# Radiation from Neutron Star impact?

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As I understand it, objects falling into an old/inactive/dead/non-pulsar neutron star would be ripped apart, form an accretion disc and eventually emit a burst of radiation as they fall onto the surface of the star.

If this happened to:

1. a comet
2. a dwarf planet
3. an ice giant

… roughly how much energy/radiation would each produce? Also, if possible, over what kind of time frame? I'm only looking for ballpark figures in all cases.

Back of the envelope? The energy released is about $$GM/R$$ per unit mass. For a 1.5 solar mass neutron star of radius 10 km, this amounts to $$2 imes 10^{16}$$ J/kg, or approximately 2/9 of the rest mass energy of the accreted material.

Of order half of this would be radiated and half would go into heating the neutron star.

Timescales are more difficult. Starting when? Once the material gets to the innermost stable circular orbit at around 15 km, then the final 5km to the surface takes a tiny fraction of a second. But from where an object is shredded (much further out), much would depend on its angular momentum.

## Astronomers Detect Prolonged X-Ray Emission from Neutron-Star Merger

In 2017, astronomers detected a gravitational-wave signal, designated GW170817, from the merger of two neutron stars. And since the detection, they have been continuously monitoring the subsequent emissions to provide the most complete picture of such an event. Their analysis provides possible explanations for X-rays that continued to radiate from the GW170817 collision long after models predicted they would stop.

This X-ray image, taken by NASA’s Chandra X-ray Observatory, shows GW170817. The central panel shows the stacked image of the field, with total exposure of 783 ks. The position of GW170817 is marked. In addition, several X-ray point sources as well as extended diffuse X-ray emissions are visible. The image stamps are centered on the location of GW170817, showing the main phases of its evolution. Image credit: Troja et al., doi: 10.1093/mnras/staa2626.

The GW170817 event was first spotted by the Laser Interferometer Gravitational-wave Observatory and its counterpart Virgo on August 17, 2017.

It occurred in the lenticular galaxy NGC 4993, which is located about 130 million light-years from Earth in the constellation Hydra.

Within hours after the detection, telescopes around the world began observing electromagnetic radiation, including gamma rays and light emitted from the merger.

It was the first and only time astronomers were able to observe the radiation associated with gravity waves, although they long knew such radiation occurs.

Seconds after GW170817 was detected, the telescopes recorded the initial jet of energy, known as a gamma-ray burst, then the slower kilonova, a cloud of gas which burst forth behind the initial jet.

Light from the kilonova lasted about three weeks and then faded. Meanwhile, nine days after the gravity wave was first detected, the telescopes observed something they’d not seen before: X-rays.

Astrophysical models predicted that as the initial jet from a neutron star collision moves through interstellar space, it creates its own shockwave, which emits X-rays, radio waves and light. This is known as the afterglow.

But such an afterglow had never been observed before. In this case, the afterglow peaked around 160 days after the gravity waves were detected and then rapidly faded away.

But the X-rays remained. They were last observed by NASA’s Chandra X-ray Observatory about 2.5 years after GW170817 was first detected.

A research team led by Dr. Eleonora Troja from the University of Maryland and NASA’s Goddard Space Flight Center propose a few possible explanations for the long-lived X-ray emissions.

One possibility is that these X-rays represent a completely new feature of a collision’s afterglow, and the dynamics of a gamma-ray burst are somehow different than expected.

“Having a collision so close to us that it’s visible opens a window into the whole process that we rarely have access to,” Dr. Troja said.

“It may be there are physical processes we have not included in our models because they’re not relevant in the earlier stages that we are more familiar with, when the jets form.”

Another possibility is that the kilonova and the expanding gas cloud behind the initial jet of radiation may have created their own shock wave that took longer to reach Earth.

“We saw the kilonova, so we know this gas cloud is there, and the X-rays from its shock wave may just be reaching us,” said Dr. Geoffrey Ryan, a postdoctoral researcher at the University of Maryland.

“But we need more data to understand if that’s what we’re seeing. If it is, it may give us a new tool, a signature of these events that we haven’t recognized before. That may help us find neutron star collisions in previous records of X-ray radiation.”

A third possibility is that something may have been left behind after the collision, perhaps the remnant of an X-ray emitting neutron star.

“Long-term monitoring of this source will be essential to test these different models,” the researchers said.

Their paper was published in the Monthly Notices of the Royal Astronomical Society.

E. Troja et al. 2020. A thousand days after the merger: Continued X-ray emission from GW170817. MNRAS 498 (4): 5643-5651 doi: 10.1093/mnras/staa2626

## Neutron Star Cooling

▪ Abstract Observation of cooling neutron stars can potentially provide information about the states of matter at supernuclear densities. We review physical properties important for cooling such as neutrino emission processes and superfluidity in the stellar interior, surface envelopes of light elements owing to accretion of matter, and strong surface magnetic fields. The neutrino processes include the modified Urca process and the direct Urca process for nucleons and exotic states of matter, such as a pion condensate, kaon condensate, or quark matter. The dependence of theoretical cooling curves on physical input and observations of thermal radiation from isolated neutron stars are described. The comparison of observation and theory leads to a unified interpretation in terms of three characteristic types of neutron stars: high-mass stars, which cool primarily by some version of the direct Urca process low-mass stars, which cool via slower processes and medium-mass stars, which have an intermediate behavior. The related problem of thermal states of transiently accreting neutron stars with deep crustal burning of accreted matter is discussed in connection with observations of soft X-ray transients. Observations imply that some stars cool more rapidly than can be explained on the basis of nonsuperfluid neutron star models cooling via the modified Urca process, whereas other star cool less rapidly. We describe possible theoretical models that are consistent with observations.

## Accretion Acceleration of Neutron Stars and Effects of Gravitational Radiation ☆,☆☆

In this paper we studied the neutron star's spin acceleration in the accretion process of the neutron star binary system, and the relation how the spin period changes with the accreted mass. We analyzed further the evolutions of both magnetic field and spin period of a neutron star, and compared the modeled results with the observational data of pulsars, to show that they are consistent with each other. Based on above studies, we investigated the effect of gravitational radiation on the spin-up process of a neutron star, and derived the change rate of the neutron star's spin period in the accretion process. We also estimated the critical angular velocity Ωcr, at which the accretion torque is balanced by that of gravitational radiation, and discussed the influence of gravitational radiation on the neutron star's spin evolution.

## Pulsating gamma rays from neutron star rotating 707 times a second

A black widow pulsar and its small stellar companion, viewed within their orbital plane. Powerful radiation and the pulsar's “wind” – an outflow of high-energy particles — strongly heat the facing side of the star to temperatures twice as hot as the sun's surface. The pulsar is gradually evaporating its partner, which fills the system with ionized gas and prevents astronomers from detecting the pulsar's radio beam most of the time. Credit: NASA's Goddard Space Flight Center/Cruz deWilde

An international research team led by the Max Planck Institute for Gravitational Physics (Albert Einstein Institute AEI) in Hannover has discovered that the radio pulsar J0952-0607 also emits pulsed gamma radiation. J0952-0607 spins 707 times in one second and is second in the list of rapidly rotating neutron stars. By analyzing about 8.5 years worth of data from NASA's Fermi Gamma-ray Space Telescope, LOFAR radio observations from the past two years, observations from two large optical telescopes, and gravitational-wave data from the LIGO detectors, the team used a multi-messenger approach to study the binary system of the pulsar and its lightweight companion in detail. Their study published in the Astrophysical Journal shows that extreme pulsar systems are hiding in the Fermi catalogs and motivates further searches. Despite being very extensive, the analysis also raises new unanswered questions about this system.

Pulsars are the compact remnants of stellar explosions which have strong magnetic fields and are rapidly rotating. They emit radiation like a cosmic lighthouse and can be observable as radio pulsars and/or gamma-ray pulsars depending on their orientation towards Earth.

The fastest pulsar outside globular clusters

PSR J0952-0607 (the name denotes the position in the sky) was first discovered in 2017 by radio observations of a source identified by the Fermi Gamma-ray Space Telescope as possibly being a pulsar. No pulsations of the gamma rays in data from the Large Area Telescope (LAT) onboard Fermi had been detected. Observations with the radio telescope array LOFAR identified a pulsating radio source and—together with optical telescope observations—allowed to measure some properties of the pulsar. It is orbiting the common center of mass in 6.2 hours with a companion star that only weighs a fiftieth of our Sun. The pulsar rotates 707 times in a single second and is therefore the fastest spinning in our Galaxy outside the dense stellar environments of globular clusters.

Searching for extremely faint signals

Using this prior information on the binary pulsar system, Lars Nieder, a Ph.D. student at the AEI Hannover, set out to see if the pulsar also emitted pulsed gamma rays. "This search is extremely challenging because the Fermi gamma-ray telescope only registered the equivalent of about 200 gamma rays from the faint pulsar over the 8.5 years of observations. During this time the pulsar itself rotated 220 billion times. In other words, only once in every billion rotations was a gamma ray observed!" explains Nieder. "For each of these gamma rays, the search must identify exactly when during each of the 1.4 millisecond rotations it was emitted."

This requires combing through the data with very fine resolution in order not to miss any possible signals. The computing power required is enormous. The very sensitive search for faint gamma-ray pulsations would have taken 24 years to complete on a single computer core. By using the Atlas computer cluster at the AEI Hannover it finished in just 2 days.

A strange first detection

"Our search found a signal, but something was wrong! The signal was very faint and not quite where it was supposed to be. The reason: our detection of gamma rays from J0952-0607 had revealed a position error in the initial optical-telescope observations which we used to target our analysis. Our discovery of the gamma-ray pulsations revealed this error," explains Nieder. "This mistake was corrected in the publication reporting the radio pulsar discovery. A new and extended gamma-ray search made a rather faint—but statistically significant—gamma-ray pulsar discovery at the corrected position."

Having discovered and confirmed the existence of pulsed gamma radiation from the pulsar, the team went back to the Fermi data and used the full 8.5 years from August 2008 until January 2017 to determine physical parameters of the pulsar and its binary system. Since the gamma radiation from J0952-0607 was so faint, they had to enhance their analysis method developed previously to correctly include all unknowns.

The pulse profile (distribution of gamma-ray photons during one rotation of the pulsar) of J0952-0607 is shown at the top. Below is the corresponding distribution of the individual photons over the ten years of observations. The greyscale shows the probability (photon weights) for individual photons to originate from the pulsar. From mid 2011 on, the photons line up along tracks corresponding to the pulse profile. This shows the detection of gamma-ray pulsations, which is not possible before mid 2011. Credit: L. Nieder/Max Planck Institute for Gravitational Physics

Another surprise: no gamma-ray pulsations before July 2011

The derived solution contained another surprise, because it was impossible to detect gamma-ray pulsations from the pulsar in the data from before July 2011. The reason for why the pulsar only seems to show pulsations after that date is unknown. Variations in how much gamma rays it emitted might be one reason, but the pulsar is so faint that it was not possible to test this hypothesis with sufficient accuracy. Changes in the pulsar orbit seen in similar systems might also offer an explanation, but there was not even a hint in the data that this was happening.

Optical observations raise further questions

The team also used observations with the ESO's New Technology Telescope at La Silla and the Gran Telescopio Canarias on La Palma to examine the pulsar's companion star. It is most likely tidally locked to the pulsar like the Moon to the Earth so that one side always faces the pulsar and gets heated up by its radiation. While the companion orbits the binary system's center of mass its hot "day" side and cooler "night" side are visible from the Earth and the observed brightness and color vary.

These observations create another riddle. While the radio observations point to a distance of roughly 4,400 light-years to the pulsar, the optical observations imply a distance about three times larger. If the system was relatively close to Earth, it would feature a never-seen-before extremely compact high density companion, while larger distances are compatible with the densities of known similar pulsar companions. An explanation for this discrepancy might be the existence of shock waves in the wind of particles from the pulsar, which could lead to a different heating of the companion. More gamma-ray observations with Fermi LAT observations should help answer this question.

Searching for continuous gravitational waves

Another group of researchers at the AEI Hannover searched for continuous gravitational wave emission from the pulsar using LIGO data from the first (O1) and second (O2) observation run. Pulsars can emit gravitational waves when they have tiny hills or bumps. The search did not detect any gravitational waves, meaning that the pulsar's shape must be very close to a perfect sphere with the highest bumps less than a fraction of a millimeter.

Rapidly rotating neutron stars

Understanding rapidly spinning pulsars is important because they are probes of extreme physics. How fast neutron stars can spin before they break apart from centrifugal forces is unknown and depends on unknown nuclear physics. Millisecond pulsars like J0952-0607 are rotating so rapidly because they have been spun up by accreting matter from their companion. This process is thought to bury the pulsar's magnetic field. With the long-term gamma-ray observations, the research team showed that J0952-0607 has one of the ten lowest magnetic fields ever measured for a pulsar, consistent with expectations from theory.

For those of you who missed my last couple of posts, allow me to introduce the neutron star: a stellar remnant similar to a white dwarf, but much denser, so dense that its protons and electrons have combined to form a neutron soup.

A neutron star forms from the collapsing core of a star between 10 and 20 M (solar masses). Its collapse produces powerful magnetic fields and extremely high temperatures, but because it becomes so small—less than the size of Los Angeles—it is very faint and radiates away its heat very slowly.

The exception to that rule comes in the form of two powerful beams of radiation that blast away from the object’s magnetic poles. As a neutron star spins—at around a hundred times per second—these radiation beams sweep across the sky like the the beams of a lighthouse.

If these beams happen to sweep over Earth, human observers see regular, rapid pulses of light. This visual phenomenon produced by neutron stars is called a pulsar.

Now that we have a basic understanding of neutron stars and pulsars, let’s explore some of the details of how these extreme objects work.

For one thing, neutron stars are not unchanging.

They mark the final state of the “lifespan” of a massive star, and once an object can be classified as a neutron star, it’s no longer a star. But neutron stars themselves evolve over time. One place where we can see this is the eventual slowing of their rotation and fading of their luminosity.

First, let’s take a closer look at why they have so much energy in the first place.

In this diagram, the dark grayish dot is the star’s core. Even after the shockwave that produces the supernova (shown here as an expanding red circle) rips outward through the star, the core has collapsed a great deal.

There’s a general rule in astronomy: when any object collapses, it generates energy, which goes into producing radiation and speeding up the object’s rotation. We see this, for example, in the collapse of an interstellar dust cloud to form a protostar.

In this case, as the dust cloud collapses, it spins faster and it also grows hotter, until the beginnings of a star—a protostar—form at its core.

A star in the main part of its life cycle—the main sequence—follows the same rule in regulating its own internal “homeostasis.” If the rate of nuclear reactions in its core ever drop too much, the core begins to collapse under the weight of the layers above, which then heats up the core and speeds up reactions again.

So…since a neutron star formed from the core of a massive star, it has a ton of energy. And because it’s so small (and a consequently small luminosity), it can’t get rid of that energy very fast.

What it can do, thanks to its powerful magnetic field, is blast radiation away at its magnetic poles. Since this energy is linked to its energy of rotation, neutron stars should gradually cool off and also slow down.

Now, remember that pulsars are specifically the visual phenomenon that happens whenever a neutron star’s beam sweeps over Earth. If it cools off to the point that it no longer produces detectable beams, we won’t see a pulsar even if its magnetic axis is pointed straight at us.

There’s still a neutron star there, though. So neutron stars can “live” longer than pulsars. Neutron stars older than about 10 million years old stop producing pulsars.

While they exist, though, pulsars are powerful. One in particular is our old friend, the Crab Nebula.

Right now, you’re looking at a visual-wavelength image of the Crab Nebula. That means you’re only seeing a tiny portion of the total light it emits.

Visible light is only a tiny portion of the light available to astronomers to study. Some of these wavelengths should be familiar to you, even if you don’t realize it—infrared is just a fancy word for heat, and ultraviolet is just a fancy word for the energy that produces sunburns.

No, heat doesn’t produce sunburns. If it did, you would feel yourself sunburning, and you’d have a tangible warning, making sunblock far less critical. Ultraviolet exposure is dangerous because you can’t feel its effects until it’s already damaged your skin (and possibly other parts of your body).

All of what you see in your daily life is just perceived with that tiny window of the electromagnetic spectrum called visible light. The rest of the spectrum actually tends to be far more valuable to astronomers. For example, only radio wavelengths can penetrate the densest interstellar dust clouds.

Anyway, the pulsar at the heart of the Crab Nebula emits photons all across the EM spectrum. That’s pretty impressive for an object that’s not producing its own energy anymore.

Here’s another thing. Remember those radiation beams that neutron stars produce? Well, would you be surprised to hear that they only account for a tiny fraction of the energy that neutron stars emit?

That’s right. Most of it—about 99.9%—is actually carried away in the form of a pulsar wind.

The pulsar wind can produce some really cool phenomena, to say the least.

Here, you can see the x-ray and infrared data paired with artist’s conceptions of what these phenomena might look like close-up.

The sphere at the center of the illustrations is the neutron star, and the two beams of radiation—especially clear in the illustration on the left—can be seen jetting away from the neutron star’s magnetic poles. The ring around the neutron star is an accretion disk, a disk of dust and gases that falls into orbit around an object.

Astronomers know that the pulsar wind is made up of high-speed atomic particles. We don’t know that these illustrations are accurate, but we do know that the pulsar wind produces small, high-energy nebulae near a young pulsar.

People…we’re talking about nebulae that emit photons like gamma rays.

Let me give you an idea of how crazy that is. Here is the interstellar medium.

You see the interstellar medium whenever you see a nebula. It’s the stuff between the stars. Generally, all nebulae are made out of the same stuff, whether they’re hazy-blue reflection nebulae, bright pink emission nebulae, shadowy dark nebulae, glowing round planetary nebulae, diverse supernova remnants, or…nebulae produced by pulsar winds.

Specifically, nebulae are a visual phenomenon—what you see when a specific portion of the interstellar medium is lit up.

This stuff isn’t naturally hot. Like I said, it’s between the stars. If it were as hot as stars, the whole night sky would glow at visible wavelengths, instead of just the stars twinkling on an otherwise dark canvas. Even when the interstellar medium emits its own light, as in the case of most nebulae, it’s usually in the infrared, radio, or visible wavelengths.

Gamma rays are ridiculously high-energy for a nebula. But, sure enough, that’s what we see where neutron stars are involved.

Since all pulsars are neutron stars, and all neutron stars are the remains of stars that went supernova (and produced supernova remnants)…shouldn’t we expect to find all pulsars at the heart of supernova remnants? And, conversely, shouldn’t we expect to find a pulsar at the heart of every supernova remnant?

The Crab Nebula has a pulsar at its center.

The weird fact is…it doesn’t actually work that way.

For one thing, not all neutron stars have radiation beams that sweep directly over Earth. These neutron stars are not called pulsars. So a neutron star found in the center of a supernova remnant wouldn’t necessarily be a pulsar.

For another thing, neutron stars don’t just spin fast, they travel fast. Supernova remnants don’t. So many neutron stars, no matter if they produced pulsars or not, leave their supernova remnant behind. We in the present day might observe a supernova remnant whose neutron star left it behind long ago.

There’s yet a third complication. Since pulsars are so small, they lose energy slowly, and they remain detectable for around 10 million years. Supernova remnants, on the other hand, tend to dissipate into the interstellar medium after around 50,000 years.

Last but not least…not all supernovas produce neutron stars.

So, the long story short? Most pulsars are actually found outside of supernova remnants, and most supernova remnants do not actually contain pulsars.

But…wait a second. How come not all supernovas produce neutron stars?

Never fear—we’ll get to that! But first, we’ll cover binary pulsars and some particularly unique neutron stars in my next couple of posts.

## Neutron star collision heralds arrival of a new era of astronomy

News of a collision between a pair of neutron stars some 130 million light years away has arrived via two completely different messengers – electromagnetic waves and gravitational waves – revealing clues to some long-standing mysteries of the universe.

Scientists are no longer just talking about it. After decades of theorizing, we're now actually in a new era of astronomy, defined by the capability to peer into the universe through multiple, distinct lenses. The first detections of gravitational waves opened the door further for this so-called multi-messenger astronomy. And now, with a highly publicized announcement Monday, astrophysicists have walked through that door.

Four times over the past two years, astronomers detected gravitational waves emanating from merging black holes. These detections were all made by scientists at the US-based Laser Interferometer Gravitational-Wave Observatory (LIGO). Its European counterpart, VIRGO, collaborated for the fourth detection. But with no light escaping black holes, astronomers using traditional telescopes – which view the universe in the electromagnetic spectrum – couldn't see anything.

The fifth detection was different this time, the colliding bodies were visible. So astrophysicists at LIGO and VIRGO detected the motion from the collision, and astronomers saw the flash of light. Scientists are already using this multi-messenger detection to unravel long-standing mysteries about the universe, such as where heavy elements like gold, platinum, and uranium form.

"It is the Rosetta Stone for all of high-energy astrophysics," says Richard O'Shaughnessy, a theoretical gravitational wave astrophysicist and LIGO researcher at the Rochester Institute of Technology. "Superlatives understate the significance of this event, and I haven't figured out a way of conveying it, even to myself."

But what exactly is it that scientists are so excited about?

### As Kamala Harris’ portfolio grows, so does the scrutiny

Some 130 million years ago, two neutron stars drawn into a gravitational dance with each other collided. This violent merger shook the fabric of space-time and sent fireworks of electromagnetic radiation out into the universe. On August 17, scientists detected the gravitational waves of this stellar event, and then less than 2 seconds later spotted a gamma-ray burst coming from the same location. Over the next few days, both Earth- and space-based telescopes picked up signals from across the electromagnetic spectrum emanating from this same event.

Neutron stars' incredible density made this event a particularly spectacular one. Neutron stars are more massive than the sun, but are just the size of a city. One teaspoon of material from a neutron star would weigh about 10 million tons. Because they are so dense, the gravity at the surface of a neutron star is billions of times stronger than the gravity at the surface of the Earth.

As the two super dense bodies spiral in toward each other, their masses produce very strong gravitational interactions. And because the neutron stars are so small, they can get very close to each other before merging, twirling around each other faster and faster as they approach each other. As they whip around, the gravitational fields of the pair of neutron stars interact, forming gravitational waves with a high-enough frequency that they can be felt on Earth.

But what are gravitational waves?

Ask a physicist and they'll tell you that gravitational waves are ripples in the fabric of space time. But what does that actually mean? Shane Larson, an astronomer at Northwestern University and a member of the LIGO Scientific Collaboration, says the explanation hinges on the law that nothing can travel faster than the speed of light.

According to Einstein's General Theory of Relativity, every object with mass bends space-time to some degree, and the greater the mass, the greater the distortion. This model explains why moons orbit planets and planets orbit the sun.

When an object moves – say, two neutron stars merge – gravity must necessarily change. And somehow information about that change in gravity has to get from that object to the other objects that experience its gravity. But because it can only travel as quickly as the finite speed of light, and that makes it impossible for things to react simultaneously, Dr. O'Shaughnessy explains, those gravitational changes propagate outward through the universe.

And this happens as quickly as it possibly could. In fact, the gravitational waves of the neutron star merger were detected on Earth first, even before the gamma ray burst.

"General relativity predicts that gravitational waves should travel at the speed of light," Dr. Larson says. That's because gravitational waves can travel through matter without slowing down.

Light, by contrast, interacts with the matter it encounters. It is scattered by matter, absorbed by matter, and reemitted by matter. In a collision of super dense neutron stars, Larson says, "the material at the core of the event is still so dense that the light can't get out as quickly as the gravitational waves … The photons and everything have to work their way out. And the energy that's energizing the matter has to work its way out to the outer layers where the photons that are emitted can reach us."

Astrophysicists have long looked for neutron star collisions, as scientists knew the celestial bodies existed in binary systems. Astronomers had spotted short gamma-ray bursts before that they thought were the signal of such a merger, but they couldn't be sure until the gravitational wave detections combined with the electromagnetic observations were made in August.

"This has been the holy grail of high-energy astrophysics," O'Shaughnessy says.

Gravitational waves and the photons of electromagnetic radiation are not the only messengers from the universe. Astronomers have also used neutrino and cosmic ray observations to learn about the universe. But with the addition of gravitational waves, scientists will now be able to develop a more complete understanding of the cosmos.

Scientists are thrilled to find out what mysteries multi-messenger astronomy might unravel now that gravitational waves have been added to the toolbox, but they're also patting themselves and their colleagues on the backs.

"Really we should all just be outrageously proud and flabbergasted that we are capable of building instruments to make these measurements," Larson says. And, he says, this wouldn't be possible without the thousands of scientists, engineers, construction workers, telecommunications workers, and others who have collaborated to build the laser interferometers.

"The fact that we can all get together and make this instrument should make all of us look at every problem that we face and go, 'Wow, why can't we solve that problem?' " Larson says. "Science is a uniquely human endeavor. As far as I know, my cats are not down in the basement building gravitational-wave detectors."

## Radio Observations Point to Likely Explanation for Neutron-Star Merger Phenomena

Three months of observations with the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) have allowed astronomers to zero in on the most likely explanation for what happened in the aftermath of the violent collision of a pair of neutron stars in a galaxy 130 million light-years from Earth. What they learned means that astronomers will be able to see and study many more such collisions.

On August 17, 2017, the LIGO and VIRGO gravitational-wave observatories combined to locate the faint ripples in spacetime caused by the merger of two superdense neutron stars. It was the first confirmed detection of such a merger and only the fifth direct detection ever of gravitational waves, predicted more than a century ago by Albert Einstein.

The gravitational waves were followed by outbursts of gamma rays, X-rays, and visible light from the event. The VLA detected the first radio waves coming from the event on September 2. This was the first time any astronomical object had been seen with both gravitational waves and electromagnetic waves.

The timing and strength of the electromagnetic radiation at different wavelengths provided scientists with clues about the nature of the phenomena created by the initial neutron-star collision. Prior to the August event, theorists had proposed several ideas — theoretical models — about these phenomena. As the first such collision to be positively identified, the August event provided the first opportunity to compare predictions of the models to actual observations.

Astronomers using the VLA, along with the Australia Telescope Compact Array and the Giant Metrewave Radio Telescope in India, regularly observed the object from September onward. The radio telescopes showed the radio emission steadily gaining strength. Based on this, the astronomers identified the most likely scenario for the merger’s aftermath.

“The gradual brightening of the radio signal indicates we are seeing a wide-angle outflow of material, traveling at speeds comparable to the speed of light, from the neutron star merger,” said Kunal Mooley, now a National Radio Astronomy Observatory (NRAO) Jansky Postdoctoral Fellow hosted by Caltech.

The observed measurements are helping the astronomers figure out the sequence of events triggered by the collision of the neutron stars.

The initial merger of the two superdense objects caused an explosion, called a kilonova, that propelled a spherical shell of debris outward. The neutron stars collapsed into a remnant, possibly a black hole, whose powerful gravity began pulling material toward it. That material formed a rapidly-spinning disk that generated a pair of narrow, superfast jets of material flowing outward from its poles.

If one of the jets were pointed directly toward Earth, we would have seen a short-duration gamma-ray burst, like many seen before, the scientists said.

“That clearly was not the case,” Mooley said.

Some of the early measurements of the August event suggested instead that one of the jets may have been pointed slightly away from Earth. This model would explain the fact that the radio and X-ray emission were seen only some time after the collision.

“That simple model — of a jet with no structure (a so-called top-hat jet) seen off-axis — would have the radio and X-ray emission slowly getting weaker. As we watched the radio emission strengthening, we realized that the explanation required a different model,” said Alessandra Corsi, of Texas Tech University.

The astronomers looked to a model published in October by Mansi Kasliwal of Caltech, and colleagues, and further developed by Ore Gottlieb, of Tel Aviv University, and his colleagues. In that model, the jet does not make its way out of the sphere of explosion debris. Instead, it gathers up surrounding material as it moves outward, producing a broad “cocoon” that absorbs the jet’s energy.

The astronomers favored this scenario based on the information they gathered from using the radio telescopes. Soon after the initial observations of the merger site, the Earth’s annual trip around the Sun placed the object too close to the Sun in the sky for X-ray and visible-light telescopes to observe. For weeks, the radio telescopes were the only way to continue gathering data about the event.

“If the radio waves and X-rays both are coming from an expanding cocoon, we realized that our radio measurements meant that, when NASA’s Chandra X-ray Observatory could observe once again, it would find the X-rays, like the radio waves, had increased in strength,” Corsi said.

Mooley and his colleagues posted a paper with their radio measurements, their favored scenario for the event, and this prediction online on November 30. Chandra was scheduled to observe the object on December 2 and 6.

“On December 7, the Chandra results came out, and the X-ray emission had brightened just as we predicted,” said Gregg Hallinan, of Caltech.

“The agreement between the radio and X-ray data suggests that the X-rays are originating from the same outflow that’s producing the radio waves,” Mooley said.

“It was very exciting to see our prediction confirmed,” Hallinan said. He added, “An important implication of the cocoon model is that we should be able to see many more of these collisions by detecting their electromagnetic, not just their gravitational, waves.”

Mooley, Hallinan, Corsi, and their colleagues reported their findings in the scientific journal Nature.

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

## Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2017 October 16
GW170817: A Spectacular Multi-Radiation Merger Event Detected
Illustrative Video Credit: NASA's Conceptual Imaging Lab

Explanation: Both gravitational and electromagnetic radiations have been detected in rapid succession for an explosive merging event for the first time. Data from the outburst fit well with a spectacular binary neutron-star death-spiral. The explosive episode was seen on August 17 in nearby NGC 4993, an elliptical galaxy only 130 million light years distant. Gravitational waves were seen first by the ground based LIGO and Virgo observatories, while seconds later the Earth-orbiting Fermi and INTEGRAL observatories detected gamma-rays, and hours after that Hubble and other observatories detected light throughout the electromagnetic spectrum. Pictured is an animated illustrative movie of the event's likely progenitors. The video depicts hot neutron stars as they spiral in toward each other and emit gravitational radiation. As they merge, a powerful jet extends that drives the short-duration gamma-ray burst, followed by clouds of ejecta and, over time, an optical supernova-type episode called a kilonova. This first coincident detection confirms that LIGO events can be associated with short-duration gamma-ray bursts. Such powerful neutron star mergers are thought to have seeded the universe with many heavy nuclei including the iodine needed for life and the uranium and plutonium needed for nuclear fission power. You may already own a souvenir of one of these explosions -- they are also thought to be the original creators of gold.

Journal articles: Lists kept by LIGO and LCO.
Tomorrow's picture: asteroid rings

## First heavy element identified from a neutron-star collision

Finding strontium in the afterglow of a neutron star collision (illustrated) provides the most direct evidence yet that such smashups set off an exotic chain of nuclear reactions that can lead to heavy-element formation.

UNIVERSITY OF WARWICK, MARK GARLICK, ESO

November 20, 2019 at 6:45 am

Astronomers have for the first time definitively ID’d the birth of a specific heavy element during a neutron-star smashup. They found strontium. And it showed up in the wavelengths of light — or spectra — making up this collision’s afterglow.

Neutron “star” is a bit of a misnomer. These objects actually are the ultra-dense leftovers of large stars that attained old age, exploded and then collapsed back on themselves. Astrophysicist Darach Watson works in Denmark at the University of Copenhagen. His team reported the strontium discovery online October 23 in Nature.

Scientists had assumed that a collision by two super-dense objects, such as neutron stars, would trigger a chain of nuclear reactions. They’re known as the r-process. In such an environment, the nuclei of atoms could rapidly gobble up neutrons. Afterward, those nuclei would become transformed in a process known as radioactive decay. The r-process was seen as a way to transform old, smaller elements into newer, bigger ones. About half of all elements heavier than iron were thought to be made in the r-process.

Finding strontium in the recent collision at last offered the most direct evidence yet that neutron-star collisions really do trigger the r-process.

#### ‘R’ you seeing it?

Physicists had long predicted that silver, gold and many other elements more massive than iron formed this way. But scientists weren’t sure where those r-process reactions took place. After all, no one had directly seen the r-process underway in a celestial event. Or they didn’t until the merger of two neutron stars in 2017. Scientists quickly analyzed light given off by that cataclysm. In it, they found evidence of the birth of a hodgepodge of heavy elements. All would seem to have come from the r-process.

Caption: Researchers announced October 16 that Advanced LIGO (the Laser Interferometer Gravitational-Wave Observatory) and its sister experiment, Advanced Virgo, have detected gravitational waves from colliding neutron stars. It was a cosmic crash also observed by more than 70 observatories around the world.

But those analyses couldn’t pinpoint precisely which elements were in that mix. Why? The researchers were examining mostly very heavy elements — ones whose complex atomic structures can generate millions of spectral features. And all of those features were not yet fully known, Watson points out. This made it extremely difficult to tease apart which elements were present, he says.

Strontium, however, is relatively light compared to other r-process elements. And its simple atomic structure creates a few strong and well-known spectral clues. So Watson and his colleagues expanded their analysis to consider it. In doing so, they turned up the clear “fingerprint” of strontium. It emerged in light collected by the Very Large Telescope in Chile within a few days of the neutron-star collision.

#### Scientists Say: Neutron star

Seeing strontium in the afterglow wasn’t all that unexpected, says Brian Metzger. He’s an astrophysicist at Columbia University in New York City and not involved in the new work. Strontium, he notes, “does tell us something interesting” about the elements formed during the neutron-star collision.

For instance, the material that produced the strontium must have had an unusually low neutron density (at least compared to what is typically seen inside a neutron star). Otherwise, such an extremely neutron-rich environment as that collision should have created much heavier r-process elements.

The strontium-making neutron-star material probably underwent some other interaction. Maybe it was bombarded by ghostly subatomic particles called neutrinos. They could have been spawned in the collision, Metzger says. “It wasn’t just [normal] neutron star guts” that provided the raw material for this strontium, he concludes.

### Power Words

astronomer A scientist who performs research involving celestial objects, space and the physical universe.

astrophysics An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

atomic Having to do with atoms, the smallest possible unit that makes up a chemical element.

cataclysm An enormous, violent, natural event. A meteor hitting Earth and wiping out most living species would qualify as a cataclysmic event.

celestial (in astronomy) Of or relating to the sky, or outer space.

colleague Someone who works with another a co-worker or team member.

density The measure of how condensed some object is, found by dividing its mass by its volume.

element A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).

heavy element (to astronomers) Any element other than hydrogen (or possibly helium).

iron A metallic element that is common within minerals in Earth’s crust and in its hot core. This metal also is found in cosmic dust and in many meteorites.

neutrino A subatomic particle with a mass close to zero. Neutrinos rarely react with normal matter. Three kinds of neutrinos are known.

neutron A subatomic particle carrying no electric charge that is one of the basic pieces of matter. Neutrons belong to the family of particles known as hadrons.

neutron star The very dense corpse of what had once been a star with a mass four to eight times that of our sun. As the star died in a supernova explosion, its outer layers shot out into space. Its core then collapsed under its intense gravity, causing protons and electrons in its atoms to fuse into neutrons (hence the star’s name). Astronomers believe neutron stars form when large stars undergo a supernova but aren’t massive enough to form a black hole. A single teaspoonful of a neutron star, on Earth, would weigh a billion tons.

nuclear reaction Events that physically alter the nucleus of an atom. (This is in contrast to chemical reactions that affect the electrons orbiting an atom.) Some nuclear reactions will transmute an atom, change it into a different chemical element, such as through fission (also known as atom splitting). Others may involve the capture of energy by bombardment with electromagnetic radiation or subatomic particles. Nuclear reactions are not affected by temperature and pressure (as chemical reactions may be). Instead, they are driven primarily by the energy of the particle that hits them or by the intensity of the radiation prompting the reaction.

online (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.

particle A minute amount of something.

physicist A scientist who studies the nature and properties of matter and energy.

radioactive An adjective that describes unstable elements, such as certain forms (isotopes) of uranium and plutonium. Such elements are said to be unstable because their nucleus sheds energy that is carried away by photons and/or and often one or more subatomic particles. This emission of energy occurs by a process known as radioactive decay.

radioactive decay The process whereby a radioactive isotope — which means a physically unstable form of some element — sheds energy and subatomic particles. In time, this shedding will transform the unstable element into a slightly different but stable element. For instance, uranium-238 (which is a radioactive, or unstable, isotope) decays to radium-222 (also a radioactive isotope), which decays to radon-222 (also radioactive), which decays to polonium-210 (also radioactive), which decays to lead-206 — which is stable. No further decay occurs. The rates of decay from one isotope to another can range from timeframes of less than a second to billions of years.

spectra (sing. spectrum) A range of related things that appear in some order. (in light and energy) The range of electromagnetic radiation types they span from gamma rays to X rays, ultraviolet light, visible light, infrared energy, microwaves and radio waves.

star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

subatomic Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).

telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.

### Citations

Journal:​ ​​ D. Watson et al. Identification of strontium in the merger of two neutron stars. Nature. Published online October 23, 2019. doi:10.1038/s41586-019-1676-3.