Gravitational Waves From Colliding Neutron Stars Detected for the First Time

Neutron stars colliding 130 million years ago produced gravitational waves and visible light that were recently detected by observatories around the world.

Scientists with the Laser Interferometer Gravitational-Wave Observatory (LIGO) Collaboration announced another detection of gravitational waves today. But this time is different.

The previous four, faint gravitational wave signals came from the merger of two distant and massive black holes. But the latest signals originated from a collision of two neutron stars — the dense cores of dead stars — in an event known as a kilonova.

Not only did this catastrophic event create gravitational waves, but it also produced electromagnetic radiation, including visible light. This is the first time light associated with gravitational waves has been detected, and the light came in several different wavelengths. The detection involved observatories around the world and in space.

“Imagine that gravitational waves are like thunder,” said Philip Cowperthwaite of the Harvard Smithsonian Center for Astrophysics, in a statement. “We’ve heard this thunder before, but this is the first time we’ve also been able to see the lightning that goes with it. The difference is that in this cosmic thunderstorm, we hear the thunder first and then get the light show afterwards.”

LIGO spokesman David Reitz, speaking at a press briefing announcing the findings, called the discovery “amazing.”

“This is the first time the cosmos has provided to us the equivalent of movies with sound,” he said. “The video is the observational astronomy across various wavelengths and the sound is gravitational waves.”

Astronomers call this event a “multi-messenger,” since it was detectable in both light and gravitational waves. They herald it as a new era in physics and astronomy, an expanded way to learn about the universe. The new detections will help shed light on many aspects of astrophysics, including the origins of elusive gamma-ray bursts. They also help resolve the debate about how the heaviest elements in the universe, such as platinum and gold, were created.

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The first detection of gravitational waves, which was announced in early 2016, confirmed Einstein’s general theory of relativity. He had predicted the waves as part of his theory proposing space-time as a concept.

Gravitational waves are ripples in space and time created when two massive, compact objects merge. Their detection has allowed scientists to look at the universe in new ways. Until recently, astronomers could only study objects in space by observing the different wavelengths of light. Being able to observe gravitational waves in conjunction with light provides a new window on the universe, especially for objects that are notoriously difficult to observe, such as black holes and neutron stars.

Neutron stars form when giant stars 10 to 30 times as big as the sun collapse into objects about the size of large cities. Eleven billion years ago, in a galaxy called NGC 4993, two massive stars went supernova, becoming neutron stars. The duo began orbiting each other in a cosmic dance. Roughly 130 millions years ago, they collided.

The aftershock of that event traveled 40 million parsecs (130 million light years) and reached Earth on August 17, 2017 at 8:41 am EDT. LIGO detected a new gravitational wave source, now called GW170817 to mark its discovery date. Just two seconds later, NASA’s Fermi satellite detected a weak pulse of gamma rays from the same location of the sky.

The detection set off observations by telescopes around the world to try and follow up on the event. The Dark Energy Camera on the Blanco telescope searched the region of sky from which the gravitational waves emanated. In less than an hour, they located a new source of visible light in the galaxy NGC 4993.

Scientists then made a series of observations that spanned the electromagnetic spectrum from X-rays to radio waves to study the aftermath of the neutron star merger. Eight papers were published today in various journals documenting the observations.

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Marcelle Soares-Santos, from Brandeis University and first author on the paper describing the optical signal of the gravitational waves, told Seeker that the optical signal offers far more precision and detail than gravitational wave signals alone.

“The optical signal lets us do the equivalent of actually going there and looking at the neutron star merger,” she said.

Soares-Santos expressed surprise at how soon scientists were able to combine electromagnetic radiation observations with gravitational wave signals, less than two years after the first detection of the ripples of space-time?

“We were eagerly anticipating this, but to be honest we were prepared for it to take much longer,” she said in an email. “We were expecting to get events that would be detected at low significance first, and then gradually increasing. To have this outstanding multitude of high significance data on the very first neutron star event we saw was really surprising.”

Later, the Very Large Array in New Mexico teamed up with the Chandra X-ray Observatory to confirm that the merger of the two neutron stars triggered a short gamma-ray burst, a brief burst of gamma rays in a jet of high-energy particles.

“This object looks far more like the theories than we had any right to expect,” said Harvard’s Kate Alexander, who led the VLA observations. She added in a video interview that the VLA will continue to track the radio emission for years to come as the material ejected from the collision slams into the surrounding medium and create other observable events.

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Other observatories that studied the kilonova included the SOAR and Magellan telescopes, the Hubble Space Telescope, and the Gemini-South telescope.

Soares-Santos explained in a statement that in the short term, these observations will yield new insights into neutron stars. But over a longer period, gravitational waves may explain the universe’s continued expansion and the composition of dark energy, an elusive, mysterious substance that makes up roughly 70 percent of the universe.

“This is a whole new window into the universe,” she said. “This is beyond my wildest dreams.”

Soares-Santos told Seeker that while she expects more gravitational wave observations to occur, these types of multi-messenger events might be hard to come by.

“We will definitely see many more events in this coming decade, perhaps up to about one per month or more,” she said. “What will be really hard to replicate is this complete set of data from so many telescopes for each of them. We will have different telescopes targeting selected subsets, but all of them at once will be hard.”

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