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Do the neutrons in neutron stars emit the radio waves?

Do the neutrons in neutron stars emit the radio waves?


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Neutrons can, especially in extreme circumstances (and large concentrations) emit electromagnetic radiation. I specifically asked about this in Physics S.E. Has a free neutron ever been shown to absorb/emit/interact with a photon?, the answer has to do with the fact that although neutrons have no detectable electric moment or dipole, they DO have a small, 'negative' magnetic moment.

But little I have read about neutron stars says which particles in or around them emits the radio waves, except one brief article mentioning infalling electrons from a surrounding plasma.


The electromagnetic radiation comes from accelerated charged particles, mainly electrons and positrons.

The surface of a neutron star is not made of neutrons. It is a totally ionised gas of nuclei plus electrons, with a composition that could range from iron-peak elements to hydrogen and helium accreted from the interstellar medium.

As a result of their rotation and strong dipolar magnetic fields then there are also very strong electric fields at the surface of the neutron star. These fields rip electrons and protons from the surface and accelerate them away - electrons from the magnetic poles and protons at lower magnetic latitudes. The classic work on this is by Julian & Goldreich (1969).

The acceleration due to these electric fields is easily sufficient to overcome gravity. The particles are forced to spiral along the magnetic field lines by the Lorentz force, and produce synchrotron or curvature radiation directly. The photons from these processes are then capable of creating secondary electron-positron pairs that are accelerated in opposite directions, releasing more photons in a cascade.

No neutrons are harmed in the making of this radiation, other than the ones in nuclei at the surface of the neutron star that get bombarded by high energy accelerated electrons and positrons.


Gravitational Waves Shed Light on Neutron Star Interiors

By: Elizabeth Howell May 9, 2018 0

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The gravitational-wave detection last year of a neutron star merger has revealed details on neutron star structure, ruling out exotic quark matter in the objects’ cores.

Artist's illustration of the final stages of a neutron-star merger.
NASA / Goddard Space Flight Center

A pair of independent studies gives new constraints on the size of neutron stars, suggesting that they are no more than 14 kilometers (8.6 miles) in radius. That's about twice the length of the Las Vegas strip. This size limit is slightly larger than previous estimates, suggesting that neutron stars might be less exotic than previously thought.

Neutron stars are the dense stellar remnants of supernova explosions. Within a tiny radius, they contain a mass of about 1.4 times that of the sun. The extreme densities and pressures smush electrons into the atomic nuclei their orbit — protons and electrons combine into neutrons, so that neutron stars are made mostly of neutrons. But there’s a possibility that the density at their cores might become so high, it breaks matter down into even smaller particles, such as quarks.

As astrophysicist Feryal Özel (University of Arizona) explained in the July 2017 issue of Sky & Telescope, for neutron stars size really does matter — the smaller the star, the higher its core density. Previous measurements have pointed to a maximum neutron star radius between 10 and 11 km. That may not sound very different from 14 km, but it would be enough to raise the central density by more than a factor of two. "This is enough to have a profound effect on the amount of repulsion the particles experience," Özel wrote, which would introduce the possibility of a quark-filled core.

The new neutron star sizes, published in two papers appearing in April 25th Physical Review Letters, are based on the August 17, 2017, LIGO/Virgo detection of gravitational waves from a pair of neutron stars merging 130 million light-years away. Massive objects, such as black holes and neutron stars, emit these ripples in spacetime as they move through space. The gravitational waves observed from the neutron star collision served as a probe of the objects’ structure. Even though the two papers used different approaches, they calculated roughly the same maximum size for neutron stars: Eemeli Annala (University of Helsinki, Finland) led a study that limited it to 13.6 km, while Farrukh J. Fattoyev (Indiana University) and colleagues limited it to 13.76 km.

Neutron Stars in the Lab

Neutron star
Casey Reed / Penn State University

Given their extremely high density, astronomers aren’t certain what neutron stars look like on the inside. Some of their ideas are based on nuclear physics, while the concept of quark matter in particular is based on the physics of high-energy particles. The various approaches can give different predictions about neutron stars’ internal structure.

Experiments at the Large Hadron Collider (LHC) at CERN and the Relativistic Heavy Ion Collider at Brookhaven National Laboratory give a sense of what a neutron star might look like in its core. Researchers at these institutions smash lead ions together at close to the speed of light to produce the high temperatures that break down protons and neutrons into a quark-gluon plasma,

"These collisions create ion-sized droplets of matter so dense that the structure of the protons and neutrons melts, and we are left with a small droplet of quark matter for a very brief moment," says theoretical physicist Aleksi Kurkela (CERN), Annala’s coauthor. "We think that this hot quark-gluon plasma is closely related to the 'cool' quark matter that we may find in the cores of neutron stars. By studying the properties of the quark-gluon plasma, we try to learn and infer what is happening in the cores of neutron stars."

If neutron stars produce quarks in their centers, they might undergo a phase change. “We could potentially observe . . . neutron stars with similar masses but with quite different radii," Kurkela explains. "Then the interpretation would be that the one with larger radius would be made of stiffer material, supposedly neutron matter. The smaller one would be made of, or at least would have a core made of, softer material which could be quark matter."

"While our current theories provide a very good description of dense matter at nuclear densities, their predictions significantly deviate when extrapolated to super-nuclear densities," adds Fattoyev (Indiana University).

Indeed, some of LIGO's observations aren't matching up with what scientists previously theorized, specifically with regards to the types of matter found inside of neutron stars, Kurkela says.

From Shape to Size

This artist's conception portrays two neutron stars at the moment of collision.
Dana Berry / SkyWorks Digital, Inc.

As two neutron stars circle each other, their respective gravitational fields create tidal forces in their partner: Gravity pulls more strongly on the side of the star closer to its companion compared to its far side. As a result, both neutron stars stretch, tidally deforming into a shape resembling a rugby ball, Kurkela explains.

The neutron stars’ shapes show what they are made of. If the matter inside of neutron stars were soft, that is, containing quarks in addition to neutrons, LIGO would see the neutron stars deform. But LIGO's observations don't fit those theories Instead, Kurkela explains, LIGO's work showed that the neutron stars were like hard, unsquishable balls, even as they merged into each other, which means they contain only neutrons in their cores. The results allowed investigators to rule out the existence of quarks inside of neutron stars.

Scientists will need more gravitational-wave observations to confirm what LIGO saw. Moreover, since neutron star collisions generate light in addition to gravitational waves, scientists hope to get more information on composition through follow-up X-ray observations, such as from the Neutron star Interior Composition Explorer (NICER) perched on the International Space Station.


Astronomers detect the most massive neutron star ever measured

Neutron stars are the compressed remains of massive stars gone supernova. WVU astronomers were part of a research team that detected the most massive neutron star to date. Credit: B. Saxton (NRAO/AUI/NSF)

West Virginia University researchers have helped discover the most massive neutron star to date, a breakthrough uncovered through the Green Bank Telescope in Pocahontas County.

The neutron star, called J0740+6620, is a rapidly spinning pulsar that packs 2.17 times the mass of the sun (which is 333,000 times the mass of the Earth) into a sphere only 20-30 kilometers, or about 15 miles, across. This measurement approaches the limits of how massive and compact a single object can become without crushing itself down into a black hole.

The star was detected approximately 4,600 light-years from Earth. One light-year is about six trillion miles.

These findings, from the National Science Foundation-funded NANOGrav Physics Frontiers Center, were published today (Sept. 16) in Nature Astronomy.

Authors on the paper include Duncan Lorimer, astronomy professor and Eberly College of Arts and Sciences associate dean for research Eberly Distinguished Professor of Physics and Astronomy Maura McLaughlin Nate Garver-Daniels, system administrator in the Department of Physics and Astronomy and postdocs and former students Harsha Blumer, Paul Brook, Pete Gentile, Megan Jones and Michael Lam.

The discovery is one of many serendipitous results, McLaughlin said, that have emerged during routine observations taken as part of a search for gravitational waves.

"At Green Bank, we're trying to detect gravitational waves from pulsars," she said. "In order to do that, we need to observe lots of millisecond pulsars, which are rapidly rotating neutron stars. This (the discovery) is not a gravitational wave detection paper but one of many important results which have arisen from our observations."

Artist impression and animation of the Shapiro Delay. As the neutron star sends a steady pulse towards the Earth, the passage of its companion white dwarf star warps the space surrounding it, creating the subtle delay in the pulse signal. Credit: BSaxton, NRAO/AUI/NSF

The mass of the pulsar was measured through a phenomenon known as "Shapiro Delay." In essence, gravity from a white dwarf companion star warps the space surrounding it, in accordance with Einstein's general theory of relativity. This makes the pulses from the pulsar travel just a little bit farther as they travel through the distorted spacetime around white dwarf. This delay tells them the mass of the white dwarf, which in turn provides a mass measurement of the neutron star.

Neutron stars are the compressed remains of massive stars gone supernova. They're created when giant stars die in supernovas and their cores collapse, with the protons and electrons melting into each other to form neutrons.

To visualize the mass of the neutron star discovered, a single sugar-cube worth of neutron-star material would weigh 100 million tons here on Earth, or about the same as the entire human population.

While astronomers and physicists have studied these objects for decades, many mysteries remain about the nature of their interiors: Do crushed neutrons become "superfluid" and flow freely? Do they breakdown into a soup of subatomic quarks or other exotic particles? What is the tipping point when gravity wins out over matter and forms a black hole?

"These stars are very exotic," McLaughlin said. "We don't know what they're made of and one really important question is, 'How massive can you make one of these stars?' It has implications for very exotic material that we simply can't create in a laboratory on Earth."

Pulsars get their name because of the twin beams of radio waves they emit from their magnetic poles. These beams sweep across space in a lighthouse-like fashion. Some rotate hundreds of times each second.

Since pulsars spin with such phenomenal speed and regularity, astronomers can use them as the cosmic equivalent of atomic clocks. Such precise timekeeping helps astronomers study the nature of spacetime, measure the masses of stellar objects and improve their understanding of general relativity.


A many-splendored explosion

The merging neutron stars powered a seconds-long gamma ray burst that beamed radiation into space. They also sparked a kilonova that glowed for days as it generated heavy elements.

The combination of gravitational waves and electromagnetic observations scored at least three significant advances. First, it explains the origins of some gamma ray bursts, the second most powerful known events in the cosmos other than merging black holes. Since the 1990s, theorists have thought that bursts shorter than two seconds originate when neutron stars merge to create a black hole. (Longer bursts, lasting minutes, are thought to spring from the collapse of individual massive stars.) The new result clinches the explanation for short bursts, says Peter Mészáros, a theorist at Pennsylvania State University in State College. "It's tremendous," he says. "If you have gravitational waves with a burst you know it has to come from a double neutron star."

Second, the event reveals a hypothesized object called a kilonova, because it briefly shines thousands of times brighter than an ordinary nova. As two neutron stars twirl together and rip each other apart, they should expel neutron-rich atomic nuclei, forming a shroud of matter totaling a few percent of a solar mass. Those nuclei beef up by gobbling neutrons in rapid succession and then quickly change their chemical identities through radioactive decay. That so-called r-process—or rapid neutron capture process—should make the shroud glow for a few days, and its light should be reddened by heavy elements that soak up blue wavelengths. That's just what astronomers saw, says Brian Metzger, a theorist at Columbia University. "It's stunning. All of a sudden the curtain lifts and what we see looks pretty close to what we expected."

The observation of a kilonova scores a third advance by solving a long-standing puzzle in nuclear physicists: the origin of half the elements heavier than iron, including silver, gold, and platinum. Nuclear physicists have long thought that those elements are generated in r-process, but haven't known where in the cosmos that happens—whether in the collapse of single stars or in merging neutron stars. The new find shows that some, and quite possibly all, of the mystery elements come from neutron-star death spirals. "For me, as a nuclear physicist, this is an extremely important result," says Witold Nazarewicz a theorist at Michigan State University in East Lansing, where experimenters are building a $730 million accelerator, in part to study the r-process.

The neutron star merger presents some puzzles of its own. For example, the gamma rays were relatively faint, even though the burst was closer than any previously measured short burst by a factor of 10, McEnery notes. That could be because researchers saw the merger from a funny angle, she says. A gamma ray burst is thought to emerge when jets of hot matter moving at near–light-speed shoot out along the rotational axis of the newborn black hole, beaming radiation into space like a lighthouse. In this case, observers on Earth may not be looking right down the jet but may be viewing it from a slight angle, McEnery says—astronomers’ first off-axis view of an astrophysical jet.

The long lag before astronomers began to pick up radio and x-ray emissions supports that picture, says Raffaella Margutti, an astrophysicist at Northwestern University in Evanston, Illinois, who studied the event with NASA's orbiting Chandra X-ray Observatory. The radio and x-ray signals come from the jet, which at first would have beamed them too narrowly along its axis to be seen from Earth. As the jet slowed, however, radiation would emerge at wider angles, making the signals detectable off-axis.

Ever since LIGO announced the first gravitational-wave event in early 2016, networks of small telescopes around the world have been poised to detect an “optical counterpart.” The race touched off by this latest event was won by Ryan Foley of the University of California (UC), Santa Cruz, and colleagues. They use 1-meter telescopes on Mount Hamilton in California and on Cerro Las Campanas in Chile to follow up LIGO/Virgo alerts. At 23:33 universal time, 10 hours and 52 minutes after the gravitational waves arrived, the team used the telescope in Chile to snap an image of NGC 4993, and Charles Kilpatrick, a postdoc at UC Santa Cruz, saw a bright spot not visible in archival images of the galaxy. "Found something," he remarked coolly in an online messaging exchange. Within the 40 minutes, four other teams had independently discovered the same optical object.

Rumor spread almost instantly over the internet. Within days, other scientists and journalists knew the outlines of the discovery, and the LIGO and Virgo teams struggled to keep a lid on the news until today's press event. That was no easy task, given the fact that astronomers tend to work in small, highly competitive teams, says Andrew Howell, an astronomer at UC Santa Barbara, and staff scientist with the Las Cumbres Observatory, which also tracked the event. Used to working as a huge team, LIGO physicists “were absolutely unprepared for the chaos that is the astronomical community," he says.

Nonetheless, astronomers and astrophysicists came together to write a single compendious paper about the event. It has been submitted to The Astrophysical Journal Letters and some researchers say it has 4600 authors—roughly one-third of all astronomers. In addition, individual groups are publishing dozens of other papers in Science , Nature , and other journals, many concurrently with the announcement.

With one spectacular event in the bag, the era of gravitational wave astronomy has begun. The next step is simply to see more such events and begin to do statistical analyses on them, astronomers say. But for the moment, the entire community is basking in the glow of the discovery and the stunning success of its models. "Sometimes I wonder whether we're all just mucking around," Howell says. "It's moments like this that reassure me that science works."


Most Massive Neutron Star Ever Detected, Almost too Massive to Exist

Astronomers using the GBT have discovered the most massive neutron star to date, a rapidly spinning pulsar approximately 4,600 light-years from Earth. This record-breaking object is teetering on the edge of existence, approaching the theoretical maximum mass possible for a neutron star.

Artist impression of the pulse from a massive neutron star being delayed by the passage of a white dwarf star between the neutron star and Earth. Credit: BSaxton, NRAO/AUI/NSF

Neutron stars – the compressed remains of massive stars gone supernova – are the densest “normal” objects in the known universe. (Black holes are technically denser, but far from normal.) Just a single sugar-cube worth of neutron-star material would weigh 100 million tons here on Earth, or about the same as the entire human population. Though astronomers and physicists have studied and marveled at these objects for decades, many mysteries remain about the nature of their interiors: Do crushed neutrons become “superfluid” and flow freely? Do they breakdown into a soup of subatomic quarks or other exotic particles? What is the tipping point when gravity wins out over matter and forms a black hole?

A team of astronomers using the Robert C. Byrd Green Bank Telescope (GBT) has brought us closer to finding the answers.

The researchers, members of the NANOGrav Physics Frontiers Center, discovered that a rapidly rotating millisecond pulsar, called J0740+6620, is the most massive neutron star ever measured, packing 2.17 times the mass of our Sun into a sphere only 30 kilometers across. This measurement approaches the limits of how massive and compact a single object can become without crushing itself down into a black hole. Recent work involving gravitational waves observed from colliding neutron stars by LIGO suggests that 2.17 solar masses might be very near that limit.

“Neutron stars are as mysterious as they are fascinating,” said Thankful Cromartie, a graduate student at the University of Virginia and Grote Reber pre-doctoral fellow at the NSF’s National Radio Astronomy Observatory in Charlottesville, Virginia. “These city-sized objects are essentially ginormous atomic nuclei. They are so massive that their interiors take on weird properties. Finding the maximum mass that physics and nature will allow can teach us a great deal about this otherwise inaccessible realm in astrophysics.”

Pulsars get their name because of the twin beams of radio waves they emit from their magnetic poles. These beams sweep across space in a lighthouse-like fashion. Some rotate hundreds of times each second. Since pulsars spin with such phenomenal speed and regularity, astronomers can use them as the cosmic equivalent of atomic clocks. Such precise timekeeping helps astronomers study the nature of spacetime, measure the masses of stellar objects, and improve their understanding of general relativity.

In the case of this binary system, which is nearly edge-on in relation to Earth, this cosmic precision provided a pathway for astronomers to calculate the mass of the two stars.

Artist impression and animation of the Shapiro Delay. As the neutron star sends a steady pulse towards the Earth, the passage of its companion white dwarf star warps the space surrounding it, creating the subtle delay in the pulse signal. Animation: BSaxton, NRAO/AUI/NSF

As the ticking pulsar passes behind its white dwarf companion, there is a subtle (on the order of 10 millionths of a second) delay in the arrival time of the signals. This phenomenon is known as “Shapiro Delay.” In essence, gravity from the white dwarf star slightly warps the space surrounding it, in accordance with Einstein’s general theory of relativity. This warping means the pulses from the rotating neutron star have to travel just a little bit farther as they wend their way around the distortions of spacetime caused by the white dwarf.

Astronomers can use the amount of that delay to calculate the mass of the white dwarf. Once the mass of one of the co-orbiting bodies is known, it is a relatively straightforward process to accurately determine the mass of the other.

Cromartie is the principal author on a paper accepted for publication in Nature Astronomy. The GBT observations were research related to her doctoral thesis, which proposed observing this system at two special points in their mutual orbits to accurately calculate the mass of the neutron star.

“The orientation of this binary star system created a fantastic cosmic laboratory,” said Scott Ransom, an astronomer at NRAO and coauthor on the paper. “Neutron stars have this tipping point where their interior densities get so extreme that the force of gravity overwhelms even the ability of neutrons to resist further collapse. Each “most massive” neutron star we find brings us closer to identifying that tipping point and helping us to understand the physics of matter at these mindboggling densities.”

These observation were also part of a larger observing campaign known as NANOGrav, short for the North American Nanohertz Observatory for Gravitational Waves, which is a Physics Frontiers Center funded by the NSF.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

The Green Bank Observatory is supported by the National Science Foundation, and is operated under cooperative agreement by Associated Universities, Inc. Any opinions, findings and conclusions or recommendations expressed in this material do not necessarily reflect the views of the National Science Foundation.

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What Are Neutron Stars?

Neutron stars are the densest objects in our universe besides black holes -- a teaspoon of neutron star material contains a mass of around a billion tons. They are formed when giant stars at least ten times more massive than our sun collapse in a spectacular supernova. The collapse is so violent that protons and electrons melt into each other to form neutrons -- hence, the name, neutron star.

NSF, LIGO, Sonoma State University, A. Simonnet

On Aug. 17, the detectors at LIGO and Virgo picked up a faint signal from a passing gravitational wave. Shortly afterwards, NASA's Fermi space telescope observed a short burst of gamma rays. After a quick analysis of the incoming gravitational waves revealed the source was likely a collision of neutron stars, the news was swiftly relayed to some 70 observatories around the world. Astronomers at the observatories pointed their telescopes toward the specified spot in the southern sky, and waited for the light show expected to arrive over the coming days and even weeks, as the succession of photons of different wavelengths produced over the course of the collision arrived at Earth.

“It was the most exciting day of my life,” said Mansi Kasliwal, a Caltech astrophysicist, and the principal investigator for GROWTH, the global relay network responsible for alerting and coordinating observatories around the world for an event like this one.

Within hours, optical photons were detected by a telescope at Las Campanas Observatory in Chile. Another hour later came the infrared photons, this time detected by the Gemini South telescope, also in Chile. Nine days later came the X-ray photons, detected by NASA's Chandra X-ray Observatory orbiting in space. Finally, 16 days after the gravitational wave detection, 27 giant, satellite dish-like radio telescopes operated by the National Radio Astronomy Observatory in New Mexico detected radio photons from the colliding neutron stars.

The detection of this event with both gravitational waves and light is an example of what scientists call multi-messenger astronomy.

“Multi-messenger events are really the holy grail, with respect to understanding issues in astrophysics,” said Mike Guidry, an astrophysicist from the University of Tennessee in Knoxville.

“We thought that multi-messenger events were possible, but my guess was that it would take a while for us to get there. It looks like I stand corrected,” said Guidry. He is currently authoring a textbook on general relativity and noted that the news might force at least a partial rewrite.

But the discovery is much more than just the first ever multi-messenger event -- GW170817 has also shed light on the process that created almost one quarter of all the elements in our universe.

The birth of gold

One of the active debates among astronomers is the origin of gold. Astrophysicists believe that about half of all the elements heavier than iron, including gold, silver, platinum and uranium, are created by a mechanism known as the r-process. Short for rapid neutron capture process, the r-process requires an abundant supply of neutrons.

For decades, astronomers have theorized about astronomical events that would produce conditions ripe for the r-process to occur. The two prevailing contenders require neutron stars. One theory suggests that r-processes can occur during a supernova explosion when a neutron star is first formed, while another theory claims that the process can also occur when two neutron stars smash into each other, casting fragments of heavy elements such as gold, silver and uranium across the universe. According to Kasliwal, supernovas are relatively common and easy to observe relative to neutron star mergers, but almost none of the observed supernova fit with the first theory. Without a way to observe neutron star mergers, the astrophysicists were stuck.

But now, with the first ever detected neutron star merger, there are now observations that support the second theory. Astronomers have now seen “unequivocal evidence of a cosmic mine that is forging about 10,000 earth-masses of heavy elements, such as gold, platinum, and neodymium," said Kasliwal in a press release.

Finding evidence for the origins of gold is just one exciting aspect of the new observations. In addition to being the birth place of gold, neutron stars, together with black holes, are also the graves of heavy stars. That’s because for stars much more massive than our sun, their final fate is to become neutron stars or black holes. The GW170817 observation, which sheds light on many neutron star properties, will revolutionize the way astrophysicists observe and study stellar evolution, according to Guidry.

“This is at least as important as the original announcement of the discovery of gravitational waves,” said Guidry.


Measuring lead nucleus tells of neutron stars

A neutron skin behaviour that affects atoms and stars.

Physicists have just collected the most accurate measurement of the thickness of the neutron ‘skin’ of a lead (Pb) nucleus, and it could help them figure out the structure and size of neutron stars.

Atoms have a nucleus filled with protons and neutrons. The number and distribution of these subatomic particles, plus how they interact with each other, determine the identity and properties of the atom.

A team of physicists at the Thomas Jefferson National Accelerator Facility, US, measured the size of the sphere of neutrons that surrounded the protons in lead it had a thickness of 0.28 millionths of a nanometre, they report in a paper, published in Physical Review Letters.

Lighter nuclei usually have a similar number of protons and neutrons, but bigger nuclei with lots of protons need extra neutrons to balance them out. For example, one isotope of lead has 82 protons and 126 neutrons.

“The question is about where the neutrons are in lead,” says Kent Paschke from the University of Virginia, US. “Lead is a heavy nucleus – there’s extra neutrons, but as far as the nuclear force is concerned, an equal mix of protons and neutrons works better.

“The protons in a lead nucleus are in a sphere, and we have found that the neutrons are in a larger sphere around them, and we call that the neutron skin.”

Neutrons are tricky to measure, so the team used a technique that measured weak nuclear force instead of electric charge.

“Protons have an electric charge and can be mapped using the electromagnetic force,” explains Paschke. “Neutrons have no electric charge, but compared to protons they have a large weak charge, and so if you use the weak interaction, you can figure out where the neutrons are.”

The team collected the measurement by shooting a beam of electrons into a sheet of cryogenically cooled lead and measuring the direction of the electrons as they rebounded off the sheet.

“On average over the entire run, we knew where the right- and left-hand beams were, relative to each other, within a width of 10 atoms,” says Krishna Kumar, of the University of Massachusetts Amherst.

“The charge radius is about 5.5 femtometres [1 nanometre = 1 million femtometres],” says Paschke. “And the neutron distribution is a little larger than that – around 5.8 femtometres, so the neutron skin is 0.28 femtometres, or about 0.28 millionths of a nanometre.”

The thickness of the skin was larger than previous measurements, which could have implications for the size of neutron stars: stars which were so big they collapsed and crushed their protons and electrons into neutrons.

“We need to know the content of the neutron star and the equation of state, and then we can predict the properties of these neutron stars,” says Kumar. “So, what we are contributing to the field with this measurement of the lead nucleus allows you to better extrapolate to the properties of neutron stars.”

As neutron stars spiral around each other, they emit gravitational waves that are detected by the Laser Interferometer Gravitational-wave Observatory (LIGO).

“As [the neutron stars] get close in the last fraction of a second, the gravitational pull of one neutron star makes the other neutron star into a teardrop – it actually becomes oblong, like an American football,” says Kumar.

“If the neutron skin is larger, then it means a certain shape for the football, and if the neutron skin were smaller, it means a different shape for the football. And the shape of the football is measured by LIGO.

“The LIGO experiment and the [current lead] experiment did very different things, but they are connected by this fundamental equation – the equation of state of nuclear matter.”

Deborah Devis

Dr Deborah Devis is a science journalist at The Royal Institution of Australia.

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Most Massive Neutron Star Ever Detected, Almost too Massive to Exist

Summary: Astronomers using the GBT have discovered the most massive neutron star Neutron Star A small compressed core of a star that has gone through supernova (star explosion). These stars are almost completely made up of only neutrons and have a strong gravitational field. to date, a rapidly spinning pulsar Pulsar A neutron star that emits regular pulses of light towards Earth. approximately 4,600 light-years from Earth. This record-breaking object is teetering on the edge of existence, approaching the theoretical maximum mass possible for a neutron star.

Neutron stars – the compressed remains of massive stars gone supernova – are the densest “normal” objects in the known universe. (Black holes are technically denser, but far from normal.) Just a single sugar-cube worth of neutron-star material would weigh 100 million tons here on Earth, or about the same as the entire human population. Though astronomers and physicists have studied and marveled at these objects for decades, many mysteries remain about the nature of their interiors: Do crushed neutrons become “superfluid” and flow freely? Do they breakdown into a soup of subatomic quarks or other exotic particles? What is the tipping point when gravity wins out over matter and forms a black hole?

A team of astronomers using the National Science Foundation’s (NSF) Green Bank Telescope (GBT) has brought us closer to finding the answers.

The researchers, members of the NANOGrav Physics Frontiers Center, discovered that a rapidly rotating millisecond pulsar, called J0740+6620, is the most massive neutron star ever measured, packing 2.17 times the mass of our Sun into a sphere only 30 kilometers across. This measurement approaches the limits of how massive and compact a single object can become without crushing itself down into a black hole. Recent work involving gravitational waves observed from colliding neutron stars by LIGO suggests that 2.17 solar masses might be very near that limit.

“Neutron stars are as mysterious as they are fascinating,” said Thankful Cromartie, a graduate student at the University of Virginia and Grote Reber doctoral fellow at the National Radio Astronomy Observatory in Charlottesville, Virginia. “These city-sized objects are essentially ginormous atomic nuclei. They are so massive that their interiors take on weird properties. Finding the maximum mass that physics and nature will allow can teach us a great deal about this otherwise inaccessible realm in astrophysics.”

Pulsars get their name because of the twin beams of radio waves they emit from their magnetic poles. These beams sweep across space in a lighthouse-like fashion. Some rotate hundreds of times each second. Since pulsars spin with such phenomenal speed and regularity, astronomers can use them as the cosmic equivalent of atomic clocks. Such precise timekeeping helps astronomers study the nature of spacetime, measure the masses of stellar objects, and improve their understanding of general relativity.

In the case of this binary system, which is nearly edge-on in relation to Earth, this cosmic precision provided a pathway for astronomers to calculate the mass of the two stars.

As the ticking pulsar passes behind its white dwarf companion, there is a subtle (on the order of 10 millionths of a second) delay in the arrival time of the signals. This phenomenon is known as “Shapiro Delay.” In essence, gravity from the white dwarf star slightly warps the space surrounding it, in accordance with Einstein’s general theory of relativity. This warping means the pulses from the rotating neutron star have to travel just a little bit farther as they wend their way around the distortions of spacetime caused by the white dwarf.

Astronomers can use the amount of that delay to calculate the mass of the white dwarf. Once the mass of one of the co-orbiting bodies is known, it is a relatively straightforward process to accurately determine the mass of the other.

Cromartie is the principal author on a paper accepted for publication in Nature Astronomy. The GBT observations were research related to her doctoral thesis, which proposed observing this system at two special points in their mutual orbits to accurately calculate the mass of the neutron star.

“The orientation of this binary star system created a fantastic cosmic laboratory,” said Scott Ransom, an astronomer at NRAO and coauthor on the paper. “Neutron stars have this tipping point where their interior densities get so extreme that the force of gravity overwhelms even the ability of neutrons to resist further collapse. Each “most massive” neutron star we find brings us closer to identifying that tipping point and helping us to understand the physics of matter at these mindboggling densities.”

These observation were also part of a larger observing campaign known as NANOGrav, short for the North American Nanohertz Observatory for Gravitational Waves, which is a Physics Frontiers Center funded by the NSF.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

The Green Bank Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.


Most Massive Neutron Star Discovered

Astronomers using the National Science Foundation’s Green Bank Telescope (GBT) have discovered the most massive neutron star yet found, a discovery with strong and wide-ranging impacts across several fields of physics and astrophysics.

“This neutron star is twice as massive as our Sun. This is surprising, and that much mass means that several theoretical models for the internal composition of neutron stars now are ruled out,” said Paul Demorest, of the National Radio Astronomy Observatory (NRAO). “This mass measurement also has implications for our understanding of all matter at extremely high densities and many details of nuclear physics,” he added.

Neutron stars are the superdense “corpses” of massive stars that have exploded as supernovae . With all their mass packed into a sphere the size of a small city, their protons and electrons are crushed together into neutrons. A neutron star can be several times more dense than an atomic nucleus, and a thimbleful of neutron-star material would weigh more than 500 million tons. This tremendous density makes neutron stars an ideal natural “laboratory” for studying the most dense and exotic states of matter known to physics.

The scientists used an effect of Albert Einstein’s theory of General Relativity to measure the mass of the neutron star and its orbiting companion, a white dwarf star. The neutron star is a pulsar , emitting lighthouse-like beams of radio waves that sweep through space as it rotates. This pulsar, called PSR J1614-2230, spins 317 times per second, and the companion completes an orbit in just under nine days. The pair, some 3,000 light-years distant, are in an orbit seen almost exactly edge-on from Earth. That orientation was the key to making the mass measurement.

As the orbit carries the white dwarf directly in front of the pulsar, the radio waves from the pulsar that reach Earth must travel very close to the white dwarf. This close passage causes them to be delayed in their arrival by the distortion of spacetime produced by the white dwarf’s gravitation. This effect, called the Shapiro Delay, allowed the scientists to precisely measure the masses of both stars.

“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,” said Scott Ransom, also of NRAO.

The astronomers used a newly-built digital instrument called the Green Bank Ultimate Pulsar Processing Instrument (GUPPI), attached to the GBT, to follow the binary stars through one complete orbit earlier this year. Using GUPPI improved the astronomers’ ability to time signals from the pulsar severalfold.

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. That much mass, they say, changes their understanding of a neutron star’s composition. Some theoretical models postulated that, in addition to neutrons, such stars also would contain certain other exotic subatomic particles called hyperons or condensates of kaons.

“Our results rule out those ideas,” Ransom said.

Demorest and Ransom, along with Tim Pennucci of the University of Virginia, Mallory Roberts of Eureka Scientific, and Jason Hessels of the Netherlands Institute for Radio Astronomy and the University of Amsterdam, reported their results in the October 28 issue of the scientific journal Nature.

Their result has further implications, outlined in a companion paper, scheduled for publication in the Astrophysical Journal Letters. “This measurement tells us that if any quarks are present in a neutron star core, they cannot be ‘free,’ but rather must be strongly interacting with each other as they do in normal atomic nuclei,” said Feryal Ozel of the University of Arizona, lead author of the second paper.

There remain several viable hypotheses for the internal composition of neutron stars, but the new results put limits on those, as well as on the maximum possible density of cold matter.

The scientific impact of the new GBT observations also extends to other fields beyond characterizing matter at extreme densities. A leading explanation for the cause of one type of gamma-ray burst — the “short-duration” bursts — is that they are caused by colliding neutron stars. The fact that neutron stars can be as massive as PSR J1614-2230 makes this a viable mechanism for these gamma-ray bursts.

Such neutron-star collisions also are expected to produce gravitational waves that are the targets of a number of observatories operating in the United States and Europe. These waves, the scientists say, will carry additional valuable information about the composition of neutron stars.

“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,” Ransom explained. “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,” he added.


Could One Type of Neutron Stars Explain Where Fast Radio Bursts Come From?

This artist’s impression shows a supernova and associated gamma-ray burst driven by a rapidly spinning neutron star with a very strong magnetic field — an exotic object known as a magnetar. Image and caption: ESO, CC BY 4.0.

In 2007, astronomers studying the sky in radio wavelengths accidentally discovered rapid ‘bursts’ of energy that lasted for a thousandth of a second each. The astronomers called them fast radio bursts (FRBs), since most of the energy released was in the radio part of the EM spectrum.

FRBs radiate hundred-billion-times more power than the Sun and last for a very short time – so short that telescopes don’t have nearly enough time to study them.

In the last 13 or so years, astronomers have proposed an extraordinary number of theories to explain their origins – all the way to aliens communicating across galaxies. But concrete answers have been wanting.

FRBs are so enigmatic because their behaviour is quite unlike that of other exotic astronomical sources.

Humans know of 112 FRBs (at least officially). Some of them are never observed a second time. Others exhibit short, intense bursts separated by long breaks, and are called repeating FRBs.

According to one theory, FRBs originate from magnetars outside the Milky Way galaxy. Physicists always prefer simpler explanations over more complicated ones, so this theory has caught the fancy of many astrophysicists. However, for many years, there was no observation to support this theory.

The recent discovery of a burst of radio-waves emission from a magnetar in our own galaxy could change this. The community of astronomers is certainly abuzz with chatter about this phenomenon.

When they run out of elements to fuse and produce energy, stars die. And stars of different masses die differently. Some of those on the heavier side blow off their outer layers in an explosion called a supernova while their cores collapse under their own gravity. If the core is heavier than a threshold, it will collapse entirely into a single point and form a black hole. If the core is heavy but not heavy enough, it will collapse until it becomes an ultra-dense ball of neutrons. These ‘neutron stars’ are so dense that a 1,000 litres of water, which would weigh 1,000 kg on Earth, would on their surface weigh 1,000,000,000,000,000 kg.

Neutron stars enveloped by extremely powerful magnetic fields – more than 10-billion-times stronger than the strongest magnets on Earth – are called magnetars.

Astronomers spot magnetars within the Milky Way by looking for the electromagnetic radiation they emit: from long-wavelength radio-waves to short-wavelength X-rays and gamma rays. Magnetars also release sudden and intense bursts of X-ray and gamma-ray radiation that astronomers study using telescopes orbiting Earth.

By studying the same magnetar at different wavelengths and power, astronomers can better understand what could have caused each emission as well as the spectrum across which each magnetar is capable of emitting energy.

On April 28, astronomers observed what they called a “forest of bursts” in X-ray wavelengths from a well-known magnetar located in the Milky Way, about 30,000 lightyears away from Earth. This was nothing new. But then, another group of astronomers observed another intense burst of radio-waves from the same magnetar on the same day, later confirmed by an independent radio telescope.

It was the first time this magnetar – named SGR 1935+2154 – had emitted radio-waves.

There was frenzy as many astronomers now asked themselves: Could magnetars be the source of FRBs?

Researchers from Columbia University, Flatiron Institute, University of California and the California Institute of Technology soon set out to find the answer, and uploaded their results as a preprint paper on May 11.

Their work suggests it may not be so simple to link FRBs with magnetars – but it isn’t impossible either.

Any theory that explains FRBs should be able to explain both repeating and non-repeating bursts. This is where the magnetar theory stumbles: repeating FRBs repeat themselves more often than the theory can make sense of.

But in an effect to explain how the same magnetar could have emitted both radio-waves and X-rays, they suggested there could actually be two kinds of magnetars: one that we’ve already seen examples of in the Milky Way and a second kind with more powerful magnetic fields.

“Explaining sources that repeat as prolifically as some extragalactic FRBs requires [us] to have magnetars with magnetic fields larger than that of SGR 1935+2154,” Ben Margalit, an astrophysicist at the University of California and an author of the study, told The Wire Science.

Under the influence of strong magnetic fields, fast-moving particles in the magnetar’s atmosphere radiate away their energy as radio-wavelength electromagnetic waves. This is how the magnetar theory tries to make sense of FRBs.

To demonstrate an association between the two types of magnetars and FRBs, the authors first derived equations to calculate how frequently the latter occur. Then, using simulations on the computer, they found that the “two-component population” neatly explained the occurrence rate of both repeating and non-repeating FRBs.

But there is a catch: for their theory to work, the second type of magnetars must be “born at a rate that is at least [100-times] lower than that of SGR 1935+2154-like magnetars”.

This in turn means the second type must be younger – “a direct consequence of them having such large magnetic fields,” according to Margalit – as well as be born through more “exotic” processes than the supernovae that birth ‘regular’ magnetars.

“I agree with the idea that there is likely a variety of magnetars, if we are to assume that they are the source of fast radio bursts,” said Kelly Gourdji, who is studying FRBs for her PhD at the Anton Pannekoek Institute of Astronomy, University of Amsterdam. “I’m not so surprised by this conclusion, and it certainly keeps FRBs interesting.” She wasn’t involved in the study.

The researchers also calculated the ratio of radio and X-ray luminosity – or radiated power – observed from SGR 1935+2154. When they compared this number with predictions based on various theories of emission, they found that it matched with one related to one elaborate theory.

Very simply speaking, when the crust of a magnetar experiences a quake or its magnetic fields twist, fast-moving particles are generated near its surface. These particles interact with each other and other particles in the ‘atmosphere’ and release energy at different wavelengths.

This theory is extremely sensitive to variations in its details, which – as the authors write in their preprint paper – “could explain why not all magnetic flares are accompanied by a luminous FRB”.

But it’s still too early to say we’ve solved the mystery of FRBs, according to Gourdji. “The comparison is based only on [the ratio], which is completely understandable since it’s the only one we have,” she said.

This limitation could go away when astronomers detect more simultaneous X-ray and radio bursts and measure their repetition rates, Paz Beniamini, an astrophysicist at the California Institute of Technology and another author of the study, said.

For now, despite the tantalising prospect of an answer, FRBs remain a step ahead.

Debdutta Paul has a PhD from the department of astronomy and astrophysics at the Tata institute of Fundamental Research, Mumbai. He is currently a freelance science communicator and journalist. He tweets at @dbdttpl.


Watch the video: what is the role of neutrons in the nucleus (May 2022).