The golden age of neutron-star physics has arrived

These stellar remnants are some of the Universe’s most enigmatic objects — and they are finally starting to give up their secrets.

Powerful magnetic and electric fields whip charged particles around, in a computer simulation of a spinning neutron star. Credit: NASA’s Goddard Space Flight Center

When a massive star dies in a supernova, the explosion is only the beginning of the end. Most of the stellar matter is thrown far and wide, but the star’s iron-filled heart remains behind. This core packs as much mass as two Suns and quickly shrinks to a sphere that would span the length of Manhattan. Crushing internal pressure — enough to squeeze Mount Everest to the size of a sugar cube — fuses subatomic protons and electrons into neutrons.

Astronomers know that much about how neutron stars are born. Yet exactly what happens afterwards, inside these ultra-dense cores, remains a mystery. Some researchers theorize that neutrons might dominate all the way down to the centre. Others hypothesize that the incredible pressure compacts the material into more exotic particles or states that squish and deform in unusual ways.

Now, after decades of speculation, researchers are getting closer to solving the enigma, in part thanks to an instrument on the International Space Station called the Neutron Star Interior Composition Explorer (NICER).

Last December, this NASA space observatory provided astronomers with some of the most precise measurements ever made of a neutron star’s mass and radius1,2, as well as unexpected findings about its magnetic field1,3. The NICER team plans to release results about more stars in the next few months. Other data are coming in from gravitational-wave observatories, which can watch neutron stars contort as they crash together. With these combined observations, researchers are poised to zero in on what fills the innards of a neutron star.

For many in the field, these results mark a turning point in the study of some of the Universe’s most bewildering objects. “This is beginning to be a golden age of neutron-star physics,” says Jürgen Schaffner-Bielich, a theoretical physicist at Goethe University in Frankfurt, Germany.

Launched in 2017 aboard a SpaceX Falcon 9 rocket, the US$62-million NICER telescope sits outside the space station and collects X-rays coming from pulsars — spinning neutron stars that radiate charged particles and energy in enormous columns that sweep around like beams from a lighthouse. The X-rays originate from million-degree hotspots on a pulsar’s surface, where a powerful magnetic field rips charged particles off the exterior and slams them back down at the opposing magnetic pole.

NICER detects these X-rays using 56 gold-coated telescopes, and time-stamps their arrival to within 100 nanoseconds. With this capability, researchers can precisely track hotspots as a neutron star whips around at up to 1,000 times per second. Hotspots are visible as they swing across the object. But neutron stars warp space-time so strongly that NICER also detects light from hotspots facing away from Earth. Einstein’s general theory of relativity provides a way to calculate a star’s mass-to-radius ratio through the amount of light-bending. That and other observations allow astrophysicists to pin down the masses and radii of the deceased stars. Those two properties could help in determining what is happening down in the cores.

Deep, dark mystery
Neutron stars get more complicated the deeper one goes. Beneath a thin atmosphere made mostly of hydrogen and helium, the stellar remnants are thought to boast an outer crust just a centimetre or two thick that contains atomic nuclei and free-roaming electrons. Researchers think that the ionized elements become packed together in the next layer, creating a lattice in the inner crust. Even further down, the pressure is so intense that almost all the protons combine with electrons to turn into neutrons, but what occurs beyond that is murky at best (see ‘Dense matter’).

Crucially, each possibility would push back in a characteristic way against a neutron star’s colossal gravity. They would generate different internal pressures and therefore a larger or smaller radius for a given mass. A neutron star with a Bose–Einstein condensate centre, for instance, is likely to have a smaller radius than one made from ordinary material such as neutrons. One with a core made of pliable hyperon matter could have a smaller radius still.

“The types of particles and the forces between them affect how soft or squashy the material is,” says Anna Watts, a NICER team member at the University of Amsterdam.

Differentiating between the models will require precise measurements of the size and mass of neutron stars, but researchers haven’t yet been able to push their techniques to fine-enough levels to say which possibility is most likely. They typically estimate masses by observing neutron stars in binary pairs. As the objects orbit one another, they tug gravitationally on each other, and astronomers can use this to determine their masses. Roughly 35 stars have had their masses measured in this way, although the figures can contain error bars of up to one solar mass. A mere dozen or so have also had their radii calculated, but in many cases, the techniques can’t determine this value to better than a few kilometres — as much as one-fifth of the size of a neutron star.

NICER’s hotspot method has been used by the European Space Agency’s XMM-Newton X-ray observatory, which launched in 1999 and is still in operation. NICER is four times more sensitive and has hundreds of times better time resolution than the XMM-Newton. Over the next two to three years, the team expects to be able to use NICER to work out the masses and radii of another half a dozen targets, pinning down their radii to within half a kilometre. With this precision, the group will be well placed to begin plotting out what is known as the neutron-star equation of state, which relates mass to radius or, equivalently, internal pressure to density.

If scientists are particularly lucky and nature happens to serve up especially good data, NICER might help eliminate certain versions of this equation. But most physicists think that, on its own, the observatory will probably narrow down rather than completely rule out models of what happens in the mysterious objects’ cores.

“This would still be a huge advance on where we are now,” says Watts.

Field lines
NICER’s first target was J0030+0451, an isolated pulsar that spins roughly 200 times per second and is 337 parsecs (1,100 light years) from Earth, in the constellation Pisces….
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Neutron Stars Rip Each Other Apart to Form Black Hole

This supercomputer simulation shows one of the most violent events in the universe: a pair of neutron stars colliding, merging and forming a black hole. A neutron star is the compressed core left behind when a star born with between eight and 30 times the sun’s mass explodes as a supernova. Neutron stars pack about 1.5 times the mass of the sun — equivalent to about half a million Earths — into a ball just 12 miles (20 km) across. Continue reading Neutron Stars Rip Each Other Apart to Form Black Hole

How CERN’s Discovery of Exotic Particles May Affect Astrophysics

The difference between a neutron star and a quark star (Chandra)

The difference between a neutron star and a quark star (Chandra)

You may have heard that CERN announced the discovery of a strange particle known as Z(4430). A paper summarizing the results has been published on the physics arxiv, which is a repository for preprint (not yet peer reviewed) physics papers. The new particle is about 4 times more massive than a proton, has a negative charge, and appears to be a theoretical particle known as a tetraquark. The results are still young, but if this discovery holds up it could have implications for our understanding of neutron stars.

A periodic table of elementary particles. Credit: Wikipedia Read more:

A periodic table of elementary particles.
Credit: Wikipedia

The building blocks of matter are made of leptons (such as the electron and neutrinos) and quarks (which make up protons, neutrons, and other particles). Quarks are very different from other particles in that they have an electric charge that is 1/3 or 2/3 that of the electron and proton. They also possess a different kind of “charge” known as color. Just as electric charges interact through an electromagnetic force, color charges interact through the strong nuclear force. It is the color charge of quarks that works to hold the nuclei of atoms together. Color charge is much more complex than electric charge. With electric charge there is simply positive (+) and its opposite, negative (-). With color, there are three types (red, green, and blue) and their opposites (anti-red, anti-green, and anti-blue).

Because of the way the strong force works, we can never observe a free quark. The strong force requires that quarks always group together to form a particle that is color neutral. For example, a proton consists of three quarks (two up and one down), where each quark is a different color. With visible light, adding red, green and blue light gives you white light, which is colorless. In the same way, combining a red, green and blue quark gives you a particle which is color neutral. This similarity to the color properties of light is why quark charge is named after colors.
Combining a quark of each color into groups of three is one way to create a color neutral particle, and these are known as baryons. Protons and neutrons are the most common baryons. Another way to combine quarks is to pair a quark of a particular color with a quark of its anti-color. For example, a green quark and an anti-green quark could combine to form a color neutral particle. These two-quark particles are known as mesons, and were first discovered in 1947. For example, the positively charged pion consists of an up quark and an antiparticle down quark.
Under the rules of the strong force, there are other ways quarks could combine to form a neutral particle. One of these, the tetraquark, combines four quarks, where two particles have a particular color and the other two have the corresponding anti-colors. Others, such as the pentaquark (3 colors + a color anti-color pair) and the hexaquark (3 colors + 3 anti-colors) have been proposed. But so far all of these have been hypothetical. While such particles would be color neutral, it is also possible that they aren’t stable and would simply decay into baryons and mesons.
There has been some experimental hints of tetraquarks, but this latest result is the strongest evidence of 4 quarks forming a color neutral particle. This means that quarks can combine in much more complex ways than we originally expected, and this has implications for the internal structure of neutron stars.
Very simply, the traditional model of a neutron star is that it is made of neutrons. Neutrons consist of three quarks (two down and one up), but it is generally thought that particle interactions within a neutron star are interactions between neutrons. With the existence of tetraquarks, it is possible for neutrons within the core to interact strongly enough to create tetraquarks. This could even lead to the production of pentaquarks and hexaquarks, or even that quarks could interact individually without being bound into color neutral particles. This would produce a hypothetical object known as a quark star.
This is all hypothetical at this point, but verified evidence of tetraquarks will force astrophysicists to reexamine some the assumptions we have about the interiors of neutron stars.

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Neutron stars – A melting pot for atomic nuclei

Neutron stars are extraordinary objects in the universe. Their density is so high that atoms melt within them and new states of matter arise. Tetyana Galatyuk and her colleagues try to simulate these conditions at the smallest scale using the FAIR particle accelerator and observe the results with the CBM detector. Thus, they not only learn more about neutron stars, but also about the innermost structure of matter.

Tetyana Galatyuk is head of a junior research group at GSI Helmholtz and the TU Darmstadt. With her group, she carries out experiments using HADES and CBM to explore compressed nuclear matter. Galatyuk studied physics in Ukraine, worked with the STAR detector at the RHIC accelerator facility in the U.S. and subsequently conducted research in Germany.

The violent birth of neutron stars

For its simulations the MPA team uses supercomputers that belong to the most powerful in the world. (a) CURIE of the TGCC-CEA computer center with 77,184 processor cores and a nominal peak performance of 1.667 Petaflop/s (1 Petaflop = 1 million billion flops). Credit: GENCI/TGCC-CEA (b) SuperMUC of the Leibniz computing center with more than 155,000 processor cores and a nominal peak performance of over 3 Petaflop/s. Credit: LRZ 2012

For its simulations the MPA team uses supercomputers that belong to the most powerful in the world. (a) CURIE of the TGCC-CEA computer center with 77,184 processor cores and a nominal peak performance of 1.667 Petaflop/s (1 Petaflop = 1 million billion flops). Credit: GENCI/TGCC-CEA (b) SuperMUC of the Leibniz computing center with more than 155,000 processor cores and a nominal peak performance of over 3 Petaflop/s. Credit: LRZ 2012

A team of researchers at the Max Planck Institute for Astrophysics conducted the most expensive and most elaborate computer simulations so far to study the formation of neutron stars at the center of collapsing stars with unprecedented accuracy. These worldwide first three-dimensional models with a detailed treatment of all important physical effects confirm that extremely violent, hugely asymmetric sloshing and spiral motions occur when the stellar matter falls towards the center. The results of the simulations thus lend support to basic perceptions of the dynamical processes that are involved when a star explodes as supernova.

Stars with more than eight to ten times the mass of our Sun end their lives in a gigantic explosion, in which the stellar gas is expelled into the surrounding space with enormous power. Such supernovae belong to the most energetic and brightest phenomena in the universe and can outshine a whole galaxy for weeks. They are the cosmic origin of chemical elements like carbon, oxygen, silicon, and iron, of which the Earth and our bodies are made of, and which are bred in massive stars over millions of years or freshly fused in the stellar explosion.
Supernovae are also the birth places of neutron stars, those extraordinarily exotic, compact stellar remnants, in which about 1.5 times the mass of our Sun is compressed to a sphere with the diameter of Munich. This happens within fractions of a second when the stellar core implodes due to the strong gravity of its own mass. The catastrophic collapse is stopped only when the density of atomic nuclei – gargantuan 300 million tons in a sugar cube – is exceeded.
What, however, causes the disruption of the star? How can the implosion of the stellar core be reversed to an explosion? The exact processes are still a matter of intense research. According to the most widely favored scenario, neutrinos, mysterious elementary particles, play a crucial role. These neutrinos are produced and radiated in tremendous numbers at the extreme temperatures and densities in the collapsing stellar core and nascent neutron star. Like the thermal radiation of a heater they heat the gas surrounding the hot neutron star and thus could “ignite” the explosion. In this scenario the neutrinos pump energy into the stellar gas and build up pressure until a shock wave is accelerated to disrupt the star in a supernova. But does this theoretical idea really work? Is it the explanation of the still enigmatic mechanism driving the explosion?

Turbulent evolution of a neutron star for six moments (0.154, 0.223, 0.240, 0.245, 0.249 and 0.278 seconds) after the beginning of the neutron star formation in a threedimensional computer simulation. The mushroom-like bubbles are characteristic of "boiling" neutrino-heated gas, whereas simultaneously the "SASI" instability causes wild sloshing and rotational motions of the whole neutrino-heated layer (red) and of the enveloping supernova shock (blue). Credit: Elena Erastova and Markus Rampp, RZG

Turbulent evolution of a neutron star for six moments (0.154, 0.223, 0.240, 0.245, 0.249 and 0.278 seconds) after the beginning of the neutron star formation in a threedimensional computer simulation. The mushroom-like bubbles are characteristic of “boiling” neutrino-heated gas, whereas simultaneously the “SASI” instability causes wild sloshing and rotational motions of the whole neutrino-heated layer (red) and of the enveloping supernova shock (blue). Credit: Elena Erastova and Markus Rampp, RZG

Unfortunately (or luckily!) the processes in the center of exploding stars cannot be reproduced in the laboratory and many solar masses of intransparent stellar gas obscure our view into the deep interior of supernovae. Research is therefore strongly dependent on most sophisticated and challenging computer simulations, in which the complex mathematical equations are solved that describe the motion of the stellar gas and the physical processes that occur at the extreme conditions in the collapsing stellar core. For this task the most powerful existing supercomputers are used, but still it has been possible to conduct such calculations only with radical and crude simplifications until recently. If, for example, the crucial effects of neutrinos were included in some detailed treatment, the computer simulations could only be performed in two dimensions, which means that the star in the models was assumed to have an artificial rotational symmetry around an axis.
Thanks to support from the Rechenzentrum Garching (RZG) in developing a particularly efficient and fast computer program, access to most powerful supercomputers, and a computer time award of nearly 150 million processor hours, which is the greatest contingent so far granted by the “Partnership for Advanced Computing in Europe (PRACE)” initiative of the European Union, the team of researchers at the Max Planck Institute for Astrophysics (MPA) in Garching could now for the first time simulate the processes in collapsing stars in three dimensions and with a sophisticated description of all relevant physics.
“For this purpose we used nearly 16,000 processor cores in parallel mode, but still a single model run took about 4.5 months of continuous computing”, says PhD student Florian Hanke, who performed the simulations. Only two computing centers in Europe were able to provide sufficiently powerful machines for such long periods of time, namely CURIE at Très Grand Centre de calcul (TGCC) du CEA near Paris (Fig. 1a) and SuperMUC at the Leibniz-Rechenzentrum (LRZ) in Munich/Garching (Fig. 1b).
Many Terabytes of simulation data (1 Terabyte are thousand billion bytes) had to be analysed and visualized before the researchers could grasp the essence of their model runs. What they saw caused excitement as well as astonishment. The stellar gas did not only exhibit the violent bubbling and seething with the characteristic rising mushroom-like plumes driven by neutrino heating in close similarity to what can be observed in boiling water. (This process is called convection.) The scientists also found powerful, large sloshing motions, which temporarily switch over to rapid, strong rotational motions (Fig. 2, movie). Such a behavior had been known before and had been named “Standing Accretion Shock Instability”, or SASI. This term expresses the fact that the initial sphericity of the supernova shock wave is spontaneously broken, because the shock develops large-amplitude, pulsating asymmetries by the oscillatory growth of initially small, random seed perturbations. So far, however, this had been found only in simplified and incomplete model simulations…..

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NASA’s Swift Reveals New Phenomenon in a Neutron Star

Astronomers using NASA’s Swift X-ray Telescope have observed a spinning neutron star suddenly slowing down, yielding clues they can use to understand these extremely dense objects.

A neutron star is the crushed core of a massive star that ran out of fuel, collapsed under its own weight, and exploded as a supernova. A neutron star can spin as fast as 43,000 times per minute and boast a magnetic field a trillion times stronger than Earth’s. Matter within a neutron star is so dense a teaspoonful would weigh about a billion tons on Earth.


An artist’s rendering of an outburst on an ultra-magnetic neutron star, also called a magnetar.
Credit: NASA’s Goddard Space Flight Center

This neutron star, 1E 2259+586, is located about 10,000 light-years away toward the constellation Cassiopeia. It is one of about two dozen neutron stars called magnetars, which have very powerful magnetic fields and occasionally produce high-energy explosions or pulses.

Observations of X-ray pulses from 1E 2259+586 from July 2011 through mid-April 2012 indicated the magnetar’s rotation was gradually slowing from once every seven seconds, or about eight revolutions per minute. On April 28, 2012, data showed the spin rate had decreased abruptly, by 2.2 millionths of a second, and the magnetar was spinning down at a faster rate…..
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Read also: An anti-glitch in a magnetar

Cosmic crashes forging gold: Nuclear reactions in space do produce the heaviest elements

Where did gold form? For a long time, the cosmic production site of this rare metal - here are shown natural gold nuggets from California and Australia - and of other very heavy chemical elements has been unknown. New theoretical models now confirm that it could be forged in the merger events of two neutron stars. Credit: Natural History Museum, London

Collisions of neutron stars produce the heaviest elements such as gold or lead. The cosmic site where the heaviest chemical elements such as lead or gold are formed has most likely been identified: Ejected matter from neutron stars merging in a violent collision provides ideal conditions. In detailed numerical simulations, scientists of the Max Planck Institute for Astrophysics and affiliated to the Excellence Cluster Universe and of the Free University of Brussels have verified that the relevant reactions of atomic nuclei do take place in this environment, producing the heaviest elements in the correct abundances….. Continue reading Cosmic crashes forging gold: Nuclear reactions in space do produce the heaviest elements

Neutrons Become Cubes Inside Neutron Stars

Intense pressure can force neutrons into cubes rather than spheres, say physicists

Trial wavefunction that interpolates between sphere (for N = 2), and cube (as N → ∞) for N = 2, 4, 8, 12.

Inside atomic nuclei, protons and neutrons fill space with a packing density of 0.74, meaning that only 26 percent of the volume of the nucleus in is empty.

That’s pretty efficient packing. Neutrons achieve a similar density inside neutron stars, where the force holding neutrons together is the only thing that prevents gravity from crushing the star into a black hole.

Today, Felipe Llanes-Estrada at the Technical University of Munich in Germany and Gaspar Moreno Navarro at Complutense University in Madrid, Spain, say neutrons can do even better.
These guys have calculated that under intense pressure, neutrons can switch from a spherical symmetry to a cubic one. And when that happens, neutrons pack like cubes into crystals with a packing density that approaches 100%.
Anyone wondering where such a form of matter might exist would naturally think if the centre of neutron stars. But there’s a problem.
On the one hand, most neutron stars have a mass about 1.4 times that of the Sun, which is too small to generate the required pressures for cubic neutrons. On the other, stars much bigger than two solar masses collapse to form black holes.
That doesn’t leave much of a mass range in which cubic neutrons can form.
As luck would have it, however, last year astronomers discovered in the constellation of Scorpius the most massive neutron star ever seen. This object, called PSR J1614-2230, has a mass 1.97 times that of the Sun.
That’s about as large as theory allows (in fact its mere existence rules out various theories about the behaviour of mass at high densities). But PSR J1614-2230 is massive enough to allow the existence of cubic neutrons.
Astrophysicists will be rubbing their hands at the prospect. The change from spherical to cubic neutrons should have a big influence on the behaviour a neutron star. It would change the star’s density, it’s stiffness and its rate of rotation, among other things.
So astronomers will be getting their lens cloths out and polishing furiously in the hope of observing this entirely new form of matter in the distant reaches of the galaxy.
Ref: Cubic Neutrons