Neutron Star Merger

There’s gold in them thar stars!

August 17, 2017 started like any other summer Thursday for Syracuse University gravitational wave researcher Duncan Brown.

Duncan Brown. Credit: Syracuse University.
Duncan Brown. Credit: Syracuse University.

He had just dropped his kid off at daycare and arrived at work a bit early. As part of the extended Laser Interferometer Gravitational-Wave Observatory (LIGO) family, Brown’s hours ahead would be spent poring over gravitational wave data, trying to figure out better ways to detect the small chirps that indicate some massive stellar disturbance has squeezed space and time itself, producing elusive gravity waves that emanate across the universe at the speed of light.

The discovery of these waves helped prove Einstein was, in fact, a genius–relatively speaking!

Though it wasn’t quite 9AM yet, Brown was already sitting in a meeting with a colleague. Out of the corner of his eye, he saw an email notification pop up on his laptop. It was an alert from the Hanford, Washington branch of LIGO. Right then and there, Brown apologized and cancelled the meeting, jumping on a call with his LIGO colleagues to see if there had been another detection of gravitational waves.

Edo Berger. Credit: Harvard University.
Edo Berger. Credit: Harvard University.

A few states north, Edo Berger was also sitting in a meeting at the start of a typical workday at Harvard University, where he is an astronomy professor. Gravitational waves are not really his thing–he’s a lighter kind of guy. Berger studies the light waves produced by huge eruptions in the cosmos, like stars exploding or colliding.

One of his main focuses is trying to figure out how elements heavier than iron–like gold–are created in the universe. Scientists suspected the massive explosions of dying stars–supernovas–smelted these heavy metals, but there was no data to prove that theory. Berger subscribed to a different idea anyway. He thought the collapsed core of a supernova–called a neutron star–was a more likely place to go panning for cosmic gold.

Besides the centers of a black holes, neutron stars are the most dense things we know of: They typically have a radius of about ten kilometer (6.2 miles) and yet have twice the mass of our Sun. Brown was especially interested in the explosion that accompanies the death spiral of two such neutron stars.

But nobody had seen such an event. Yet.

Back at Syracuse, Brown was on the phone with other LIGO scientists. “Very quickly, we realized that there’s something interesting here,” said Brown.

The signal detected at 8:41AM Eastern that day was cleaner and longer than any of the other four previously-detected gravitational wave events. Brown tried to practice what he preaches to his students: what scientists observe is often not at all what they expect. Yet, here he was, staring at a blip in the fabric of spacetime that had all the telltale signs of a neutron star merger. It was indeed exactly what he had hoped to see.

The signal’s characteristic shape offered a clue as to its origin. “It turns out that the gravitational waveform is very, very sensitive to the masses of the objects in a binary,” said Brown. “The heavier those objects are, the more quickly they can spiral.”

For example, the gravitational waves seen for the first time in 2015 were caused by the merging of two black holes, each about 30 times more massive than our Sun. Because those cosmic objects are so large, they spiraled into each other very quickly, only emitting a chirp of gravitational waves–a ripple in spacetime all 0.2 seconds long. Don’t blink!

“This signal, the computer could see it for almost 100 seconds. I burned out my hearing at too many concerts when I was younger so I only hear the last few seconds,” said Brown, laughing. “My younger graduate students assure me they can hear more than that.”

WATCH: The gravitational wave signal intensifies as the neutron stars circle into one another. Credit: LIGO/Virgo.

This finding meant that the two objects circling each other in the new signal were less massive and orbited each other more lazily, meaning they were lighter neutron stars instead of heavier black holes.

The signal was also louder than the other gravitational wave signals previously detected, so the event occurred closer to Earth than the others. But, where exactly?

Other observatories provided supporting data that would narrow down the location of the event. Virgo, a European laser interferometer based in Italy that is similar to LIGO, also recorded the gravitational wave disturbance. Up in orbit, NASA’s Fermi satellite had detected a short burst of high energy light, gamma rays, 2 seconds after the gravitational wave signal.

“Short hard gamma ray bursts, nobody knows where they come from,” said Brown. “Until now.” One of working theories was that they are produced by neutron star collisions, which added to the feeling that it was extremely unlikely the gamma ray burst detected by Fermi was just a coincidental event.

Brown was thrilled, but there was work to be done to localize where in the sky this event occurred. A glitch in the other LIGO observatory branch, in Livingston, Louisiana, had caused it to miss the signal. Brown and his colleagues would have to go in and reanalyze the data by hand.

They got to work, but first they sent out a message to the larger astrophysics community: LIGO and Virgo detected a gravitational wave event, and the Fermi satellite detected a short hard gamma ray burst. It looked like it might be an elusive binary neutron star merger.

Up at Harvard, Berger ignored call after call, trying to pay attention in his meeting. “After the fourth time, I was like, ‘Okay, I’ve got to answer this, what’s going on?’ I get this barrage of text messages and voicemails saying: ‘Check your email, LIGO found a neutron star binary.”

Berger had been waiting for just this moment. He and his team had signed up for confidential access to the newest LIGO data, back when it was just starting, in the hopes that perhaps one day there would be both a gravitational wave signal and a light wave signal that would indicate a neutron star collision. A few years back, Berger had written a review on the topic, saying he hoped that within a decade, as LIGO and others got upgraded, they’d see something like this. That it happened so quickly was exciting news, and Berger was ready to go.

It took half a day for Brown and the LIGO team to triangulate the signal. When they did, they had a strong indication of where this gravitational wave occurred: a small patch of sky visible from the southern hemisphere. Because the gravitational wave signal was so loud and the team also had the Fermi gamma ray measurements with which to cross-reference, this estimate was the most accurate of any previous gravity wave detection.

A good lead was exactly what Berger needed. Previous gravitational wave signals were too short and weak to triangulate accurately. Not that it would have been much help for Berger–black hole mergers are dark affairs, producing only gravitational waves and no light. Even if scientists like Brown could tell Berger exactly where those black hole mergers occurred, telescopes would only see a dark, seemingly empty patch of sky.

With a collision of two neutron stars, however, light should be able to escape. If Brown could get Berger a good estimate of stellar coordinates for this event, Berger might be able to scan the area with telescopes and find the explosion. For hours, he had been waiting for the star map. Finally, it came and the baton in one of astronomy’s greatest relays was passed for the first time from gravitational wave scientists to light wave scientists–spring loaded to deploy an armada of instruments to take pictures of what they knew was coming.

“We spent 5 minutes staring in awe at the picture, then we knew we had to get into action,” said Berger. “We already had plans to use other observatories in the case of such an eventuality.”

Dark Energy Camera spots a bright new phenomenon: the collision of two neutron stars.
Dark Energy Camera spots a bright new phenomenon: the collision of two neutron stars.

One of the first to pinpoint the exact location was an instrument called the Dark Energy Camera, an instrument on the huge, 12 foot Blanco telescope in the mountains of Chile, which swept across the patch of sky right after sunset. Looking through the images, Brown and colleagues noticed a bright point of light in the galaxy NGC 4993, one that wasn’t there before. Bingo!

For the first time in history, we were seeing the scintillating explosion from the collision of two neutron stars.

“Even when I say it now, I find it kind of astounding,” said Brown. “I thought it would take us days of painstaking hard work to go through the data. Instead, we did it in about 45 minutes from getting the observations from the telescope.”

Berger and his team launched the protocol they had developed for just this kind of event, triggering high-priority requests in observatories all over the world. Berger called the Space Telescope Science Institute, which is in charge of the Hubble Space Telescope. “We’ve got to go,” Berger recalled saying, “this is real.”

The Hubble Space Telescope catches a glimpse of the explosion as well.
The Hubble Space Telescope catches a glimpse of the explosion as well.

Within half an hour, the Hubble was pointed at the new source of light in the sky. Within a few hours, a radio telescope was also listening for radio waves coming from the explosion. And a day later, x-ray observatories were also enlisted.

Soon, all those were getting signals from the vicinity of NGC 4993. Light across the whole electromagnetic spectrum was pouring out of the system, carrying with it vast amounts of information. It was a treasure trove of data, and Berger’s team was especially prepared to collect it.

“We had all these things lined up, so when people were struggling to get this telescope going, get another telescope going, we were gathering all this data,” Berger said. “And the outcome of this is Monday morning, at 10AM, we’re going to have 8 papers coming out.”

Just two months have passed since the first detection of this event, but scientists have already managed to conduct ground-breaking science.

The gravitational wave signal itself was unlike any ever seen before, proving that LIGO and Virgo can pick up more than just the mergers of two black holes. This was the closest gravity wave and the closest short gamma ray burst ever detected, just 130 million light years away. Previously an orphaned astronomical phenomenon, short gamma ray bursts now have an identifiable source.

But the real showstopper was finding that some of the light produced was actually not caused by the explosion itself, but by the subsequent creation of heavy elements.

Supernova explosions can make all elements up to iron, but they don’t have enough neutrons to keep making heavier elements through radioactive reactions. Berger and many others subscribed to the theory that the collision of two neutron stars spews enough energy and neutrons to create everything past iron: gold, silver, uranium–you name it. The near-infrared light they collected in subsequent days confirmed that suspicion.

All the gold in every wedding ring, all the uranium and plutonium in our nuclear weapons, all the special heavy metal in your smartphone: all of it was cooked in the collisions of neutron stars.

Brown called the discovery “quite literally a gold mine.”

“We have now completed our understanding of how our periodic table actually formed,” said Berger. “We only had half the story and now we have the whole story.”

Beyond the specifics of the discovery, this event is a great day for the scientific endeavor in general. Thousands of people and around 70 observatories worked in perfect harmony to answer some fundamental questions about the nature of the world around us.

“We can look out into the universe with these facilities and telescopes that took a lot of money and effort to build, money that the taxpayers put into them, and make really incredible discoveries by collaborating and working as scientists,” Berger said. Who knows how this basic research will affect the average person in a few decades? It’s always a good bet, though.

Two months later, observatories all over the world are still seeing the afterglow of the explosion. There is already so much data, however, that scientists will be busy for years to come.

Brown estimates there are already close to 50 scientific papers written and slated for publication. Berger and his team, quick on the draw, have 8 coming out Monday alone. Hundreds, perhaps thousands of astrophysics graduate students will end up writing their dissertations about some aspect of this neutron star collision.

Eleven billion years ago, two stars in a distant galaxy formed and started their cosmic waltz. For billions of years, the two danced closer and closer together, until one day, 130 million years ago, they crashed into each other. Waves, both gravitational and electromagnetic, raced toward Earth at the speed of light, where scientists around the world managed to catch the oscillations.

Under the flickering light of a stellar trainwreck, a new era of astronomy is born.

Banner image credit: NASA.

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