Even the best scientists in the world are stuck here on Earth—which means figuring out that, say, two neutron stars collided 130 million light-years away and created metals like gold is a little tricky. But that’s precisely the announcement scientists affiliated with the Laser Interferometer Gravitational-Wave Observatory, or LIGO, made earlier today.
Throughout the discovery, scientists were aided by some incredibly lucky timing, starting with the fact that this signal fell in a tiny window of time when scientists could gather data from all three gravitational wave detectors built to date. The two LIGO detectors shut down just eight days after the signal for a year of upgrades, and their European counterpart, Virgo, only came online at the beginning of August.
“We all thought, ‘Oh, it’s just for fun, we’re not going to detect anything more,'” Wen-fai Fong, an astrophysicist at Northwestern University involved with the detection, told Newsweek.
Instead of nothing, they got the pieces they needed to not only identify the first ever detected collision of neutron stars but also to answer all sorts of questions about what happened—and to ask a whole lot more.
The gravitational waves
The gravitational waves signal arrived early in the morning of August 17, when LIGO’s detector based in Washington heard a new kind of “chirp.” But scientists had to confirm the signal with the detector’s Louisiana branch since the instruments can create false alarms every few hours, Peter Saulson, a physicist at Syracuse University who works on the LIGO detectors, told Newsweek. In fact, one cut off the very last second of the detection’s signal at the Livingston facility. Scientists had to analyze the Louisiana detector’s data by hand to confirm that the two machines had been recording in perfect harmony until the glitch.
From just that signal, scientists already knew that this wasn’t another black-hole collision. Those “chirps,” sweeping from low to high pitches like a siren, happen much faster—in less than a second—whereas this one lingered in low pitches for almost a minute that the detector could record.
To scientists, that suggested that the collision involved much less celestial stuff than a black-hole merger—that it was instead two neutron stars, each about one and a half times the mass of our sun. It also told scientists how fast the stars were moving when they collided: at one third the speed of light.
But LIGO’s new counterpart, the Virgo detector in Italy, hadn’t registered a signal to match LIGO’s. With other mechanisms in place to confirm the LIGO detections were accurate, Virgo’s silence turned out to be helpful, Saulson says. “In fact, it was a fabulous and instant clue” that told the team the collision had happened in Virgo’s blind spot. That’s actually the whole point of having three of these detectors spread across the globe, Saulson added. “It’s really good for finding where on the sky the signal came from, where the event happened.”
Then the race was on to get astronomers a map of where the signal came from so they could develop a plan to try to spot the event. Saulson’s colleague Duncan Brown helped design the software that pinpoints a signal, which had just been rewritten. “We were under a lot of pressure to get the sky map out,” he says, leading to a strange role reversal: “My former graduate student was yelling at me that I wasn’t doing something quickly enough.”
The short gamma ray burst
Just two seconds after LIGO’s detection, scientists had already snagged another piece of the puzzle: a satellite flagged a giant short gamma ray burst of high-energy light, “a firehouse of material,” as Fong describes it. Astrophysicists have typically focused on long gamma ray bursts, she adds, calling the short variety “kind of like the hipster class of gamma ray bursts.”
Previously, the closest short burst we had ever spotted to Earth took place 1.8 billion light-years away; the burst on August 17 was traced to just 130 million light-years away, about 15 times closer.
And scientists had guessed short gamma rays bursts might come from neutron star mergers, so the pairing of LIGO and Fermi’s data already convinced some scientists of what they were seeing. “Within two seconds, a gigantic scientific mystery was solved,” Saulson says.
The light signature
There was just one type of data left to track down: a photograph of the event. Unlike the black-hole mergers LIGO had previously spotted, which are invisible, neutron star mergers should show up in telescopes. So the LIGO team sent out alerts and their map of where to look.
Ben Shappee, an astronomer with the Carnegie Institution for Science, happened to be in Chile on August 17. He had arranged for observing time months ago to check in on a supernova he had discovered last year, the brightest ever spotted. “I was planning on spending most of the night just observing that one object,” he says, but instead he woke up to the LIGO alert. His supernova would have to wait.
Shappee and his colleagues put together a list of galaxies in the right neighborhood that they considered likely suspects for this sort of event. As soon as the sun started to set, they started checking up on each one, looking in a range of different types of light since they had three telescopes at their disposal.
That turned out to be particularly lucky. Astronomers had predicted a neutron star collision would glow red; instead, Shappee and his colleagues spotted a huge flash of blue light in a galaxy called NGC 4993.
Previously, NGC 4993’s claim to fame was having first been spotted by William Herschel, the 18th-century British astronomer who also discovered Uranus. “It’s a pretty nondescript-looking galaxy,” Saulson says, “but because it’s relatively close, it’s been observed many times.” That includes a photo taken just three weeks before the collision, which shows “a boring galaxy.”
The bright flash of blue light in the August 17 photo became the first-ever sighting of a kilonova, the explosion of matter and light that results from the smashing together of two neutron stars. Thanks to a trick of chemistry, different elements create flashes of light with different fingerprints, so astronomers could analyze the kilonova’s contents. They were able to identify the signature of what’s called “r process” elements, which include heavy metals like gold and platinum.
At the same time, telescopes on Earth were turning toward NGC 4993, so was one in space, the Chandra X-ray Observatory. Space telescopes are much less nimble than their earthbound counterparts, so it took about two and half days to reorient it. And when it did turn toward NGC 4993, it saw—nothing.
“It was extremely exciting that we saw nothing!” says Raffaella Margutti, an astrophysicist at Northwestern University. “We decided to keep staring at that location in the sky.” About two weeks later, they were rewarded with a signal. The delay meant that they could calculate about how closely the burst of X-rays produced by the collision came to Earth, and establish that the jet was pointed between 20 and 40 degrees off Earth.
And that signal reached us just in time, since, in yet another coincidence, the sun soon blocked our view of the explosion. Astronomers are blind until December—but if their streak of good luck holds, there will still be something to see.