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A cosmic first! Gravitational waves and telescopes reveal the clash and flash of neutron stars

Alan Boyle
Neutron star merger
An artist’s conception shows two neutron stars merging, and sending out radiation as well as gravitational waves in the process. (NSF / LIGO / Sonoma State University Illustration / A. Simonnet)

For the first time ever, researchers have recorded the cataclysmic smash-up of two neutron stars by virtue of their gravitational waves as well as their electromagnetic emissions, producing data that could unlock cosmic secrets galore.

The findings from the Aug. 17 event, detailed today in more than a dozen research papers, represent the best example of “multi-messenger astronomy.”

More than 70 observatories and thousands of scientists contributed to the findings, headed by the Laser Interferometer Gravitational-wave Observatory, or LIGO.

“We did it again — but this time, we all did it,” David Reitze, executive director of the LIGO Laboratory, said at today’s news briefing announcing the results.

By combining the gravitational-wave readings with observations in wavelengths ranging from radio signals to gamma rays, scientists are gaining new insights into how neutron stars evolve, and how gold and other heavy elements are forged in their furnaces.

They’re also coming up with new questions.

“Theorists will have to go away and come up with some new models to explain what we saw,” Penn State’s Jamie Kennea, head of science operations for NASA’s Swift Gamma-Ray Burst Mission, told GeekWire.

The key to the observational campaign was a roughly 100-second-long “chirp” picked up at 5:41 a.m. PT on Aug. 17 by LIGO’s detector in Hanford, Wash. The signal registered a gravitational disturbance in the fabric of spacetime, originating somewhere in the distant universe.

The chirp initially went unnoticed by researchers monitoring LIGO’s other detector in Livingston, La. But Hanford’s signal sounded the alarm. The Livingston team, and the team in charge of Europe’s Virgo gravitational-wave observatory in Italy, quickly added in their results to narrow down the source of the chirp, known as GW170817, to a patch in the southern constellation Hydra.

Meanwhile, NASA’s Fermi space telescope picked up on a brilliant flash of light in the form of gamma rays from the same direction, two seconds after the gravitational waves faded. The European Space Agency’s Integral satellite detected the gamma rays as well. Those detections narrowed down the search area even further.

Scientific Minutemen

Astronomers put out the call via an email-and-text communication tool called the Gamma-ray Burst Coordinates Network for telescopes to follow up on the observations. Scores of scientific teams responded, like a Minuteman militia.

“We said, ‘OK, all the things we planned to observe are out the window,'” Aaron Tohuvavohu, science operations and research assistant for the Swift mission team at Penn State, told GeekWire.

The LIGO team had previously detected four confirmed ripples of gravitational waves from the collision of binary black holes — but GW170817 was clearly different. The fact that the gravitational clash also produced a gamma-ray flash argued against black holes, because computer models suggest that no light escapes from a black hole merger.

“It immediately appeared to us the source was likely to be neutron stars, the other coveted source we were hoping to see — and promising the world we would see,” MIT’s David Shoemaker, spokesperson for the LIGO Scientific Collaboration, said in a news release.

Neutron stars are leftovers from stellar explosions, consisting of exotic matter that has been crushed so densely that a teaspoon’s worth would weigh millions of tons. They’re typically no more than 12 miles wide, but are more massive than the sun.

The buildup to a neutron star merger had never been observed before. But scientists previously suspected that at least some gamma-ray bursts were caused by neutron star smash-ups, and the LIGO data fit the models. The gravitational-wave pattern pointed to the merger of two stars, 1.1 and 1.6 times as massive as the sun, at a distance of 130 million light-years from Earth.

The all-points bulletin for astronomical observations quickly produced paydirt. Within hours, astronomers using the Carnegie Observatories’ Swope Telescope in Chile reported seeing an optical counterpart to the gamma-ray flash and the gravitational-wave crash in a galaxy called NGC 4993. That narrowed down the target to a precise spot.

For almost two months now, astronomers have been monitoring the spot in wavelengths ranging from radio and infrared to X-rays and gamma rays. The readings consistently point to a neutron star merger as the cause of the event on the morning of Aug. 17.

“The detection of gravitational waves from a binary neutron star is something that we have spent decades preparing for,” Alan Weinstein, head of Caltech’s astrophysical data analysis group for LIGO, said in a news release. “On that morning, all of our dreams came true.”

New window on the cosmos

The gravitational-wave observations have opened a new window for tracing the fate of neutron stars and the causes of gamma-ray bursts.

Before LIGO, astronomers could only infer that the bursts were caused by neutron star smash-ups.

“It’s like you’re seeing a broken egg and you try to put the egg back together,” Penn State’s Tohuvavohu explained. Continuing the metaphor, scientists at LIGO and Virgo are now able to pick up the gravitational-wave signature of two cosmic eggs moments before they break.

Studies of the smash-up support the view that neutron star collisions result in a rapidly evolving, supernova-like phenomenon known as a “kilonova.”

Follow-up studies have confirmed long-held suspicions that such collisions create heavy elements, including some that are essential for life, and spread them into the wider cosmos. The evidence is contained in spectral signatures of chemical elements that have been seen in infrared observations of the blast site.

“For the very first time, we see unequivocal evidence of a cosmic mine that is forging about 10,000 Earth-masses of heavy elements such as gold, platinum and neodymium,” said Caltech’s Mansi Kasliwal, leader of a 18-telescope network known as the Global Relay of Observatories Watching Transients Happen, or GROWTH.

To illustrate the contribution of neutron star collisions to life as we know it, LIGO’s Reitze held out his great-grandfather’s century-old gold watch during today’s briefing.

“The gold in this watch was very likely produced in the collision of two neutron stars approximately billions of years ago,” he said.

Mysteries remain: For example, the Swift mission’s scientists didn’t see the immediate burst of X-rays that they expected. The X-ray and radio signatures of the blast weren’t picked up until more than a week after it was detected in other wavelengths.

Theorists suspect that the delayed emissions, and other twists such as the relative dimness of the gamma-ray burst, came about due to the blast’s orientation and its interactions with material that was thrown off by the stars. Nailing down that connection will be the subject of further study.

Eleonora Troja, a researcher at NASA’s Goddard Space Flight Center, said the smashed-up stars “very likely” ended up as a black hole, but other scientists said that was still up for debate.

One thing is certain: This won’t be the last clash-and-flash combination picked up by the scientists at LIGO, Virgo and their partners on astronomical teams. Over the next year or so, the LIGO detectors will be getting upgrades to make them more sensitive to neutron star collisions.

“We even more eagerly anticipate the detection of gravitational waves from different kinds of known, extremely energetic astrophysical objects, like rapidly spinning pulsars, supernovae and neutron-star quakes,” Weinstein said, “and especially from heretofore-unknown astrophysical objects.”

Research papers about the GW170817 neutron star merger appear in the journals Science and Nature, Physical Review Letters and the Astrophysical Journal Letters. Briefings about the findings are being webcast by the LIGO-Virgo Collaboration.

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