compton in the early 1990s is a quasiperiodic oscillation. The mix of frequencies that rise and fall in the signal can be deciphered to describe the final moments of massive objects as they circle one another and then collide.
From what the researchers can tell, the collisions each produced an object around 20 percent larger than the current record-holder heavyweight neutron star – a pulsar calculated at 2.14 times the mass of our Sun. They were also twice the diameter of a typical neutron star.
Interestingly, the objects were rotating at an extraordinary pace of nearly 78,000 times a minute, far faster than the record-holding pulsar J1748–2446ad, which manages a mere 707 turns a second.
The few rotations each neutron star managed to pull off in its brief lifetime of a fraction of a second could have been powered by just enough angular momentum to combat their gravitational implosion.
How this may apply to other neutron star mergers, further blurring the boundaries of stellar collapse and black hole generation, is a question for future research.
This research was published in Nature.
Wild New Study Reveals Neutron Stars Are Actually Like a Box of Chocolates : ScienceAlert
Life isn’t really like a box of chocolates, but it seems that something out there is. Neutron stars – some of the densest objects in the Universe – can have structures very similar to chocolates, with either gooey or hard centers.
What kinds of particle configurations those centers consist of is still unknown, but new theoretical work revealing this surprising result could put us a step closer to understanding the strange guts of these dead stars, and the wild extremes possible in our Universe.
Neutron stars are pretty incredible. If we consider black holes to be objects of immense (if not infinite) concentrations of matter, neutron stars win second place in the Universe’s Most Dense Award. Once a star with a mass of around 8 to 30 times that of the Sun’s runs out of matter to fuse in its core, it’s no longer supported by heat’s outward pressure, allowing the core to collapse under gravity as its shell of surrounding gases drift off into space.
The resulting neutron star has a reduced mass of up to around 2.3 times the mass of the Sun, but it’s squeezed into a sphere around just 20 kilometers (12 miles) across. These things are capital-letters DENSE – and what exactly happens to matter under such mind-blowing pressures is something scientists are dying to know.
Some studies propose that nuclei crowd together until they form shapes that resemble pasta. Others suggest even deeper inside the star, pressures become so extreme that atomic nuclei cease to exist altogether, condensing into a “soup” of quark matter.
Now, theoretical physicists led by Luciano Rezzolla of Goethe University in Germany have discovered how neutron stars might be akin to chocolates with different fillings.
The team combined theoretical nuclear physics and astrophysical observations to develop a set of more than a million ‘equations of state’. These are equations that relate the pressure, temperature, and volume of a given system, in this case a neutron star.
Using these, the team developed a scale-dependent description of the speed of sound in neutron stars. And this is where it gets interesting. The speed of sound in a given object, be it a star or a planet, can reveal the structure of its interior.
Just as seismic waves on Earth and Mars propagate differently through materials of different density, revealing structures and layers, acoustic waves that bounce around in stars can reveal what’s going on inside them.
When the team used their equations of state to study the speed of sound in neutron stars, their structures were not uniform across the board. Rather, the neutron stars on the lower end of the mass range, below 1.7 times the mass of the Sun, seemed to have a squishy mantle and harder core, while those above 1.7 solar masses had a hard mantle and a squishy core.
“This result is very interesting because it gives us a direct measure of how compressible the center of neutron stars can be,” Rezzolla says.
“Neutron stars apparently behave a bit like chocolate pralines: light stars resemble those chocolates that have a hazelnut in their center surrounded by soft chocolate, whereas heavy stars can be considered more like those chocolates where a hard layer contains a soft filling.”
This seems to fit with both the nuclear pasta and quark soup interpretations of neutron star innards, but it also provides new information that could help model neutron stars across a range of masses in future work.
This could also explain how, regardless of their masses, all neutron stars have roughly the same diameter of around 20-kilometers.
“Our extensive numerical study not only allows us to make predictions for the radii and maximum masses of neutron stars, but also to set new limits on their deformability in binary systems, that is, how strongly they distort each other through their gravitational fields,” says physicist Christian Ecker of the University of Goethe.
“These insights will become particularly important to pinpoint the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars.”
Chocolate praline nuclear pasta quark soup, anyone?
The research has been published in two papers in The Astrophysical Journal Letters. They can be found here and here.
Neutron Stars Are Basically Giant Cosmic Pralines, Astrophysicists Say
Astrophysicists modeling the insides of neutron stars have found that the extremely compact objects have different internal structures, depending on their mass. They suggest thinking of the stars as different types of chocolate praline, a delicious treat—but that’s where the similarities end, at least as far as we know.
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Neutron stars are the extraordinarily dense corpses of massive stars that imploded; they’re second only to black holes in terms of their density. Neutron stars are so-named because their gravitational force causes their atoms’ electrons to collapse onto the protons, creating an object that is almost entirely composed of neutrons.
Neutron stars’ gravitational fields are super intense. If a human observer went near one, they’d be torn apart at an atomic level. Their gravitations fields are so strong that a “mountain” on a neutron star would stand less than a millimeter tall.
The recent research team constructed millions of models to try to discern the internal workings of these stars, which are remarkably difficult to study and, as a result, are more the domain of theory than observation.
The researchers found that lighter neutron stars—those with masses about 1.7 times that of our Sun and under—should have soft mantles and stiff cores. Heavier neutron stars have the opposite, according to the team’s findings, which were published today in The Astrophysical Journal Letters.
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Luciano Rezzolla, an astrophysicist at the Institute of Theoretical Physics and who led the research, likened the stars’ structure to chocolate pralines.
“Light stars resemble those chocolates that have a hazelnut in their centre surrounded by soft chocolate, whereas heavy stars can be considered more like those chocolates where a hard layer contains a soft filling,” Rezzolla said in a Goethe University Frankfurt release.
The researchers modeled over a million possible scenarios for neutron star makeup, based on expectations for the star’s mass, pressure, volume, and temperature, as well as astronomical observations of the objects.
Modeling is a crucial means of interrogating neutron stars, because only a few contraptions on Earth—CERN’s Large Hadron Collider and SLAC’s Matter in Extreme Conditions instrument, for two—are capable of mimicking such intense physics.
To determine the consistencies of the stars, the researchers modeled how the speed of sound would travel through the objects. Sound waves are also used to understand the internal structure of planets, as the InSight lander has intrepidly done on Mars.
“What we have shown, by constructing millions of equation of state models (from which the sound speed can be computed), is that maximally massive neutron stars have a lower sound speed in the core region than in their outer layers,” said Christian Ecker, an astrophysicist at Goethe University, in an email to Gizmodo.
“This hints to some material change in their cores, like for example a transition from baryonic to quark matter,” Ecker added.
The researchers also found that all neutron stars are probably about 7.46 miles (12 km) across, regardless of their mass. That measurement is less than half that of a 2020 finding that the typical neutron star was about 13.6 miles (22 km) across. Despite that size, the average neutron star mass is around half a million Earths. There’s dense, and then there’s dense.
While the findings offer some insight about the diversity of neutron stars in terms of their consistency, the researchers did not investigate the stars’ ingredients or how they fit together. (If you’ve gotten this far, neutron stars are not actually made of chocolate.) Some suspect that neutron stars are neutrons all the way down; others believe that the centers of the stars are factories for exotic, hitherto unidentified particles.
But for the most part, these superdense enigmas remain just that. Thankfully, there are observatories set up to collect more direct data. Mergers (i.e. violent collisions) between neutron stars and with black holes can reveal the mass of the involved objects, as well as the nature of neutron star material.
Projects like NICER, NANOGrav, the CHIME radio telescope, and the LIGO and Virgo scientific collaborations are all teaching physicists about neutron star size and structure.
More observational data can be fed into models for better estimates of the stars’ aspects. Ecker added that very massive neutron stars (in the ballpark of two solar masses) would be particularly helpful in better constraining expectations of the physical characteristics of these extreme objects.
With any luck, we may soon get more details of the exact ingredients of these giant cosmic pralines—and how their recipes may differ depending on their size.
More: Extremely Massive Neutron Star May Be the Largest Ever Spotted
Cosmic chocolate pralines? General neutron star structure revealed
So far, little is known about the interior of neutron stars, those extremely compact objects that can form after the death of a star. The mass of our sun or even more is compressed into a sphere with the diameter of a large city. Since their discovery more than 60 years ago, scientists have been trying to decipher their structure.
The greatest challenge is to simulate the extreme conditions inside neutron stars, as they can hardly be recreated on Earth in the laboratory. There are therefore many models in which various properties—from density and temperature—are described with the help of so-called equations of state. These equations attempt to describe the structure of neutron stars from the stellar surface to the inner core.
Now, physicists at Goethe University Frankfurt have succeeded in adding further crucial pieces to the puzzle. The working group, led by Prof. Luciano Rezzolla at the Institute of Theoretical Physics, has developed more than a million different equations of state that satisfy the constraints set by data obtained from theoretical nuclear physics on the one hand, and by astronomical observations on the other. Their work is published in The Astrophysical Journal Letters.
When evaluating the equations of state, the working group made a surprising discovery: “Light” neutron stars (with masses smaller than about 1.7 solar masses) seem to have a soft mantle and a stiff core, whereas “heavy” neutron stars (with masses larger than 1.7 solar masses) instead have a stiff mantle and a soft core.
“This result is very interesting because it gives us a direct measure of how compressible the center of neutron stars can be,” says Prof. Luciano Rezzolla, “Neutron stars apparently behave a bit like chocolate pralines: Light stars resemble those chocolates that have a hazelnut in their center surrounded by soft chocolate, whereas heavy stars can be considered more like those chocolates where a hard layer contains a soft filling.”
Crucial to this insight was the speed of sound, a study focus of Bachelor’s student Sinan Altiparmak. This quantity measure describes how fast sound waves propagate within an object and depends on how stiff or soft matter is. Here on Earth, the speed of sound is used to explore the interior of the planet and discover oil deposits.
By modeling the equations of state, the physicists were also able to uncover other previously unexplained properties of neutron stars. For example, regardless of their mass, they very probably have a radius of only 12 km. Thus, they are just as large in diameter as Goethe University’s hometown of Frankfurt.
Study author Dr. Christian Ecker explains, “Our extensive numerical study not only allows us to make predictions for the radii and maximum masses of neutron stars, but also to set new limits on their deformability in binary systems, that is, how strongly they distort each other through their gravitational fields. These insights will become particularly important to pinpoint the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars.”
More information:
On the Sound Speed in Neutron Stars, The Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac9b2a
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‘Overweight’ neutron star defies a black hole theory, say astronomers | Black holes
An “overweight” neutron star has been observed by astronomers, who say the mysterious object confounds astronomical theories.
The hypermassive star was produced by the merger of two smaller neutron stars. Normally such collisions result in neutron stars so massive that they collapse into a black hole almost instantaneously under their own gravity. But the latest observations revealed the monster star hovering in view for more than a day before it faded out of sight.
“Such a massive neutron star with a long life expectancy is not normally thought to be possible,” said Dr Nuria Jordana-Mitjans, an astronomer at the University of Bath. “It is a mystery why this one was so long-lived.”
The observations also raise questions about the source of incredibly energetic flashes, known as short gamma-ray bursts (GRBs), that accompany neutron star mergers. These outbursts – the most energetic events in the universe since the big bang – were widely assumed to be launched from the poles of the newly formed black hole. But in this case, the observed gamma-ray burst must have emanated from the neutron star itself, suggesting that an entirely different process was at play.
Neutron stars are the smallest, densest stars in existence, occupying a sweet spot between conventional stars and black holes. They are about 12 miles wide, and so dense that a teaspoon of material would have a mass of 1bn tonnes. They have a smooth crust of pure neutrons, 10bn times stronger than steel.
“They’re such weird exotic objects,” said Prof Carole Mundell, an astronomer at the University of Bath and co-author of the study. “We can’t gather this material and bring it back to our lab so the only way we can study it is when they do something in the sky that we can observe.”
In this case, Mundell said, something appears to have prevented the neutron star “noticing how massive it is”. One possibility is that the star was spinning so fast and with such immense magnetic fields that its collapse was delayed – something like how water stays inside a tilted bucket if it is swung around fast enough.
“This is the first direct glimpse that we may have of a hypermassive spinning neutron star in nature,” said Mundell. “My hunch is we’ll be finding more of them.”
The unexpected sightings were made using Nasa’s orbiting Neil Gehrels Swift Observatory, which detected the initial gamma-ray burst coming from a galaxy about 10.6bn light years away. A robotic observatory, the Liverpool Telescope, situated in the Canary Islands, then automatically swivelled to view the aftermath of the merger. These observations revealed telltale signatures of a highly magnetised, rapidly spinning neutron star.
This suggests that the neutron star itself launched the gamma-ray burst, rather than it occurring after its gravitational collapse. Until now, the exact sequence of events has been hard to figure out.
“We were excited to catch the very early optical light from this short gamma-ray burst – something that is still largely impossible to do without using a robotic telescope,” said Mundell. “Our discovery opens new hope for upcoming sky surveys with telescopes such as the Rubin Observatory LSST, with which we may find signals from hundreds of thousands of such long-lived neutron stars before they collapse to become black holes.”
Stefano Covino, an astronomer at the Brera Astronomical Observatory in Milan, who was not involved in the research, said: “The team found evidence of the existence of a meta-stable hypermassive neutron star, which is a really important finding.”
He said the work could provide new insights into the interior structure of neutron stars, which are assumed to have a core of exotic matter, though the exact form that this takes is unknown.
The findings are published in the Astrophysical Journal.
New Tool Uses Gravitational Waves to Peer Inside Neutron Stars
Credit: NASA’s Goddard Space Flight Center/CI Lab
Imagine taking a star with twice the mass of the Sun and crushing it down to the size of Manhattan. The result would be a neutron star—one of the densest objects found anywhere in the Universe. In fact, they exceed the density of any material found naturally on Earth by a factor of tens of trillions. Although neutron stars are remarkable astrophysical objects in their own right, their extreme densities may also allow them to function as laboratories for studying fundamental questions of nuclear physics, under conditions that could never be reproduced on Earth.
Neutron stars are so dense, that a single teaspoon of one would have a mass of about a trillion kilograms.
Because of these exotic conditions, scientists still do not understand what exactly neutron stars themselves are made from, their so-called “equation of state” (EoS). Determining this is a major goal of modern astrophysics research. A new piece of the puzzle, constraining the range of possibilities, has been discovered by a pair of scholars at the Institute for Advanced Study (IAS): Carolyn Raithel, John N. Bahcall Fellow in the School of Natural Sciences; and Elias Most, Member in the School and John A. Wheeler Fellow at
Neutron star merger and the gravity waves it produces. Credit: NASA/Goddard Space Flight Center
Ideally, astrophysicists would like to look inside these exotic objects, but they are too small and distant to be imaged with standard telescopes. Researchers instead rely on indirect properties that they can measure—such as the mass and radius of a
Doomed neutron stars whirl toward their demise in this animation. Gravitational waves (pale arcs) bleed away orbital energy, causing the stars to move closer together and merge. As the stars collide, some of the debris blasts away in particle jets moving at nearly the speed of light, producing a brief burst of gamma rays (magenta). In addition to the ultra-fast jets powering the gamma rays, the merger also generates slower-moving debris. An outflow driven by accretion onto the merger remnant emits rapidly fading ultraviolet light (violet). A dense cloud of hot debris stripped from the neutron stars just before the collision produces visible and infrared light (blue-white through red). The UV, optical, and near-infrared glow is collectively referred to as a kilonova. Later, once the remnants of the jet directed toward us had expanded into our line of sight, X-rays (blue) were detected. This animation represents phenomena observed up to nine days after GW170817. Credit:
New tool allows scientists to peer inside neutron stars
Imagine taking a star twice the mass of the sun and crushing it to the size of Manhattan. The result would be a neutron star—one of the densest objects found anywhere in the universe, exceeding the density of any material found naturally on Earth by a factor of tens of trillions. Neutron stars are extraordinary astrophysical objects in their own right, but their extreme densities might also allow them to function as laboratories for studying fundamental questions of nuclear physics, under conditions that could never be reproduced on Earth.
Because of these exotic conditions, scientists still do not understand what exactly neutron stars themselves are made from, their so-called “equation of state” (EoS). Determining this is a major goal of modern astrophysics research. A new piece of the puzzle, constraining the range of possibilities, has been discovered by a pair of scholars at IAS: Carolyn Raithel, John N. Bahcall Fellow in the School of Natural Sciences; and Elias Most, Member in the School and John A. Wheeler Fellow at Princeton University. Their work was recently published in The Astrophysical Journal Letters.
Ideally, scientists would like to peek inside these exotic objects, but they are too small and distant to be imaged with standard telescopes. Scientists rely instead on indirect properties that they can measure—like the mass and radius of a neutron star—to calculate the EoS, the same way that one might use the length of two sides of a right-angled triangle to work out its hypotenuse. However, the radius of a neutron star is very difficult to measure precisely. One promising alternative for future observations is to instead use a quantity called the “peak spectral frequency” (or f2) in its place.
But how is f2 measured? Collisions between neutron stars, which are governed by the laws of Einstein’s Theory of Relativity, lead to strong bursts of gravitational wave emission. In 2017, scientists directly measured such emissions for the first time. “At least in principle, the peak spectral frequency can be calculated from the gravitational wave signal emitted by the wobbling remnant of two merged neutron stars,” says Most.
It was previously expected that f2 would be a reasonable proxy for radius, since—until now—researchers believed that a direct, or “quasi-universal,” correspondence existed between them. However, Raithel and Most have demonstrated that this is not always true. They have shown that determining the EoS is not like solving a simple hypotenuse problem. Instead, it is more akin to calculating the longest side of an irregular triangle, where one also needs a third piece of information: the angle between the two shorter sides. For Raithel and Most, this third piece of information is the “slope of the mass-radius relation,” which encodes information about the EoS at higher densities (and thus more extreme conditions) than the radius alone.
This new finding will allow researchers working with the next generation of gravitational wave observatories (the successors to the currently operating LIGO) to better utilize the data obtained following neutron star mergers. According to Raithel, this data could reveal the fundamental constituents of neutron star matter. “Some theoretical predictions suggest that within neutron star cores, phase transitions could be dissolving the neutrons into sub-atomic particles called quarks,” stated Raithel. “This would mean that the stars contain a sea of free quark matter in their interiors. Our work may help tomorrow’s researchers determine whether such phase transitions actually occur.”
Gravitational waves could prove the existence of the quark-gluon plasma
More information:
Carolyn A. Raithel et al, Characterizing the Breakdown of Quasi-universality in Postmerger Gravitational Waves from Binary Neutron Star Mergers, The Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac7c75
Carolyn A. Raithel et al, Characterizing the Breakdown of Quasi-universality in Postmerger Gravitational Waves from Binary Neutron Star Mergers, The Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac7c75
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New tool allows scientists to peer inside neutron stars (2022, October 17)
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“Black widow” neutron star devoured its mate to become heaviest found yet
Enlarge / A spinning neutron star periodically swings its radio (green) and gamma-ray (magenta) beams past Eart. A black widow pulsar heats the facing side of its stellar partner to temperatures twice as hot as the Sun’s surface and slowly evaporates it.
NASA’s Goddard Space Flight Center
Astronomers have determined the heaviest neutron star known to date, weighing in at 2.35 solar masses, according to a recent paper published in the Astrophysical Journal Letters. How did it get so large? Most likely by devouring a companion star—the celestial equivalent of a black widow spider devouring its mate. The work helps establish an upper limit on just how large neutron stars can become, with implications for our understanding of the quantum state of the matter at their cores.
Neutron stars are the remnants of supernovae. As Ars Science Editor John Timmer wrote last month:
The matter that forms neutron stars starts out as ionized atoms near the core of a massive star. Once the star’s fusion reactions stop producing enough energy to counteract the draw of gravity, this matter contracts, experiencing ever-greater pressures. The crushing force is enough to eliminate the borders between atomic nuclei, creating a giant soup of protons and neutrons. Eventually, even the electrons in the region get forced into many of the protons, converting them to neutrons.
This finally provides a force to push back against the crushing power of gravity. Quantum mechanics prevent neutrons from occupying the same energy state in close proximity, and this prevents the neutrons from getting any closer and so blocks the collapse into a black hole. But it’s possible that there’s an intermediate state between a blob of neutrons and a black hole, one where the boundaries between neutrons start to break down, resulting in odd combinations of their constituent quarks.
Short of 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, like 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 nucleus, but when you have 1.5 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.”
This animation shows a black widow pulsar together with its small stellar companion. Powerful radiation and the pulsar’s “wind”—an outflow of high-energy particles—strongly heat the facing side of the companion, evaporating it over time.
The neutron star featured in this latest paper is a pulsar, PSR J0952-0607—or J0952 for short—located in the constellation Sextans between 3,200 and 5,700 light-years away from Earth. Neutron stars are born spinning, and the rotating magnetic field emits beams of light in the form of radio waves, X-rays, or gamma rays. Astronomers can spot pulsars when their beams sweep across Earth. J0952 was discovered in 2017 thanks to the Low-Frequency Array (LOFAR) radio telescope, following up on data on mysterious gamma ray sources collected by NASA’s Fermi Gamma-ray Space Telescope.
Your average pulsar spins at roughly 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 thus far. The current favored hypothesis is that these kinds of pulsars were once part of binary systems, gradually stripping down their companion stars until the latter evaporated away. That’s why such stars are known as black widow pulsars—what Filippenko calls a “case of cosmic ingratitude”:
The evolutionary pathway is absolutely fascinating. Double exclamation point. 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.
This process would explain how J0952 became so heavy. And such systems are a boon to scientists like Filippenko and his colleagues keen to weigh neutron stars precisely. The trick is to find neutron star binary systems in which the companion star is small but not too small to detect. Of the dozen or so black widow pulsars the team has studied over the years, only six met that criteria.
Enlarge / 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.
W. M. Keck Observatory, Roger W. Romani, Alex Filippenko
J0952’s companion star is 20 times the mass of Jupiter and tidally locked in orbit with the pulsar. The side facing J0952 is thus quite hot, reaching temperatures of 6,200 Kelvin (10,700° F), making it bright enough to be spotted with a large telescope.
Fillipenko et al. spent the last four years making six observations of J0952 with the 10-meter Keck telescope in Hawaii to catch the companion star at specific points in its 6.4-hour orbit around the pulsar. They then compared the resulting spectra to the spectra of similar Sun-like stars 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 limit to how large neutron stars can become before collapsing into black holes, as well as winnowing down competing theories on the nature of the quark soup at their cores. “We can keep looking for black widows and similar neutron stars that skate even closer to the black hole brink,” Filippenko said. “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.”
DOI: Astrophysical Journal Letters, 2022. 10.3847/2041-8213/ac8007 (About DOIs).