Neutron Stars

[Mighty_Emperor]

......things get vey weird indeed:

Bizarre Matter Found in a Neutron Star

Summary - (Sep 9, 2004) Scientists have theorized that the inside of a neutron star - the remnant from a star that has collapsed under its own gravity - is a special place where the laws of physics begin to break down; atoms are squeezed so tightly by gravity that all protons and electrons are crushed into neutrons which swirl around like a liquid, but without friction (called a superfluid). This theory has gotten some confirmation according to new research from NASA which observed neutron star EXO 0748-676, located 30,000 light years away. Using various instruments, NASA scientists determined that it's approximately 11.5 km (7 miles) in diameter, and contains 1.75 solar masses. With this much mass packed into a small area, the observations match the theory that neutron stars exist in this superfluidic state, but without being crushed further.

Full Story - Scientists have obtained their best measurement yet of the size and contents of a neutron star, an ultra-dense object containing the strangest and rarest matter in the Universe.

This measurement may lead to a better understanding of nature's building blocks -- protons, neutrons and their constituent quarks -- as they are compressed inside the neutron star to a density trillions of times greater than on Earth.

Dr. Tod Strohmayer of NASA's Goddard Space Flight Center in Greenbelt, Md., and his colleague, Adam Villarreal, a graduate student at the University of Arizona, present these results today during a Web-based press conference in New Orleans at the meeting of the High Energy Astrophysics Division of the American Astronomical Society.

They said their best estimate of the radius of a neutron star is 7 miles (11.5 kilometers), plus or minus a stroll around the French Quarter. The mass appears to be 1.75 times that of the Sun, more massive than some theories predict. They made their measurements with NASA's Rossi X-ray Timing Explorer and archived X-ray data

The long-sought mass-radius relation defines the neutron star's internal density and pressure relationship, the so-called equation of state. And this, in turn, determines what kind of matter can exist inside a neutron star. The contents offer a crucial test for theories describing the fundamental nature of matter and energy and the strength of nuclear interactions.

"We would really like to get our hands on the stuff at the center of a neutron star," said Strohmayer. "But since we can't do that, this is about the next best thing. A neutron star is a cosmic laboratory and provides the only opportunity to see the effects of matter compressed to such a degree."

A neutron star is the core remains of a star once bigger than the Sun. The interior contains matter under forces that perhaps existed at the moment of the Big Bang but which cannot be duplicated on Earth. The neutron star in today's announcement is part of a binary star system named EXO 0748-676, located in the constellation Volans, or Flying Fish, about 30,000 light-years away, visible in southern skies with a large backyard telescope.

In this system, gas from a "normal" companion star plunges onto the neutron star, attracted by gravity. This triggers thermonuclear explosions on the neutron star surface that illuminate the region. Such bursts often reveal the spin rate of the neutron star through a flickering in the X-ray light emitted, called a burst oscillation. (Refer to Items 1 - 6 for an artist's concept of this process. A movie and a detailed caption can be found in the blue column on the right.)

The scientists detected a 45-hertz burst oscillation frequency, which corresponds to a neutron star spin rate of 45 times per second. This is a leisurely pace for neutron stars, which are often seen spinning over 300 times per second.

The scientists next capitalized on EXO 0748-676 observations with the European Space Agency's XMM-Newton satellite from 2002, led by Dr. Jean Cottam of NASA Goddard. Cottam's team had detected spectral lines emitted by hot gas, similar in look to the lines of a cardiogram. These lines had two features. First, they were Doppler shifted. This means the energy detected was an average of the light spinning around the neutron star, moving away from us and then towards us. Second, the lines were gravitationally redshifted. This means that gravity pulled on the light as it tried to escape the region, stealing a bit of its energy.

Strohmayer and Villarreal determined that the 45-hertz frequency and the observed line widths from Doppler shifting are consistent with a neutron star radius between 9.5 and 15 kilometers, with the best estimate at 11.5 kilometers. The relationship among burst frequency, Doppler shifting and radius is that the velocity of gas swirling around the star's surface depends on the star's radius and its spin rate. In essence, a faster spin corresponds to a wider spectral line (a technique similar to how a state trooper can detect speeding cars).

Cottam team's gravitational redshift measurement offered the first measure of a mass-radius ratio, albeit without knowledge of a mass and radius. This is because the degree of redshifting (strength of gravity) depends on the mass and radius of the neutron star. Some scientists had questioned this measurement, for the spectral lines detected seemed too narrow. The new results strengthen the gravitational redshift interpretation of the Cottam team's spectral lines (and thus the mass-radius ratio) because a slower-spinning star can easily produce such relatively narrow lines.

So, ever more confident of the mass-radius ratio and now knowing the radius, the scientists could calculate the neutron star's mass. The value was between 1.5 and 2.3 solar masses, with the best estimate at 1.75 solar masses.

The result supports the theory that matter in the neutron star in EXO 0748-676 is packed so tightly that almost all protons and electrons are squeezed into neutrons, which swirl about as a superfluid, a liquid that flows without friction. Yet the matter isn't packed so tightly that quarks are liberated, a so-called quark star.

"Our results are really starting to put the squeeze on the neutron star equation of state," said Villareal. "It looks like equations of state which predict either very large or very small stars are nearly excluded. Perhaps more exciting is that we now have an observational technique that should allow us to measure the mass-radius relations in other neutron stars."

A proposed NASA mission called the Constellation X-ray Observatory would have the ability to make such measurements, but with much greater precision, for a number of neutron star systems.

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Original Source: NASA News Release

http://www.universetoday.com/am/publish/bizarre_matter_found_neutron_star.html?992004

NASA news release:
http://www.gsfc.nasa.gov/topstory/2004/0908nsmatter.html

 

[Rubyait]

Hot spots on neutron stars tracked for first time

Hot spots on neutron stars have been tracked for the first time, by astronomers using an X-ray space telescope.

The breakthrough result will help improve the understanding of neutron stars’ internal structure and the dominant role of their strong magnetic fields. It also provides the first measurement of very small-sized features on objects hundreds to thousands of light years away, astronomers say.

Neutron stars are the remnants of some supernovae explosions, formed when the stellar core collapses and compresses protons and electrons into neutrons. The stars are very dense, only about 20 kilometres across and they also spin rapidly, which creates intense magnetic fields.

Italian astronomers used the European Space Agency’s XMM-Newton spacecraft to measure the X-rays from three neutron stars. They divided each star into 10 segments and measured their temperature. As the stars spun, at about five times per second, astronomers were able to track their hot spots, which were three or four times hotter than the surface surrounding them.

The hot spots may be where each star’s magnetic field accelerates particles and funnels them back to the surface - focusing electromagnetic energy on a specific region. On Earth, charged particles from the Sun are channeled through concentrated magnetic field lines at the poles, giving rise to aurora.

“[A hot spot] could be the pole of the magnetic field of the neutron star,” says Patrizia Caraveo, at the Istituto Nazionale di Astrofisica in Milan, Italy.

Unexpectedly large
Astronomers thought that neutron stars should have hot spots, but these had not been observed until now. The acceleration of charged particles along magnetic field lines is one explanation for the hot spots. Heat transfer from the stars’ core to the surface is another.

But as Caraveo’s team notes, there are some problems with the study. For example, one of the neutron stars they examined - PSR B0656+14 - would have to have a diameter of 42 km to fit in with their observations, says Coleman Miller, based at the University of Maryland, US. That would be unexpectedly large for a neutron star, as most have diameters between 16 km and 24 km.

In addition, the observed size of stars’ hot spots varies from 50 metres to almost 2 km - something researchers cannot explain.

The researchers suggest that several of the neutron stars may not have two conventional poles, but rather, two or more hot spots close to one another, giving the impression of one large “super spot”.

http://www.newscientist.com/article.ns?id=dn7307

 

[KeyserXSoze]

Starquake!Source

Huge Quake Cracks Star

By Bjorn Carey
LiveScience Staff Writer
posted: 27 September 2005
12:01 am ET

Astronomers have found the first evidence of cracks in a neutron star's crust. The star cracked when it was rocked by the strongest "starquake" ever recorded, researchers said last week.

Last December, astronomers worldwide monitored the explosion that caused this starquake. The eruption was huge – in the first 200 milliseconds of the event the star released energy equivalent to what our Sun produces in 250,000 years. It was the brightest explosion ever detected outside of the Milky Way.

Now scientists have used a collection of data from various satellites to provide the first observational evidence that the blast caused the star to form cracks several miles long. Scientists hope these cracks will provide a window into the mysterious interiors of neutron stars.

There are millions of neutron stars in the Milky Way galaxy alone, and some of these have magnetic fields trillions of times stronger than Earth's, the strongest of which are called magnetars.

This particular magnetar – SGR 1806-20 – is surrounded by the strongest magnetic field known in the universe. This could explain why the starquake – caused when the magnetar's crust could no longer contain the magnetic stress building in the star's interior – was so intense.

A magnetar's interior is a dense, liquid-like mix of neutrons, protons, and electrons – making it a terrific conductor of electricity. Because it has the characteristics of a fluid, it moves around a lot. The magnetar's magnetic field loops around the star, and all this movement in the interior messes with the field's shape, winding it up like you might do with a rubber band.

But the magnetar's exterior crust is not so pliable. The crust is made mostly of iron. The magnetic field passes through it in places, which isn't a problem for normal neutron stars. But in magnetars, the field interacts with the core and shifts around erratically, causing crustal stress. Eventually, the stress reaches the point where the crust cracks.

"Imagine threading a rubber band between two cards, and then twisting the middle," study leader Steve Schwartz of the Imperial College of London told SPACE.com. "All those twisting stresses accumulate at the points where the rubber band threads through the card to the outside. Keep twisting long enough and you will rip the card."

The first crack to form was three miles (five kilometers long) – significant since this magnetar is only six miles (10 kilometers) in diameter. Radiation spewed from this crack, causing a steep initial increase in detectable radiation.

But that was just the beginning. Radiation continued to spill out of the star, but at a much slower rate than the initial burst. This suggests that

"Whether this is a set of long, [three mile] cracks, or a multitude of much smaller ones isn't obvious to me," Schwartz said. "My hunch is therefore: one big one, followed by lots and lots of ongoing smaller ones."

What this means for SGR 1806-20 isn't clear, but it seems that cracks form more to relieve pressure than as a sign that the star is blowing apart.

"The result of the cracking is to relax the interior and exterior field to a less twisted state," Schwartz said. "This has very little impact on the star itself, other than the fact that it will take time to twist up the field again."

SGR 1806-20 is 50,000 light-years away, but the blast was so huge it temporarily blinded some satellites and briefly altered Earth's upper atmosphere. A similar blast occurring within 10 light-years of our planet would fry Earth's ozone layer. But don't worry – the closest magnetar is 13,000 light-years away.

Two satellites designed to study the Earth's magnetosphere – the European Space Agency's Cluster and Double Star satellites – didn't go offline and recorded the entire event. Data from these two satellites was combined with observations from around the world to uncover the cracks.

So far, nine magnetars have been firmly identified, and four of these repeatedly emit bursts of X-rays and gamma rays. SGR 1806-20, which has a magnetic field more powerful than any other object in the universe, is one of these so-called soft gamma repeaters.

Researchers still don't know why SGR 1806-20's burst was so incredible, but they hope that a look into its cracks will help solve the mystery.

8)

 

[rynner2]

Rare dead star found near Earth

Astronomers have spotted a space oddity in Earth's neighbourhood: a dead star with some unusual characteristics.
The object - known as a neutron star - was studied using space telescopes and ground-based observatories.

But this one, located in the constellation Ursa Minor, seems to lack some key characteristics found in other neutron stars.

Details of the study, by a team of American and Canadian researchers, will appear in the Astrophysical Journal.

If confirmed, it would be only the eighth known "isolated neutron star" - meaning a neutron star that does not have an associated supernova remnant, binary companion, or radio pulsations.

The object has been nicknamed Calvera, after the villain in the 1960s western film The Magnificent Seven.

"The seven previously known isolated neutron stars are known collectively as The Magnificent Seven within the community," said co-author Derek Fox, of Pennsylvania State University, US.

"So the name Calvera is a bit of an inside joke on our part."

The authors estimate that the object is 250 to 1,000 light-years away. This would make Calvera one of the closest neutron stars to Earth - and possibly the closest.

Neutron stars are one of the possible end points for a star. They are created when stars with masses greater than four to eight times those of our Sun exhaust their nuclear fuel, and undergo a supernova explosion.

This explosion blows off the outer layers of the star, forming a supernova remnant. The central region of the star collapses under gravity, causing protons and electrons to combine to form neutrons - hence the name "neutron star".

Data search

Robert Rutledge of McGill University in Montreal, Canada, originally noticed the object.

He compared a catalogue of 18,000 X-ray sources from the German-American Rosat satellite, which operated from 1990 to 1999, with catalogues of objects that appeared in visible light, infrared light, and radio waves.

Professor Rutledge realized that a Rosat source, known as 1RXS J141256.0+792204, did not appear to have a counterpart at any other wavelength.

The group aimed Nasa's Swift satellite at the object in August 2006. Swift's X-ray telescope showed that the source was still there, and was emitting about the same amount of X-ray energy as it had during the Rosat era.

The Swift observations enabled the group to pinpoint the object's position more accurately, and showed that it was not associated with any known astronomical object.

The researchers followed up with the 8.1m Gemini North Telescope in Hawaii and a short observation by Nasa's Chandra X-ray Observatory.

Unusual properties

Exactly what type of neutron star Calvera is remains a mystery. According to Dr Rutledge, there are no widely accepted alternative theories to explain objects such as this that are bright in X-rays and faint in visible light.

"Either Calvera is an unusual example of a known type of neutron star, or it is some new type of neutron star, the first of its kind," said Dr Rutledge.

Calvera's location high above the plane of our Milky Way galaxy is also a mystery. The researchers believe the object is the remnant of a star that lived in our galaxy's starry disc before exploding as a supernova.

In order to reach its current position, it had to wander some distance out of the disc.

http://news.bbc.co.uk/1/hi/sci/tech/6955769.stm

 

[rynner2]

Home computers discover rare star

By putting their home computers to work when they would otherwise be idle, three "citizen scientists" have discovered a rare astronomical object.

The unusual find is called a "disrupted binary pulsar"; these pulsars can be created when a massive star collapses.

The discoverers, from the US and Germany found the object with the help of the Einstein@Home project.

It asks users to donate time on their computers, allowing them to be used for searching through scientific data.

This type of project is known as "distributed computing". Einstein@Home harnesses the power of home machines in order to process large amounts of data.

Credited with the discovery are Chris and Helen Colvin, both information technology professionals from Iowa, US, and systems analyst Daniel Gebhardt from Mainz in Germany.

Their computers, along with 500,000 others from around the world, are being used to analyse data for Einstein@Home.

Users download a screensaver which, among other things, shows the area of sky being processed.

The newly discovered radio pulsar, given the designation PSR J2007+2722, is a fast-spinning neutron star which can be formed in certain types of supernovae, or stellar explosions.

This lone pulsar rotates 41 times per second and has an unusually low magnetic field.

Jim Cordes, professor of astronomy at Cornell University in Ithaca, US, said the object once had a companion star from which it acquired mass. But this companion has since exploded; this has kicked the surviving object free.

"We think there should be more of these disrupted binary pulsars, but there haven't been that many found," said Professor Cordes.

"No matter what else we find out about it, this pulsar is bound to be extremely interesting for understanding the basic physics of neutron stars and how they form."

He said the discovery demonstrated the power of such distributed computing networks to collect and sort through vast amounts of data.

Einstein@Home was originally organised to find gravitational waves - ripples in space-time. The latest find was made by searching through radio astronomy data collected the Arecibo Observatory in Puerto Rico, US.

About one-third of Einstein@Home's computing capacity is used to search Arecibo data.

It is the first deep space discovery made by the project.

http://www.bbc.co.uk/news/science-environment-10959590

 

[rynner2]

Black hole mystery solved by magnetic star discovery
By Howard Falcon-Lang, Science reporter

The discovery of a rare magnetic star - or magnetar - is challenging theories about the origin of black holes.

Magnetars are a special type of neutron star with a powerful magnetic field.

They are formed by gravitational collapse after the original, or progenitor star, dies and forms a catastrophic supernova.

For this newly discovered magnetar, astronomers calculated that the mass of the progenitor must have been at least 40 times greater than that of our Sun.

Collapsing stars of this size should form a black hole. The fact that this one resulted in a neutron star, challenges established theory.

The study, led by Dr Ben Ritchie of the Open University, is published in the journal Astronomy and Astrophysics.

The new magnetar was found in an extraordinary star cluster known as Westerlund 1, located 16,000 light years away in the southern constellation of Ara (the Altar). This region contains numerous massive stars.

Dr Ritchie remarked that if the Earth was "located at the heart of this remarkable cluster, our night sky would be full of hundreds of stars as bright as the full Moon".

To calculate the mass of the progenitor star, the research team estimated its lifespan. Massive stars collapse earlier than small stars because the pressure on their core is greater, causing them to burn up their hydrogen fuel more rapidly.

The astronomers assumed that this star formed at the same time as others in the same cluster.

So the fact that this star had already collapsed shows that it must have been more massive than the other stars that still exist there.

Stars that are more than 25 times more massive than our Sun normally collapse to form black holes.

Dr Negueruela of the University of Alicante in Spain, a co-author on the study, said that the mystery of the missing black hole might be explained if the progenitor star got rid "of nine tenths of its mass before exploding as a supernova".

One way of achieving this "diet plan" would be if the progenitor was part of cosmic double-act known as a "binary star", and its companion pulled off some of its mass. This would have allowed it to avoid the fate of becoming a black hole.

Professor Mike Cruise, an astrophysicist at the UK's University of Birmingham, who was not involved in the study, told BBC News that the new research was "a brilliant piece of detective work".

He commented: "What is especially attractive about this paper is the way the researchers' arguments are based on robust measurements, not just theory."

http://www.bbc.co.uk/news/science-environment-11011118

 

[rynner2]

Biggest neutron star ever detected is twice the mass of the sun
The biggest neutron star ever detected in the universe that is nearly twice the mass of the sun has been identified by astronomers.
By Andrew Hough
Published: 11:00PM BST 27 Oct 2010

Researchers found that despite being comparatively small and around the size of a small city the star is so tightly crushed that its weight is immense.

Just a thimbleful of material from the star, named PSR J1614-223, would weigh 500 million tonnes.

Astronomers discovered that the neutron star, the "corpse" that remains after a star has undergone a supernova, is twice as massive as our sun.

It is located about 3,000 light-years away in the direction of the constellation Scorpio, a newly spotted neutron star is the largest ever discovered to date.

Scientists believe its discovery could have wide ranging implications for our understanding of physics.
Because they are so dense, the stars are an ideal natural lab for scientists to study some of the most exotic states of matter known to physics, which can only exist in such extreme environments.

The star was found by the National Science Foundation's Green Bank Telescope (GBT).

Scientists used an effect of Einstein's theory of relativity to measure its mass and that of its orbiting companion, a white dwarf star.

"This neutron star is twice as massive as our Sun," said Paul Demorest, from the National Radio Astronomy Observatory (NRAO), in Charlottesville, Virginia.

"This is surprising and that much mass means that several theoretical models for the internal composition of neutron stars now are ruled out.
"This mass measurement also has implications for our understanding of all matter at extremely high densities and many details of nuclear physics."

The neutron star is a pulsar, which emits beams of radio waves like a lighthouse as it spins.
The orbit of the white dwarf takes it directly in front of the pulsar, causing a delay for the time the radio waves take to reach earth.
Because this delay, called the Shapiro Effect, is caused by gravity, scientists can use it to precisely caclulate the mass of both stars.

Scott Ransom, an astronomer also from the NRAO, added: "We got very lucky with this system.
"The rapidly-rotating pulsar gives us a signal to follow throughout the orbit, and the orbit is almost perfectly edge-on.
"In addition, the white dwarf is particularly massive for a star of that type. This unique combination made the Shapiro Delay much stronger and thus easier to measure."

The researchers expected the neutron star to have roughly one and a half times the mass of the Sun. Instead, their observations revealed it to be twice as massive as the Sun.

This means that previously held ideas about the compisition of neutron stars cannot be right, say the team, writing in the journal Nature.

Mr Ransom said: "Pulsars in general give us a great opportunity to study exotic physics, and this system is a fantastic laboratory sitting out there, giving us valuable information with wide-ranging implications.
"It is amazing to me that one simple number - the mass of this neutron star - can tell us so much about so many different aspects of physics and astronomy."

http://www.telegraph.co.uk/science/spac ... e-sun.html

 

[OneWingedBird]

Way over my space cadet head... still interesting, I never knew that pulsars could glitch before!

The maximum glitch observed in a pulsar systematically constrains its mass

Pulsar glitches, sudden jumps in frequency in otherwise steadily spinning down radio pulsars, offer a unique glimpse into the superfluid interior of neutron stars. The exact trigger of these events remains, however, elusive and this has hampered attempts to use glitch observations to constrain fundamental physics. In this paper we propose a new method to measure the mass of glitching pulsars, using observations of the maximum glitch recorded in a star, together with state of the art microphysical models of the pinning interaction between superfluid vortices and ions in the crust. Studying systematically all the presently observed large glitchers, we find an inverse correlation between size of the maximum glitch and the pulsar mass. Our procedure will allow current and future observations of glitching pulsars to constrain not only the physics of glitch models but also the equation of state of dense matter in neutron star interiors.
Comments: 4 figures, 1 table

Arxiv

 

[Comfortably Numb]

Radical Change’ Needed After Latest Neutron Star Collision

Source: quantamagazine.org
Date: 20 February, 2020

A recent neutron star merger has defied astronomers’ expectations, leading them to question longstanding ideas about neutron stars and the supernovas that create them. “We have to go back to the drawing board.”

[...]

Last summer, the gravitational wave observatory known as LIGO caught its second-ever glimpse of two neutron stars merging. The collision of these incredibly dense objects — the hulking cores of long-ago supernova explosions — sent shudders through space-time powerful enough to be detected here on Earth. But unlike the first merger, which conformed to expectations, this latest event has forced astrophysicists to rethink some basic assumptions about what’s lurking out there in the universe. “We have a dilemma,” said Enrico Ramirez-Ruiz of the University of California, Santa Cruz.

The exceptionally high mass of the two-star system was the first indication that this collision was unprecedented. And while the heft of the stars alone wasn’t enough to cause alarm, it hinted at the surprises to come.

In a paper recently posted to the scientific preprint site arxiv.org, Ramirez-Ruiz and his colleagues argue that GW190425, as the two-star system is known, challenges everything we thought we knew about neutron star pairs. This latest observation appears to be fundamentally incompatible with scientists’ current understanding of how these stars form, and how often. As a result, researchers may need to rethink years of accepted knowledge.

https://www.quantamagazine.org/radical-change-needed-after-latest-neutron-star-collision-20200220/

 

[EnolaGaia]

Radical Change’ Needed After Latest Neutron Star Collision

... And one of those radical changes is the emerging suspicion the material within the cores of certain neutron stars consists of "quark soup."

There's Now Strong Evidence That an Exotic Type of Matter Exists Inside Neutron Stars

Neutron stars are rising to the top of the list of the most delicious objects in the Universe. First, it was dense "nuclear pasta" beneath their crusts. Now, we have fresh evidence that the cores of the most massive neutron stars are made up of an exotic 'soup' of subatomic particles called quarks.

Physicists have produced new calculations using data from gravitational waves first detected from a neutron star collision in August 2017, along with observations of surprisingly massive neutron stars. Their conclusion hints at an exciting result - the cores of the most massive neutron stars are so dense, atomic nuclei cease to exist, condensing into quark matter.

It is, the researchers say, an important milestone in understanding the strange innards of these extreme objects.

"Confirming the existence of quark cores inside neutron stars has been one of the most important goals of neutron star physics ever since this possibility was first entertained roughly 40 years ago," said theoretical physicist Aleksi Vuorinen of the University of Helsinki and the Helsinki Institute of Physics. ...

For a few decades, astronomers have hypothesised that, under high-enough heat and density, neutrons break down even further into their constituent quarks, creating a sort of quark soup. ...

It's not an absolute slam-dunk; but the calculations indicate that something really peculiar would have to be going on, if the cores of these stars are not quark matter.

"There is still a small but nonzero chance that all neutron stars are composed of nuclear matter alone," Vuorinen explained.

"What we have been able to do however, is quantify what this scenario would require. In short, the behaviour of dense nuclear matter would then need to be truly peculiar. For instance, the speed of sound would need to reach almost that of light."

The discovery of quark matter inside neutron stars wouldn't just be amazing for its own sake - it could help us learn more about the very earliest moments of our Universe. ...

FULL STORY:
https://www.sciencealert.com/collid...led-a-primitive-kind-of-matter-in-their-cores

PUBLISHED RESEARCH REPORT:
https://www.nature.com/articles/s41567-020-0914-9