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How did Fermi manage to see neutron star merger?

How did Fermi manage to see neutron star merger?


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When the GW170817 neutron stars merger was observed by LIGO/Virgo, the Fermi gamma-ray telescope observed the event 2s after the merge. How did it know where and when to look? It must take some time to rotate the satellite and I guess that evaluation of data from LIGO/Virgo also isn't instantaneous. Or is it? And does it automatically send suspicious signals to the Fermi operation centre to point it to the right location?


The Fermi instruments have a very wide field of view; the gamma ray burst detector covers the whole sky not occulted by the earth with low angular precision, and the Large Area Telescope covers about 1/5 of the entire sky with arc-minute precision. I don't know if the LAT happened to be pointing in the right direction (I guess ~20% chance?) but the burst detector would at least be able to identify the timing of the gamma ray signal from the collision from nearly anywhere in the sky.


First observations of merging neutron stars mark a new era in astronomy

Two months ago, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) notified astronomers around the world of the possible detection of gravitational waves from the merger of two neutron stars. From that moment on August 17, the race was on to detect a visible counterpart, because unlike the colliding black holes responsible for LIGO's four previous detections of gravitational waves, this event was expected to produce a brilliant explosion of visible light and other types of radiation.

A small team led by Ryan Foley, an assistant professor of astronomy and astrophysics at UC Santa Cruz, was the first to find the source of the gravitational waves, located in a galaxy 130 million light-years away called NGC 4993. Foley's team captured the first images of the event with the 1-meter Swope Telescope at the Carnegie Institution's Las Campanas Observatory in Chile.

"This is a huge discovery," Foley said. "We're finally connecting these two different ways of looking at the universe, observing the same thing in light and gravitational waves, and for that alone this is a landmark event. It's like being able to see and hear something at the same time."

Theoretical astrophysicist Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz and a member of Foley's team, said the observations have opened a new window into understanding the physics of neutron star mergers. Among other things, the results could resolve a hotly debated question about the origins of gold and other heavy elements in the universe, which Ramirez-Ruiz has been studying for years.

"I think this can prove our idea that most of these elements are made in neutron star mergers," he said. "We are seeing the heavy elements like gold and platinum being made in real time."

Foley's team is publishing four papers October 16 in Science based on their observations and analysis, as well as three papers in Astrophysical Journal Letters, and they are coauthors of several more papers in Nature and other journals, including two major papers led by the LIGO collaboration. The key Science papers include one presenting the discovery of the first optical counterpart to a gravitational wave source, led by UCSC graduate student David Coulter, and another, led by postdoctoral fellow Charles Kilpatrick, presenting a state-of-the-art comparison of the observations with theoretical models to confirm that it was a neutron-star merger. Two other Science papers were led by Foley's collaborators at the Carnegie Institution for Science.

By coincidence, the LIGO detection came on the final day of a scientific workshop on "Astrophysics with gravitational wave detections," which Ramirez-Ruiz had organized at the Niels Bohr Institute in Copenhagen and where Foley had just given a talk. "I wish we had filmed Ryan's talk, because he was so gloomy about our chances to observe a neutron star merger," Ramirez-Ruiz said. "But then he went on to outline his strategy, and it was that strategy that enabled his team to find it before anyone else."

Foley's strategy involved prioritizing the galaxies within the search field indicated by the LIGO team, targeting those most likely to harbor binary pairs of neutron stars, and getting as many of those galaxies as possible into each field of view. Other teams covered the search field more methodically, "like mowing the lawn," Foley said. His team found the source in the ninth field they observed, after waiting 10 hours for the sun to set in Chile.

"As soon as the sun went down, we started looking," Foley said. "By finding it as quickly as we did, we were able to build up a really nice data set."

He noted that the source was bright enough to have been seen by amateur astronomers, and it likely would have been visible from Africa hours before it was visible in Chile. Gamma rays emitted by the neutron star merger were detected by the Fermi Gamma-ray Space Telescope at nearly the same time as the gravitational waves, but the Fermi data gave no better information about the location of the source than LIGO did.

Foley's team took the first image of the optical source 11 hours after the LIGO detection and, after confirming their discovery, announced it to the astronomy community an hour later. Dozens of other teams quickly followed up with observations from other telescopes. Foley's team also obtained the first spectra of the source with the Magellan Telescopes at Carnegie's Las Campanas Observatory.

The gravitational wave source was named GW170817, and the optical source was named Swope Supernova Survey 2017a (SSS17a). By about seven days later, the source had faded and could no longer be detected in visible light. While it was visible, however, astronomers were able to gather a treasure trove of data on this extraordinary astrophysical phenomenon.

"It's such a rich data set, the amount of science to come from this one thing is incredible," Ramirez-Ruiz said.

Neutron stars are among the most exotic forms of matter in the universe, consisting almost entirely of neutrons and so dense that a sugar cube of neutron star material would weigh about a billion tons. The violent merger of two neutron stars ejects a huge amount of this neutron-rich material, powering the synthesis of heavy elements in a process called rapid neutron capture, or the "r-process."

The radiation this emits looks nothing like an ordinary supernova or exploding star. Astrophysicists like Ramirez-Ruiz have developed numerical models to predict what such an event, called a kilonova, would look like, but this is the first time one has actually been observed in such detail. Kilpatrick said the data fit remarkably well with the predictions of theoretical models.

"It doesn't look like anything we've ever seen before," he said. "It got very bright very quickly, then started fading rapidly, changing from blue to red as it cooled down. It's completely unprecedented."

A theoretical synthesis of data from across the spectrum, from radio waves to gamma rays, was led by Ariadna Murguia-Berthier, a graduate student working with Ramirez-Ruiz, and published in Astrophysical Journal Letters, providing a coherent theoretical framework for understanding the full range of observations. Their analysis indicates, for example, that the merger triggered a relativistic jet (material moving at near the speed of light) that generated the gamma-ray burst, while matter torn from the merger system and ejected at lower speeds drove the r-process and the kilonova emissions at ultraviolet, optical, and infrared wavelengths.

Ramirez-Ruiz has calculated that a single neutron-star merger can generate an amount of gold equal to the mass of Jupiter. The team's calculations of heavy element production by SSS17a suggest that neutron star mergers can account for about half of all the elements heavier than iron in the universe.

The detection came just one week before the end of LIGO's second observing run, which had begun in November 2016. Foley was in Copenhagen, taking advantage of his one afternoon off to visit Tivoli Gardens with his partner, when he got a text from Coulter alerting him to the LIGO detection. At first, he thought it was a joke, but soon he was pedaling his bicycle madly back to the University of Copenhagen to begin working with his team on a detailed search plan.

"It was crazy. We barely got it done, but our team was incredible and it all came together," Foley said. "We got lucky, but luck favors the prepared, and we were ready."

Foley's team at UC Santa Cruz includes Ramirez-Ruiz, Coulter, Kilpatrick, Murguia-Berthier, professor of astronomy and astrophysics J. Xavier Prochaska, postdoctoral researcher Yen-Chen Pan, and graduate students Matthew Siebert, Cesar Rojas-Bravo and Enia Xhakaj. Other team members include Maria Drout, Ben Shappee, and Tony Piro at the Observatories of the Carnegie Institution for Science UC Berkeley astronomer Daniel Kasen and Armin Rest at the Space Telescope Science Institute.

Their team is called the One-Meter, Two-Hemisphere (1M2H) Collaboration because they use two one-meter telescopes, one in each hemisphere: the Nickel Telescope at UC's Lick Observatory and Carnegie's Swope Telescope in Chile. The UCSC group is supported in part by the National Science Foundation, Gordon and Betty Moore Foundation, Heising-Simons Foundation, and Kavli Foundation fellowships for Foley and Ramirez-Ruiz from the David and Lucile Packard Foundation and for Foley from the Alfred P. Sloan Foundation a Niels Bohr Professorship for Ramirez-Ruiz from the Danish National Research Foundation and the UC Institute for Mexico and the United States (UC MEXUS).

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.


Doomed Neutron Stars Create Blast of Light and Gravitational Waves

Shortly after 8:41 a.m. EDT on Aug. 17, NASA's Fermi Gamma-ray Space Telescope picked up a pulse of high-energy light from a powerful explosion, which was immediately reported to astronomers around the globe as a short gamma-ray burst. The scientists at the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves dubbed GW170817 from a pair of smashing stars tied to the gamma-ray burst, encouraging astronomers to look for the aftermath of the explosion. Shortly thereafter, the burst was detected as part of a follow-up analysis by ESA’s (European Space Agency’s) INTEGRAL satellite.

NASA's Swift, Hubble, Chandra and Spitzer missions, along with dozens of ground-based observatories, including the NASA-funded Pan-STARRS survey, later captured the fading glow of the blast's expanding debris.

Neutron stars are the crushed, leftover cores of massive stars that previously exploded as supernovas long ago. The merging stars likely had masses between 10 and 60 percent greater than that of our Sun, but they were no wider than Washington, D.C. The pair whirled around each other hundreds of times a second, producing gravitational waves at the same frequency. As they drew closer and orbited faster, the stars eventually broke apart and merged, producing both a gamma-ray burst and a rarely seen flare-up called a "kilonova."

Neutron star mergers produce a wide variety of light because the objects form a maelstrom of hot debris when they collide. Merging black holes -- the types of events LIGO and its European counterpart, Virgo, have previously seen -- very likely consume any matter around them long before they crash, so we don't expect the same kind of light show.

Within hours of the initial Fermi detection, LIGO and the Virgo detector at the European Gravitational Observatory near Pisa, Italy, greatly refined the event's position in the sky with additional analysis of gravitational wave data. Ground-based observatories then quickly located a new optical and infrared source -- the kilonova -- in NGC 4993.

To Fermi, this appeared to be a typical short gamma-ray burst, but it occurred less than one-tenth as far away as any other short burst with a known distance, making it among the faintest known. Astronomers are still trying to figure out why this burst is so odd, and how this event relates to the more luminous gamma-ray bursts seen at much greater distances.

NASA’s Swift, Hubble and Spitzer missions followed the evolution of the kilonova to better understand the composition of this slower-moving material, while Chandra searched for X-rays associated with the remains of the ultra-fast jet.

Credit: NASA's Goddard Space Flight Center/CI Lab

Credit: NASA's Goddard Space Flight Center/CI Lab

Credit: NASA's Goddard Space Flight Center/CI Lab

Credit: NASA's Goddard Space Flight Center/CI Lab

Music: "Exploding Skies" from Killer Tracks

This version is the raw 3840x2160, 60 fps animation and includes frames for download.

Credit: NASA's Goddard Space Flight Center/CI Lab

Credit: NASA's Goddard Space Flight Center

Credit: NASA's Goddard Space Flight Center

Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet


Finding the signal

On August 17th at 8:41AM ET, just before LIGO and Virgo were scheduled to stop observations after a months-long run, both of LIGO’s observatories in Washington and Louisiana picked up what looked to be a gravitational wave signal. Immediately, astronomers suspected that it was from two neutron stars colliding, since the wave perturbed LIGO’s instruments for over a minute and a half (much longer than previous signals from black holes, which lasted just fractions of a second). It was a sign that the merging objects were much smaller than black holes. “Neutron stars are so much smaller than black holes, so they get much closer together before they merge,” Laura Cadonati, a LIGO collaborator and professor of physics at Georgia Institute of Technology, tells The Verge. “So you can observe the waves for a long time, and get a nice, long, beautiful signal.”

A rendering of the neutron star merger at the moment of impact. Image: Carnegie Institution for Science

At the same time LIGO got its signal, NASA’s Fermi space telescope (in orbit around Earth) detected an intense burst of high-energy light, known as a gamma ray burst, coming from deep space. Astronomers have suspected that neutron stars may create these beams of high-energy radiation when they collide because the explosions are so hot and powerful. Detecting a burst at the same time as a wave signal made the astronomers confident they were seeing two neutron stars merge.

Meanwhile, astronomers initially thought Virgo had missed the signal, since it wasn’t showing up in the observatory’s data. But after a further look, scientists realized Virgo had picked it up the wave signal was just incredibly faint. It turned out the merger occurred in a part of the sky that is a bit of a blindspot for Virgo, which is a byproduct of the observatory’s location on Earth. “Virgo in a way missed it, because it happened to be in a narrow part of the sky where Virgo couldn’t quite catch it,” says Kalogera.

But the fact that Virgo missed it actually helped astronomers figure out where the signal was coming from: the scientists knew the exact spot in the southern sky that Virgo could not see. That knowledge, combined with the data from LIGO’s two observatories, helped the collaboration to pinpoint exactly where the waves were coming from, narrowing the signal’s home to a patch of sky of just 30 square degrees. That’s a small sample of the night sky, which is 40,000 square degrees.


A new era in astronomy: First observations of merging neutron stars

The merger of two neutron stars generated a bright kilonova observed by UC Santa Cruz astronomers, as depicted in this artist's illustration.

Two months ago, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) notified astronomers around the world of the possible detection of gravitational waves from the merger of two neutron stars.

The first optical image of a gravitational wave source was taken by a team led by UC Santa Cruz astronomer Ryan Foley. This image of Swope Supernova Survey 2017a (SSS17a, indicated by arrow) shows the light emitted from the cataclysmic merger of two neutron stars.
Credit: 1M2H Team/UC Santa Cruz and Carnegie Observatories/Ryan Foley

From that moment on August 17, the race was on to detect a visible counterpart, because unlike the colliding black holes responsible for LIGO's four previous detections of gravitational waves, this event was expected to produce a brilliant explosion of visible light and other types of radiation.

A small team led by Ryan Foley, an assistant professor of astronomy and astrophysics at UC Santa Cruz, was the first to find the source of the gravitational waves, located in a galaxy 130 million light-years away called NGC 4993. Foley's team captured the first images of the event with the 1-meter Swope Telescope at the Carnegie Institution's Las Campanas Observatory in Chile.

"This is a huge discovery," Foley said. "We're finally connecting these two different ways of looking at the universe, observing the same thing in light and gravitational waves, and for that alone this is a landmark event. It's like being able to see and hear something at the same time."

New window

Theoretical astrophysicist Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz and a member of Foley's team, said the observations have opened a new window into understanding the physics of neutron star mergers. Among other things, the results could resolve a hotly debated question about the origins of gold and other heavy elements in the universe, which Ramirez-Ruiz has been studying for years.

"I think this can prove our idea that most of these elements are made in neutron star mergers," he said. "We are seeing the heavy elements like gold and platinum being made in real time."

Foley's team is publishing four papers October 16 in Science based on their observations and analysis, as well as three papers in Astrophysical Journal Letters, and they are coauthors of several more papers in Nature and other journals, including two major papers led by the LIGO collaboration. The key Science papers include one presenting the discovery of the first optical counterpart to a gravitational wave source, led by UC Santa Cruz graduate student David Coulter, and another, led by postdoctoral fellow Charles Kilpatrick, presenting a state-of-the-art comparison of the observations with theoretical models to confirm that it was a neutron-star merger. Two other Science papers were led by Foley's collaborators at the Carnegie Institution for Science.

By coincidence, the LIGO detection came on the final day of a scientific workshop on "Astrophysics with gravitational wave detections," which Ramirez-Ruiz had organized at the Niels Bohr Institute in Copenhagen and where Foley had just given a talk. "I wish we had filmed Ryan's talk, because he was so gloomy about our chances to observe a neutron star merger," Ramirez-Ruiz said. "But then he went on to outline his strategy, and it was that strategy that enabled his team to find it before anyone else."

Strategy

The UC Santa Cruz team found SSS17a by comparing a new image of the galaxy N4993 (right) with images taken four months earlier by the Hubble Space Telescope (left).
Credit: Left, Hubble/STScI Right, 1M2H Team/UC Santa Cruz and Carnegie Observatories/Ryan Foley

Foley's strategy involved prioritizing the galaxies within the search field indicated by the LIGO team, targeting those most likely to harbor binary pairs of neutron stars, and getting as many of those galaxies as possible into each field of view. Other teams covered the search field more methodically, "like mowing the lawn," Foley said. His team found the source in the ninth field they observed, after waiting 10 hours for the sun to set in Chile.

"As soon as the sun went down, we started looking," Foley said. "By finding it as quickly as we did, we were able to build up a really nice data set."

He noted that the source was bright enough to have been seen by amateur astronomers, and it likely would have been visible from Africa hours before it was visible in Chile. Gamma rays emitted by the neutron star merger were detected by the Fermi Gamma-ray Space Telescope at nearly the same time as the gravitational waves, but the Fermi data gave no better information about the location of the source than LIGO did.

Foley's team took the first image of the optical source 11 hours after the LIGO detection and, after confirming their discovery, announced it to the astronomy community an hour later. Dozens of other teams quickly followed up with observations from other telescopes. Foley's team also obtained the first spectra of the source with the Magellan Telescopes at Carnegie's Las Campanas Observatory.

Rich data set

The 1M2H Collaboration discovered SSS17a using the 1-meter Swope Telescope at the Carnegie Institution's Las Campanas Observatory in Chile. (
Credit: Observatories of the Carnegie Institution for Science

The gravitational wave source was named GW170817, and the optical source was named Swope Supernova Survey 2017a (SSS17a). By about seven days later, the source had faded and could no longer be detected in visible light. While it was visible, however, astronomers were able to gather a treasure trove of data on this extraordinary astrophysical phenomenon.

"It's such a rich data set, the amount of science to come from this one thing is incredible," Ramirez-Ruiz said.

Neutron stars are among the most exotic forms of matter in the universe, consisting almost entirely of neutrons and so dense that a sugar cube of neutron star material would weigh about a billion tons. The violent merger of two neutron stars ejects a huge amount of this neutron-rich material, powering the synthesis of heavy elements in a process called rapid neutron capture, or the "r-process."

The radiation this emits looks nothing like an ordinary supernova or exploding star. Astrophysicists like Ramirez-Ruiz have developed numerical models to predict what such an event, called a kilonova, would look like, but this is the first time one has actually been observed in such detail. Kilpatrick said the data fit remarkably well with the predictions of theoretical models.

The violent merger of two neutron stars is thought to involve three main energy-transfer processes, shown in this diagram, that give rise to the different types of radiation seen by astronomers, including a gamma-ray burst and a kilonova explosion seen in visible light. As the two stars spiral toward one another and merge to form a "hyper-massive" neutron star, a small fraction of the matter is ejected in a tidal tail (labeled "red component" in the diagram). The merger generates a short gamma-ray burst resulting from twin jets of material moving out from the rotational poles of the merger at close to the speed of light, likely triggered after the collapse of the remnant onto a black hole. In addition, an intense outflow of neutrinos from the hyper-massive neutron star drives a wind of material moving at about one-tenth the speed of light (labeled "blue component" in the diagram). The blue and red wavelengths that dominate the light from the kilonova at different stages result from different elements in the ejected material, which is heated by radioactive decay processes.
Credit: Murguia-Berthier et al., Science

"It doesn't look like anything we've ever seen before," he said. "It got very bright very quickly, then started fading rapidly, changing from blue to red as it cooled down. It's completely unprecedented."

A theoretical synthesis of data from across the spectrum, from radio waves to gamma rays, was led by Ariadna Murguia-Berthier, a graduate student working with Ramirez-Ruiz, and published in Astrophysical Journal Letters, providing a coherent theoretical framework for understanding the full range of observations. Their analysis indicates, for example, that the merger triggered a relativistic jet (material moving at near the speed of light) that generated the gamma-ray burst, while matter torn from the merger system and ejected at lower speeds drove the r-process and the kilonova emissions at ultraviolet, optical, and infrared wavelengths.

Ramirez-Ruiz has calculated that a single neutron-star merger can generate an amount of gold equal to the mass of Jupiter. The team's calculations of heavy element production by SSS17a suggest that neutron star mergers can account for about half of all the elements heavier than iron in the universe.

Not a joke

The detection came just one week before the end of LIGO's second observing run, which had begun in November 2016. Foley was in Copenhagen, taking advantage of his one afternoon off to visit Tivoli Gardens with his partner, when he got a text from Coulter alerting him to the LIGO detection. At first, he thought it was a joke, but soon he was pedaling his bicycle madly back to the University of Copenhagen to begin working with his team on a detailed search plan.

"It was crazy. We barely got it done, but our team was incredible and it all came together," Foley said. "We got lucky, but luck favors the prepared, and we were ready."

Foley's team at UC Santa Cruz includes Ramirez-Ruiz, Coulter, Kilpatrick, Murguia-Berthier, professor of astronomy and astrophysics J. Xavier Prochaska, postdoctoral researcher Yen-Chen Pan, and graduate students Matthew Siebert, Cesar Rojas-Bravo, and Enia Xhakaj. Other team members include Maria Drout, Ben Shappee, and Tony Piro at the Observatories of the Carnegie Institution for Science UC Berkeley astronomer Daniel Kasen and Armin Rest at the Space Telescope Science Institute.

Their team is called the One-Meter, Two-Hemisphere (1M2H) Collaboration because they use two one-meter telescopes, one in each hemisphere: the Nickel Telescope at UC's Lick Observatory and Carnegie's Swope Telescope in Chile. The UC Santa Cruz group is supported in part by the National Science Foundation, Gordon and Betty Moore Foundation, Heising-Simons Foundation, and Kavli Foundation fellowships for Foley and Ramirez-Ruiz from the David and Lucile Packard Foundation and for Foley from the Alfred P. Sloan Foundation a Niels Bohr Professorship for Ramirez-Ruiz from the Danish National Research Foundation and the UC Institute for Mexico and the United States (UC MEXUS).

Learn more about UC Santa Cruz astrophysicist Enrico Ramirez-Ruiz's theories about how neutron mergers could generate gold below.


On the Merger of Neutron Stars

I had thought to go straight back into current news after Centauri Dreams’ recent hiatus, but that’s never a fully satisfactory solution, especially when major events happen while I’m away. I don’t want to simply repeat what everyone has already read about the gravitational wave event GW170817, but there are a few things that caught my eye that we can discuss this morning. After all, we’re dealing with a new phenomenon — kilonovae — that has been predicted but never observed. Nor have we ever before tied gravitational wave events to visible light.

Image: Artist’s impression of merging neutron stars. Credit: ESO.

Now we’re seeing the combination of gravitational wave and electromagnetic astronomy in what promises to be a fertile new ground of study. The fifth GW event ever observed, GW170817 was detected on August 17 of this year by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US, working with the Virgo Interferometer in Italy. In less than two seconds, the Fermi Gamma-ray Space Telescope and ESA’s INTErnational Gamma Ray Astrophysics Laboratory (INTEGRAL) detected a gamma ray burst from the same area of sky.

The collaborative nature of the observations that ensued is quite an achievement. What our gravitational wave detectors can give us is a broad window on the sky within which the event can be confined, one that contains millions of stars and is localized to an area of about 35 square degrees. Telescopes searching for the source included ESO’s Visible and Infrared Survey Telescope for Astronomy (VISTA) and VLT Survey Telescope (VST) at the Paranal Observatory, the Italian Rapid Eye Mount (REM) telescope at ESO’s La Silla Observatory, the LCO 0.4-meter telescope at Las Cumbres Observatory, and the American DECam at Cerro Tololo Inter-American Observatory. Pan-STARRS and Subaru in Hawaii quickly joined in.

I won’t keep listing them here, but about 70 observatories around the world went to work on GW170817, with the Swope 1-meter instrument in Chile announcing a new point of light close to the lenticular galaxy NGC 4993 in Hydra, quickly verified by VISTA observations in the infrared. Distance estimates agreed with the 130 million light year distance of NGC 4993, so what we have is the closest gravitational wave event yet detected and a relatively close gamma-ray burst.

Image: A map of the approximately 70 light-based observatories that detected the gravitational-wave event called GW170817. On August 17, the LIGO and Virgo detectors spotted gravitational waves from two colliding neutron stars. Light-based telescopes around the globe observed the aftermath of the collision in the hours, days, and weeks following. They helped pinpoint the location of the neutron stars and identified signs of heavy elements, such as gold, in the collision’s ejected material. Credit: LIGO-Virgo.

This is the first time I have encountered the term ‘kilonova,’ though the idea behind it is decades old. The properties of this event track theoretical predictions that have been used to explain short gamma-ray bursts, events a thousand times brighter than a typical nova. The simultaneous detection of gravitational waves and gamma rays from this source provides powerful evidence. The merger of two neutron stars, thought to be the cause of the event, has produced a burst of heavy elements moving outbound as fast as .20 c.

Each neutron star in the binary that merged weighed between 1 and 2 solar masses, making the detection of their merger tricky because they are so much smaller than the black holes events we have previously observed. Thus Eliot Quataert (UC-Berkeley):

“We were anticipating LIGO finding a neutron star merger in the coming years but to see it so nearby – for astronomers – and so bright in normal light has exceeded all of our wildest expectations. And, even more amazingly, it turns out that most of our predictions of what neutron star mergers would look like as seen by normal telescopes were right!”

Thus we expand our understanding of how heavy elements are created, a process that can happen in stellar cores through fusion up to iron. Just how elements beyond iron are produced has always been a major issue in astrophysics, but neutron star mergers offer a solution. It was evidently Quataert, working with Brian Metzger (Columbia University) and Daniel Kasen (UC-Berkeley) who applied the term ‘kilonova’ to such events in a 2010 paper (see this UC-Berkeley news release). Kasen refers to neutron star merger debris as ‘weird stuff — a mixture of precious metals and radioactive waste.’

And Columbia postdoc Jennifer Barnes, who had worked with Kasen at Berkeley, helped nail down what a kilonova should look like:

“When we calculated the opacities of the elements formed in a neutron star merger, we found a lot of variation. The lighter elements were optically similar to elements found in supernovae, but the heavier atoms were more than a hundred times more opaque than what we’re used to seeing in astrophysical explosions,” said Barnes. “If heavy elements are present in the debris from the merger, their high opacity should give kilonovae a reddish hue.”

Image: A team of UC Santa Cruz astronomers led by former UC Berkeley graduate student Ryan Foley was the first to detect the light from the neutron star merger 11 hours after the gravitational waves from the collision reached Earth. The left image shows that the glow (red arrow) was not there four months earlier. Credit: UC-SC, Swopes Telescope and Hubble.

The observations fit the calculations, with the color of the merger event shading from an earlier blue to the reddish signature showing the heavier elements of the inner debris cloud.

Image: Three days after the merger and explosion, the bright blue glow from lighter elements in the outer polar regions is beginning to fade, giving way to the red glow from the heavier elements in the surrounding doughnut and spherical core. The red glow persisted for more than two weeks. Credit: Dan Kasen.

About 6 percent of a solar mass of heavy elements came out of all this, a yield of gold alone that was more than 200 Earth masses, and a platinum yield of nearly 500 Earth masses. Neutron star mergers, it appears, can account for all the gold in the visible universe. So much for the idea that what we can now call ‘ordinary’ supernovae can account for the heavy elements beyond iron. Clearly, neutron star mergers are a key player. Gravitational wave astronomy is already paying off.

Those wanting to dig into the papers on GW170817 can start with Kasen et al., “Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event,” Nature 16 October 2017 (abstract), as well as Arcavi et al., “Optical emission from a kilonova following a gravitational-wave-detected neutron-star merger,” Nature 16 October 2017 (abstract) and work from there. The UC-Berkeley news release cited above contains links to a variety of key sources.

Comments on this entry are closed.

Wow. I always though the supernova’s made the gold. This is why gold is so rare not the supernovas. I read that the lighter atoms absorb light more towards the ultra violet and the heavier atoms absorb more towards the infra red. source. Absolutely Small. I can see that idea works through the remote sensing of the elements with electromagnetism. I never heard of the word Kilonova for the collision of neutrons stars also.

To complete the LIGO set we’re now waiting to detect a black hole merger with a neutron star, and the direct detection of waves from a supernova or hypernova. Yet more refined checks on the accuracy of GR will result. Both of these candidates should have optical counterparts and so will put on quite the show.

I still have not heard any explanation for the almost-two-second delay between the merger (as signaled by the gravity waves) and the emission of the gamma rays. Anybody understand this?

I think it’s called the Shapiro Effect. GW’s are unaffected by magnetic fields, and therefore travel in an ABSOLUTE STRAIT LINE, whereas magnetic fields cause EMW’s to corkscrew in an extremely minute way, meaning that the overall DISTANCE the EMW’s travel is very very VERY slightly GREATER than the distance the GW’s travel.

“waiting to detect a black hole merger with a neutron star, and the direct detection of waves from a supernova or hypernova”

The mass ratio between a BH and neutron star is likely to be large, therefore the gravitational wave emission would be weak so the event would have to be close. Supernovae etc. require a great deal of asymmetry to result in any appreciable gravitational waves. And the lower frequency is (as I recall) outside LIGO’s sensitivity range. All of these would be nice to see but don’t hold your breath.

I found the chart I was looking for. It plots amplitude (by detectable strain) and frequency by GW instrument and event class. I had an old copy on my computer but had to do a search to find where it’s from. This provides a better answer than I gave earlier.

Paul Gilster: Countless theories about MOND and dark energy, but after pinning down the time differential(

1.7 secomds)between the GW and the GRB, only a VERY FEW remain viable. Since these two subjects are BOTH way above my head, a guest post on this website by an EXPERT in those fields would be greatly appreciated, unless, of course, you choose to delve into it yourself.

A very tough subject indeed, Harry! And while I might delve, I’m not the expert to write the subject up. If I can find someone who is, I’ll pass along the request for some thoughts. My thought right now is that dark energy is such an evolving study that we’re not going to be able to draw firm conclusions now or for quite some time, but perhaps GW analysis will eventually offer insights. Anyway, I’m glad to look for someone to probe this with the right credentials.

That would be interesting to hear. I expect we will soon be hearing theories put forward on the detailed dynamics of the merger process, even if based only on one event. The dynamics will depend on whether the merger can result in a black hole, perhaps one residual neutron star with a large amount of exploding ordinary matter or destruction of both neutron stars. EM would only come from ordinary matter, not a BH.

Since the mass range for a stable neutron star is so narrow and the borderline merger mass to result in a BH is so poorly understood we are on the cusp of discovering some new and really interesting stuff. Go LIGO!

What if two such stars were to collide nearby–but behind an End of life Red Giant?

Might that act as a catcher’s mitt–protect Earth from radiation, and give a nice rich field of metalks and gas?

Not my field is conformal gravity in or out?

I looked “conformal gravity” up on Wikipedia, and was completely lost after the fifth sentence. That’s why we need an expert to post something here, either as a blog, or in the comments section somewhere.

I had heard of conformal gravity but knew almost nothing about it since it was off the mainstream. The referenced paper presents a formal falsification of the theory. It’s more explanatory of what the theory is and does than the unhelpful Wikipedia article. At least the article points to this paper (via CERN). I’d have to read it to learn the details, but I don’t know if I’ll bother since it looks like another oddball way to explain away dark matter and dark energy with suspect parameter insertion and adjustment.

Can someone calculate how many kilonovae per galaxy it takes to account for all the heavy elements in, say, our galaxy? Were they more common closer to the Big Bang then they are now or will be in the future? Should we worry that they may be the answer to Fermi’s Paradox? Do we need to start building a few deep underground cities just in case?

According to the analysis the quantity of the measured elements is such that not many of these events is enough to account for all the heavy elements in a galaxy. However since I have not yet read the paper I don’t know the details of how they determined this.

What puzzles me is that the gold is ejected from the Kilonova at a speed of 30% light speed. How do you slow the stuff down so that it can be incorporated into a star-forming nebula or a stellar nursery?

Good question. Not all the nuclei will be traveling that speed: there will be a large range of velocities because of the shock waves on turbulence that result. What range exactly I don’t know. Many of these particles will travel cosmological distance and become cosmic rays. Neutron star collisions were posited as a major source of cosmic rays, and this event supports that.

Luckily there is a large surplus of heavy elements from these collisions. I suppose the small proportion that hit stars, planets and even nebulae would account for what we find. The heaviest elements on Earth may have mostly come from distant galaxies.

A number of orbiting neutron pairs have been observed. I believe the first was observed several decades ago. Was this pair detected prior to their merger or have any images been found on old plates?

The known neutron star pairs are within our galaxy… this merger event are in a galaxy 130 million lyrs away. It’s not yet possible to see pairs this far away.

I would be interested to know what the effects would be if this happened at the distance from Alpha Centauri to our solar system? This would be distorting the space we live in, but would we even notice and it would also be distorting time!

A kilonova explosion only four light years away is probably bad. There has to be many radioactive elements including Uranium due to the heat and pressure from the momentum or kinetic and potential gravitational energy which is also released as gamma rays etc. The gamma rays would destroy the ozone layer in our atmosphere if they hit it and some of those radioactive elements would reach Earth in a hundred years due to their high speed. It depends if there is an accretion blocking them or not and the angle of it towards Earth.

I have read that two black holes merging releases enough radiation alone to sterilize worlds for hundreds of light years around, if not more.

Where did you read that? There is no EM radiation or particles from the merger of two BH. Destructive effects of the radiated GW is short range.

A science book from circa 1981. I cannot remember the title.

Although there is no direct EW from a BH merger gas and dust can be compressed by the GW if close by, a Sun’s mass in GW energy is a serious amount compression.

Forgot to add compresses the gas and dust to high energy EW emissions.

Yes, but unlikely. Over the long time the BH spiraled inward all the ordinary matter, including remaining accretion disks, would have been either violently expelled or fallen into one of the BH. That is, there would be no stable or quasi-stable orbits in that crazy place.

There is always material around these BH’s, they have no stellar winds so interstellar gas and dust can collect over eons. The energy released is over milliseconds and it is vast, plenty to compress and vaporise material to great distances.

As well as the spectacular results of neutron star binaries destroying themselves, it is interesting to consider how they formed in the first place. The sequence of events required to ensure that the system does not become unbound as a result of the supernovae, or end up with the first neutron star falling into the core of the second star and becoming a Thorne-Żytkow object puts some fairly strict constraints on the progenitor binary.

Michael, I fear it would be the radiation effects and not the gravitational effects one would have to worry about if one were only four light years from such an event. Can someone calculate it? I think the whole solar system would be fried. Maybe gold dust might eventually come trickling down from space, but I expect nobody would be around to sweep it up.

Quoting myself a few posts above in this thread, I have read that two black holes merging releases enough radiation alone to sterilize worlds for hundreds of light years around, if not more.

Waves of joy: why astronomers are ecstatic about colliding neutron stars

Witnessing the collision of a pair of neutron stars was the biggest science event of 2017. Lauren Fuge reflects on why astronomers are so excited.


Two neutron stars collided near the solar system 4.6 billion years ago

According to a new study published in the May 2, 2019 issue of Nature, 4.6 billion years ago, two neutron stars collided near the early Solar System (actually about 1000 light-years from the gas cloud that eventually formed the Solar System). This violent collision has created heavy elements like silver, gold, platinum, cesium, and uranium. The study says 0.3% of the Earth’s heaviest elements have been created by this event.

Researchers concluded that 4.6 billion years ago, about 100 million years before the formation of Earth, two neutron stars collided about 1000 light-years away Since our galaxy, the Milky Way is at least 100,000 light-years in diameter, this distance can be easily treated as the “cosmic neighborhood”.

Scientists say “if a comparable event happened today at a similar distance from the Solar System, the ensuing radiation could outshine the entire night sky”.

To arrive at their conclusion, the authors of the study, astrophysicists Szabolcs Marka at Columbia University and Imre Bartos at the University of Florida, compared the composition of meteorites to numerical simulations of the Milky Way.

There is a process called “r-process” (rapid neutron-capture process) which is responsible for the creation (nucleosynthesis) of approximately half the abundances of the atomic nuclei heavier than iron.

As neutron-star mergers occur infrequently, their deposition of radioactive isotopes into the pre-solar nebula could have been dominated by a few nearby events. Although short-lived r-process isotopes (with half-lives shorter than 100 million years) are no longer present in the Solar System, their abundances in the early Solar System are known because their daughter products were preserved in high-temperature condensates found in meteorites.

Researchers assert that that abundances of short-lived r-process isotopes in the early Solar System point to their origin in neutron-star mergers, and indicate substantial deposition by a single nearby merger event.

By comparing numerical simulations with the early Solar System abundance ratios of actinides produced exclusively through the r-process, researchers constrain the rate of occurrence of their Galactic production sites to within about 1-100 per million years. This is consistent with observational estimates of neutron-star merger rates, but rules out supernovae and stellar sources, Marka ad Bartos say.

Researchers further find that there was probably a single nearby merger that produced much of the curium and a substantial fraction of the plutonium present in the early Solar System. Such an event may have occurred about 300 parsecs (978 light-years) away from the pre-solar nebula (see notes 1), approximately 80 million years before the formation of the Solar System.

Since our current technology depends heavily on these rare elements, these findings will have interesting effects on the quest for extraterrestrial civilizations. If heavy elements are even more scarce, there could be no intelligent species in the Universe with the level of technological growth we have had.

This could be one possible explanation of why we haven’t heard from ET yet, or as Enrico Fermi put it: “Where is everybody?“.


Gravitational waves from merging neutron stars

For the first time, researchers have simultaneously measured the gravitational waves and the light from two merging neutron stars. 'Multi-messenger astronomy', combining the observation of gravitational waves and electromagnetic radiation, begins with this event, registered on 17 August 2017 at 14:41:04 CEST. Together, the complementary methods will considerably increase our understanding of extreme astrophysical events. For example, this discovery confirms that two merging neutron stars are indeed the precursor to short gamma-ray bursts and that the explosion following this merger – known as a kilonova – is the source of heavy elements in the Universe. During the 17 August observations, researchers at the Max Planck Institutes for Gravitational Physics in Potsdam and Hannover, and the Max Planck Institute for Astrophysics and for Extraterrestrial Physics in Garching, played a central role.

Dance of the heavyweights: two neutron stars orbit each other, spiralling ever closer together, radiating gravitational waves in the process. Astrophysicists have long been theorizing that neutron stars – the cores of burnt-out, massive suns waves in the process. Astrophysicists have long been theorizing that neutron stars – the cores of burnt-out, massive suns – could create gamma rays burst. This image of the real event GW170817 comes from a numerical-relativistic simulation.

© Numerical Relativity Simulation: T. Dietrich (Max Planck Institute for Gravitational Physics) and the BAM collaboration Scientific Visualization: T. Dietrich, S. Ossokine, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics)

The two LIGO detectors in Hanford (Washington state, USA) and Livingston (Louisiana) observed the signal referred to as GW170817 for around 100 seconds. The measurements made by the Virgo detector in Tuscany, near Pisa, Italy, improved localization in the heavens substantially and allowed the researchers to constrain the origin of the wave to a spot in the southern skies of only 28 square degrees – around 130 times the apparent size of the full moon.

Only 1.7 seconds later the Gamma-ray Burst Monitor (GBM) on board the Fermi satellite registered a gamma-ray burst, known as GRB 170817A, from roughly the same direction as the gravitational wave signal. The probability that this encounter was purely coincidental is one to 200 million – the odds of having six correct numbers in the lottery are considerably better!

&aposAt first, the detection of this new gamma-ray burst by Fermi did not really seem extraordinary,&apos says Andreas von Kienlin, scientist at the Max Planck Institute for Extraterrestrial Physics, who also helped build the instrument. &aposWe observe four or five new gamma-ray bursts per week.&apos Only later did the researcher learn about the LIGO and Virgo observatory measurements: &aposWe immediately knew that this was a historical event.&apos

A further surprise is that the gamma-rays and the gravitational waves were not detected at exactly the same time, but with an approximately two-second time difference. &aposThese and the additional observations provide us with unique insights into the physics in and around this event&apos, says von Kienlin.

Integral, another gamma-ray mission with a Max Planck instrument on board – the SPI – also observed high-energy radiation from the merger. &aposThe signal we observed may not have been very bright, but we can confirm the Fermi result with a completely independent gamma-ray detection,&apos says Roland Diehl, scientist at the Max Planck Institute for Extraterrestrial Physics and co-investigator for the SPI. &aposWith regard to the link between gamma-ray burst and gravitational waves, we are on safe ground here. Moreover, we can clearly connect a short gamma ray burst to a neutron star collision for the first time&apos, says Diehl.

The very precise localization of the LIGO/Virgo observation allowed a handful of observatories around the globe to search the area of the heavens from where the signal originates only a few hours later. Optical telescopes, including the GROND instrument on the 2.2 metre Max Planck Society telescope in the La Silla observatory at the European Southern Observatory (ESO), discovered a new point of light, similar to a star, near the NGC 4993 galaxy. This lenticular galaxy system is approximately 130 million light-years from Earth.

Three-step observation: the first sign of the neutron-star merger on 17 August 2017 was a brief burst of gamma rays discovered by US satellite Fermi (above). Soon after, scientists from the LIGO-Virgo cooperation reported that their detectors had recorded the gravitational waves 1.7 seconds before the Fermi burst (middle). A little later, scientists reported that the burst was also observed with an instrument aboard the European satellite Integral.

© NASA's Goddard Space Flight Center, Caltech/MIT/LIGO Lab and ESA

For the astronomers, one thing is certain: the optical object, the gamma-ray burst and the gravitational waves all originate from one and the same source: the merging of a pair of neutron stars. Eventually, more than 70 ground- and space-based observatories noted the event in the x-ray, ultra-violet, visible, infra-red and radio wave spectra.

&aposThese agreeing observations give us a detailed picture of this event from three minutes prior to the merger up to several weeks later,&apos says Jochen Greiner, the Max Planck Institute for Extraterrestrial Physics scientist responsible for building GROND.

This opinion is shared by the scientists at the Max Planck Institute for Gravitational Physics in Hannover and Potsdam: &aposIn itself, the first evidence of gravitational waves emitted by merging neutron stars is already extremely exiting. However, the combination with dozens of follow-up observations in the electromagnetic spectrum makes it truly revolutionary&apos, say Alessandra Buonanno and her Director colleagues Bruce Allen and Karsten Danzmann.

Identification of GW170817 as a binary star system consisting of two neutron stars, and observations of electromagnetic radiation following their collision, allow conclusions on the previously mysterious origin of the short gamma-ray bursts to be reached.

&aposWhile we did have strong hints that merging neutron stars are indeed the progenitors for short gamma ray bursts, we did not have final proof. The simultaneous observation of this two-second burst by Integral and Fermi, and by the gravitational wave detectors, is the first conclusive evidence that at least some of these gamma ray bursts are indeed powered by neutron star mergers,&apos says Rashid Sunyaev, Director at the Max Planck Institute for Astrophysics.

Luminous light point: this image shows the galaxy NGC 4993, some 130 million light years away from Earth, in the constellation Hydra, in which the two neutron stars exploded. This so-called kilonova appears as a bright star. Such objects are the main source for very heavy chemical elements such as gold and platinum in the universe.

Luminous light point: this image shows the galaxy NGC 4993, some 130 million light years away from Earth, in the constellation Hydra, in which the two neutron stars exploded. This so-called kilonova appears as a bright star. Such objects are the main source for very heavy chemical elements such as gold and platinum in the universe.

Analyses of the LIGO data returned a relatively small distance of the merging neutron stars from the earth of around 85 to 160 million light years, in agreement with the 130 million light years to the assumed parent galaxy NGC 4993. In contrast to previous gravitational wave observations, the scientists calculated the masses of the merging objects as 1.1 to 1.6 times that of our Sun, comparable to that of known neutron stars and dissimilar to that of black holes.

Neutron stars are the extremely dense, exhausted remains of massive stars, with diameters of only around 20 kilometres. Astrophysicists have studied the merger of two such star corpses in theory for a long time – and have enjoyed confirmation in the observation of the GW170817 signal: &aposThe physical parameters of the transient event match the theoretical predictions for a so-called kilonova from a neutron star merger remarkably well&apos, says Anders Jerkstrand from the Max Planck Institute for Astrophysics.

&aposIn particular, the rate at which the light from the source dimmed over the ten days following the merger exactly corresponds to the forecast that the ejecta are dominated by radioactive elements much heavier than iron&apos says Jerkstrand.

Two neutron stars merging in a gamma ray burst liberates enormous quantities of energy. At the same time, dense matter is expelled at high velocities. Because the ejected matter displays a high concentration of free neutrons, the heaviest elements in the universe can form from them. The process involved is referred to as rapid neutron capture or, more briefly, the r-process.

&aposThe origin of the heaviest chemical elements in the universe was a mystery to us scientists for a long time&apos, says Hans-Thomas Janka, lead scientist at the Max Planck Institute for Astrophysics. &aposNow we have the first observational proof that neutron star collisions may be the sources of these elements. They may even be the main source of the r-process elements.&apos

The observation and characterization of minutes-long signals from merging neutron stars, concealed in detector noise, demands extremely precise waveform models. Members of the &aposAstrophysical and Cosmological Relativity&apos department at the Max Planck Institute for Gravitational Physics developed models which were deployed as templates in the optimal filter searches that discovered GW170817.

Cosmic energy factory: when neutron stars collide, the explosion shoots huge jets of glowing matter into space at high velocities, as shown in this artistic representation. The jets produce a short gamma-ray burst (shown in magenta). The cloud around the black hole in the centre can be observed with visible and near-infrared wavelengths.

© NASA's Goddard Space Flight Center/CI Lab

In addition, Max Planck researchers played a central role in the development and implementation of the search algorithms used to observe GW170817. As members of the team that investigated the signal immediately after its discovery, they immediately removed a temporary spurious signal from data delivered by the LIGO Livingston instrument.

This allowed the position in the heavens to be so precisely pinpointed that astronomers were able to observe a quickly fading visible light glow within twelve hours of GW170817&aposs discovery. They had thus identified the parent galaxy and redshift, leading to measurement of the Hubble constant and therefore to the distance.

The Max Planck Institute for Gravitational Physics also helped to develop and apply analysis algorithms and waveform models, allowing them to identify the source of GW170817 as a binary neutron star system. In an additional insight, the results contradict theories that assume highly repulsive nuclear forces, because they predict relatively large and therefore highly deformable neutron stars.

In Potsdam, both analytical methods and numerical simulations were adopted to construct modern waveform models which predict weaker repulsive nuclear forces in GW170817&aposs neutron stars. Moreover, Max Planck scientists have investigated electromagnetic signals created by the ejection of matter as the stars merged. These signals contain information on the creation of heavy elements in the universe, as discussed above.

&aposIn a domino effect, GW170817 has set off a spectacular sequence of astrophysical observations, simultaneously solving long-standing mysteries and presenting us with new ones&apos, says Alessandra Buonanno, Director at the Max Planck Institute in Potsdam. &aposRemarkably, GW170817 has also delivered insights into the nature of ultradense matter in the interior of the most fascinating and extreme objects in the universe – neutron stars.&apos

Scientists in the &aposObservation-based Relativity and Cosmology&apos department at the institute in Hannover played a central role during the first hours of analyses, as well as in the characterization of the gravitational wave signal and understanding the source. &aposEven in my wildest dreams I would not have dared to hope that we would simultaneously demonstrate the first discovery of a binary neutron star by gravitational waves, the corresponding gamma-ray burst and the electromagnetic signals. I imagined we would only see such a thing after 20 or more observations of two merging neutron stars. This is fantastic!&apos, as Director Bruce Allen stated.

&aposThis is the beginning of multi-messenger astronomy and a greater understanding of our universe. We are very proud to play a central role in measuring gravitational waves, because our lasers, developed and tested in the GEO600 project, are at the heart of all gravitational wave observatories&apos, summarizes Max Planck Director Karsten Danzmann, Allen&aposs colleague at the Hannover institute.


Gravitational Waves From Merging Neutron Stars

Dance of the heavyweights: two neutron stars orbit each other, spiralling ever closer together, radiating gravitational waves in the process. Astrophysicists have long been theorizing that neutron stars, the cores of burnt-out, massive suns, could create gamma rays burst. The image of the real event GW170817 comes from a numerical-relativistic simulation.

For the first time, researchers have simultaneously measured the gravitational waves and the light from two merging neutron stars.

'Multi-messenger astronomy', combining the observation of gravitational waves and electromagnetic radiation, begins with this event, registered on 17 August 2017 at 14:41:04 CEST. Together, the complementary methods will considerably increase our understanding of extreme astrophysical events. For example, this discovery confirms that two merging neutron stars are indeed the precursor to short gamma-ray bursts and that the explosion following this merger - known as a kilonova - is the source of heavy elements in the Universe. During the 17 August observations, researchers at the Max Planck Institutes for Gravitational Physics in Potsdam and Hannover, and the Max Planck Institute for Astrophysics and for Extraterrestrial Physics in Garching, played a central role.

The two LIGO detectors in Hanford (Washington state, USA) and Livingston (Louisiana) observed the signal referred to as GW170817 for around 100 seconds. The measurements made by the Virgo detector in Tuscany, near Pisa, Italy, improved localization in the heavens substantially and allowed the researchers to constrain the origin of the wave to a spot in the southern skies of only 28 square degrees - around 130 times the apparent size of the full moon.

Only 1.7 seconds later the Gamma-ray Burst Monitor (GBM) on board the Fermi satellite registered a gamma-ray burst, known as GRB 170817A, from roughly the same direction as the gravitational wave signal. The probability that this encounter was purely coincidental is one to 200 million - the odds of having six correct numbers in the lottery are considerably better!

'At first, the detection of this new gamma-ray burst by Fermi did not really seem extraordinary,' says Andreas von Kienlin, scientist at the Max Planck Institute for Extraterrestrial Physics, who also helped build the instrument. 'We observe four or five new gamma-ray bursts per week.' Only later did the researcher learn about the LIGO and Virgo observatory measurements: 'We immediately knew that this was a historical event.'

A further surprise is that the gamma-rays and the gravitational waves were not detected at exactly the same time, but with an approximately two-second time difference. 'These and the additional observations provide us with unique insights into the physics in and around this event', says von Kienlein.

Integral, another gamma-ray mission with a Max Planck instrument on board - the SPI - also observed high-energy radiation from the merger. 'The signal we observed may not have been very bright, but we can confirm the Fermi result with a completely independent gamma-ray detection,' says Roland Diehl, scientist at the Max Planck Institute for Extraterrestrial Physics and co-investigator for the SPI. 'With regard to the link between gamma-ray burst and gravitational waves, we are on safe ground here. Moreover, we can clearly connect a short gamma ray burst to a neutron star collision for the first time', says Diehl.

The very precise localization of the LIGO/Virgo observation allowed a handful of observatories around the globe to search the area of the heavens from where the signal originates only a few hours later. Optical telescopes, including the GROND instrument on the 2.2 metre Max Planck Society telescope in the La Silla observatory at the European Southern Observatory (ESO), discovered a new point of light, similar to a star, near the NGC 4993 galaxy. This lenticular galaxy system is approximately 130 million light-years from Earth.

For the astronomers, one thing is certain: the optical object, the gamma-ray burst and the gravitational waves all originate from one and the same source: the merging of a pair of neutron stars. Eventually, more than 70 ground- and space-based observatories noted the event in the x-ray, ultra-violet, visible, infra-red and radio wave spectra.

'These agreeing observations give us a detailed picture of this event from three minutes prior to the merger up to several weeks later,' says Jochen Greiner, the Max Planck Institute for Extraterrestrial Physics scientist responsible for building GROND.

This opinion is shared by the scientists at the Max Planck Institute for Gravitational Physics in Hannover and Potsdam: 'In itself, the first evidence of gravitational waves emitted by merging neutron stars is already extremely exiting. However, the combination with dozens of follow-up observations in the electromagnetic spectrum makes it truly revolutionary', say Alessandra Buonanno and her Director colleagues Bruce Allen and Karsten Danzmann.

Identification of GW170817 as a binary star system consisting of two neutron stars, and observations of electromagnetic radiation following their collision, allow conclusions on the previously mysterious origin of the short gamma-ray bursts to be reached.

'While we did have strong hints that merging neutron stars are indeed the progenitors for short gamma ray bursts, we did not have final proof. The simultaneous observation of this two-second burst by Integral and Fermi, and by the gravitational wave detectors, is the first conclusive evidence that at least some of these gamma ray bursts are indeed powered by neutron star mergers,' says Rashid Sunyaev, Director at the Max Planck Institute for Astrophysics.

Analyses of the LIGO data returned a relatively small distance of the merging neutron stars from the earth of around 85 to 160 million light years, in agreement with the 130 million light years to the assumed parent galaxy NGC 4993. In contrast to previous gravitational wave observations, the scientists calculated the masses of the merging objects as 1.1 to 1.6 times that of our Sun, comparable to that of known neutron stars and dissimilar to that of black holes.

Neutron stars are the extremely dense, exhausted remains of massive stars, with diameters of only around 20 kilometres. Astrophysicists have studied the merger of two such star corpses in theory for a long time - and have enjoyed confirmation in the observation of the GW170817 signal: 'The physical parameters of the transient event match the theoretical predictions for a so-called kilonova from a neutron star merger remarkably well', says Anders Jerkstrand from the Max Planck Institute for Astrophysics.

'In particular, the rate at which the light from the source dimmed over the ten days following the merger exactly corresponds to the forecast that the ejecta are dominated by radioactive elements much heavier than iron' says Jerkstrand.

Two neutron stars merging in a gamma ray burst liberates enormous quantities of energy. At the same time, dense matter is expelled at high velocities. Because the ejected matter displays a high concentration of free neutrons, the heaviest elements in the universe can form from them. The process involved is referred to as rapid neutron capture or, more briefly, the r-process.

'The origin of the heaviest chemical elements in the universe was a mystery to us scientists for a long time', says Hans-Thomas Janka, lead scientist at the Max Planck Institute for Astrophysics. 'Now we have the first observational proof that neutron star collisions may be the sources of these elements. They may even be the main source of the r-process elements.'

The observation and characterization of minutes-long signals from merging neutron stars, concealed in detector noise, demands extremely precise waveform models. Members of the 'Astrophysical and Cosmological Relativity' department at the Max Planck Institute for Gravitational Physics developed models which were deployed as templates in the optimal filter searches that discovered GW170817.

In addition, Max Planck researchers played a central role in the development and implementation of the search algorithms used to observe GW170817. As members of the team that investigated the signal immediately after its discovery, they immediately removed a temporary spurious signal from data delivered by the LIGO Livingston instrument.

This allowed the position in the heavens to be so precisely pinpointed that astronomers were able to observe a quickly fading visible light glow within twelve hours of GW170817's discovery. They had thus identified the parent galaxy and redshift, leading to measurement of the Hubble constant and therefore to the distance.

The Max Planck Institute for Gravitational Physics also helped to develop and apply analysis algorithms and waveform models, allowing them to identify the source of GW170817 as a binary neutron star system. In an additional insight, the results contradict theories that assume highly repulsive nuclear forces, because they predict relatively large and therefore highly deformable neutron stars.

In Potsdam, both analytical methods and numerical simulations were adopted to construct modern waveform models which predict weaker repulsive nuclear forces in GW170817's neutron stars. Moreover, Max Planck scientists have investigated electromagnetic signals created by the ejection of matter as the stars merged. These signals contain information on the creation of heavy elements in the universe, as discussed above.

'In a domino effect, GW170817 has set off a spectacular sequence of astrophysical observations, simultaneously solving long-standing mysteries and presenting us with new ones', says Alessandra Buonanno, Director at the Max Planck Institute in Potsdam. 'Remarkably, GW170817 has also delivered insights into the nature of ultradense matter in the interior of the most fascinating and extreme objects in the universe - neutron stars.'

Scientists in the 'Observation-based Relativity and Cosmology' department at the institute in Hannover played a central role during the first hours of analyses, as well as in the characterization of the gravitational wave signal and understanding the source. 'Even in my wildest dreams I would not have dared to hope that we would simultaneously demonstrate the first discovery of a binary neutron star by gravitational waves, the corresponding gamma-ray burst and the electromagnetic signals. I imagined we would only see such a thing after 20 or more observations of two merging neutron stars. This is fantastic!', as Director Bruce Allen stated.

'This is the beginning of multi-messenger astronomy and a greater understanding of our universe. We are very proud to play a central role in measuring gravitational waves, because our lasers, developed and tested in the GEO600 project, are at the heart of all gravitational wave observatories', summarizes Max Planck Director Karsten Danzmann, Allen's colleague at the Hannover institute.


How did Fermi manage to see neutron star merger? - Astronomy

Press Release From: University of California Berkeley
Posted: Monday, October 16, 2017

The first detection of gravitational waves from the cataclysmic merger of two neutron stars, and the observation of visible light in the aftermath of that merger, finally answer a long-standing question in astrophysics: Where do the heaviest elements, ranging from silver and other precious metals to uranium, come from?

Based on the brightness and color of the light emitted following the merger, which closely match theoretical predictions by University of California, Berkeley, and Lawrence Berkeley National Laboratory physicists, astronomers can now say that the gold or platinum in your wedding ring was in all likelihood forged during the brief but violent merger of two orbiting neutron stars somewhere in the universe.

This is the first detection of a neutron star merger by the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in the United States, whose leaders were awarded the Nobel Prize in Physics two weeks ago, and the Virgo detector in Italy. LIGO had previously detected gravitational waves from four black hole mergers, and Virgo one, but such events should be completely dark. This is the first time that light associated with a source of gravitational waves has been detected.

"We have been working for years to predict what the light from a neutron merger would look like," said Daniel Kasen, an associate professor of physics and of astronomy at UC Berkeley and a scientist at Berkeley Lab. "Now that theoretical speculation has suddenly come to life."

The neutron star merger, dubbed GW170817, was detected on August 17 and immediately telegraphed to observers around the world, who turned their small and large telescopes on the region of the sky from which it came. The ripples in spacetime that LIGO/Virgo measured suggested a neutron star merger, since each star of the binary weighed between 1 and 2 times the mass of our Sun. Apart from black holes, neutron stars are the densest objects known in the universe. They are created when a massive star exhausts its fuel and collapses onto itself, compressing a mass comparable to that of the Sun into a sphere only 10 miles across.

Only 1.7 seconds after the gravitational waves were recorded, the Fermi space telescope detected a short burst of gamma rays from the same region, evidence that concentrated jets of energy are produced during the merger of neutron stars. Less than 11 hours later, observers caught their first glimpse of visible light from the source. It was localized to a known galaxy, NGC 4993, situated about 130 million light-years from Earth in the direction of the constellation Hydra.

The detection of a neutron star merger was surprising, because neutron stars are much smaller than black holes and their mergers produce much weaker gravitational waves than do black hole mergers. According to Berkeley professor of astronomy and physics Eliot Quataert, "We were anticipating LIGO finding a neutron star merger in the coming years but to see it so nearby -- for astronomers -- and so bright in normal light has exceeded all of our wildest expectations. And, even more amazingly, it turns out that most of our predictions of what neutron star mergers would look like as seen by normal telescopes were right!"

The LIGO/Virgo observations of gravitational waves and the detection of their optical counterpart will be discussed at a 10 a.m. EDT press conference on Monday, Oct. 16, at the National Press Club in Washington, D.C. Simultaneously, several dozen papers discussing the observations will be published online by Nature [http://www.nature.com], Science [http://science.sciencemag.org] and the Astrophysical Journal Letters [http://apjl.aas.org].

While hydrogen and helium were formed in the Big Bang 13.8 billion years ago, heavier elements like carbon and oxygen were formed later in the cores of stars through nuclear fusion of hydrogen and helium. But this process can only build elements up to iron. Making the heaviest elements requires a special environment in which atoms are repeatedly bombarded by free neutrons. As neutrons stick to the atomic nuclei, elements higher up the periodic table are built.

Where and how this process of heavy element production occurs has been one of the longest-standing questions in astrophysics. Recent attention has turned to neutron star mergers, where the collision of the two stars flings out clouds of neutron-rich matter into space, where they could assemble into heavy elements.

Speculation that astronomers might see light from such heavy elements traces back to the 1990s, but the idea had mostly been gathering dust until 2010, when Brian Metzger, then a freshly minted graduate student at UC Berkeley, now a professor of astrophysics at Columbia University, co-authored a paper with Quataert and Kasen in which they calculated the radioactivity of the neutron star debris and estimated its brightness for the first time.

"As the debris cloud expands into space," Metzger said, "the decay of radioactive elements keeps it hot, causing it to glow."

Metzger, Quataert, Kasen and collaborators showed that this light from neutron star mergers was roughly one thousand times brighter than normal nova explosions in our galaxy, motivating them to name these exotic flashes "kilonovae."

Still, basic questions remained as to what a kilonova would actually look like.

"Neutron star merger debris is weird stuff -- a mixture of precious metals and radioactive waste," Kasen said.

Astronomers know of no comparable phenomena, so Kasen and collaborators had to turn to fundamental physics and solve mathematical equations describing how the quantum structure of heavy atoms determines how they emit and absorb light.

Jennifer Barnes, an Einstein postdoctoral fellow at Columbia, worked as a Berkeley graduate student with Kasen to make some of the first detailed predictions of what a kilonova should look like.

"When we calculated the opacities of the elements formed in a neutron star merger, we found a lot of variation. The lighter elements were optically similar to elements found in supernovae, but the heavier atoms were more than a hundred times more opaque than what we're used to seeing in astrophysical explosions," said Barnes. "If heavy elements are present in the debris from the merger, their high opacity should give kilonovae a reddish hue."

"I think we bummed out the entire astrophysics community when we first announced that," Kasen said. "We were predicting that a kilonova should be relatively faint and redder than red, meaning it would be an incredibly difficult thing to find. On the plus side, we had defined a smoking-gun -- you can tell that you are seeing freshly produced heavy elements by their distinctive red color."

That is just what astronomers observed.

The August LIGO/Virgo discovery of a neutron star merger meant that "judgment day for the theorists would come sooner than expected," Kasen said.

"For years the idea of a kilonova had existed only in our theoretical imagination and our computer models," he said. "Given the complex physics involved, and the fact that we had essentially zero observational input to guide us, it was an insanely treacherous prediction -- the theorists were really sticking their necks out."

But as the data trickled in, one night after the next, the images began to assemble into a surprisingly familiar picture.

On the first couple nights of observations, the color of the merger event was relatively blue with a brightness that matched the predictions of kilonova models strikingly well if the outer layers of the merger debris are made of light precious elements such as silver. However, over the ensuing days the emission became increasingly red, a signature that the inner layers of the debris cloud also contain the heaviest elements, such as platinum, gold and uranium.

"Perhaps the biggest surprise was how well-behaved the visual signal acted compared to our theoretical expectations," Metzger noted. "No one had ever seen a neutron star merger up close before. Putting together the complete picture of such an event involves a wide range of physics -- general relativity, hydrodynamics, nuclear physics, atomic physics. To combine all that and come up with a prediction that matches the reality of nature is a real triumph for theoretical astrophysics."

Kasen, who was also a member of observational teams that discovered and conducted follow-up observations of the source, recalled the excitement of the moment: "I was staying up past 3 a.m. night after night, comparing our models to the latest data, and thinking, 'I can't believe this is happening I'm looking at something never before seen on Earth, and I think I actually understand what I am seeing.'"

Kasen and his colleagues have presented updated kilonova models and theoretical interpretations of the observations in a paper released Oct. 16 in advance of publication in Nature. Their models are also being used to analyze a wide-ranging set of data presented in seven additional papers appearing in Nature, Science and the Astrophysical Journal Letters.

Not only did the observations confirm the theoretical predictions, but the modeling allowed Kasen and his colleagues to calculate the amount and chemical makeup of the material produced. The scientists inferred that around 6 percent of a solar mass of heavy elements were made. The yield of gold alone was around 200 Earth masses, and that of platinum nearly 500 Earth masses.

Initially, astrophysicists thought ordinary supernovae might account for the heavy elements, but there have always been problems with that theory, said co-author Enrico Ramirez-Ruiz, a professor of astronomy and astrophysics at UC Santa Cruz. According to Ramirez-Ruiz, the new observations support the theory that neutron star mergers can account for all the gold in the universe, as well as about half of all the other elements heavier than iron.

"Most of the time in science you are working to gradually advance an established subject," Kasen said. "It is rare to be around for the birth of an entirely new field of astrophysics. I think we are all very lucky to have had the chance to play a role."