A rare cosmic collision observed by astronomers around the world — including a team at the University of Hawaii at Manoa — has generated new insight into the formation of dense elements.
The event was the Aug. 17 merger of two neutron stars, the remnants of stars that have used up their hydrogen and other chemical fuels. These are the stellar equivalent of peach pits.
That collision resulted in a burst of gravity waves that was detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO), which has detectors in Hanford, Wash., and Livingston, La., and NASA’s Fermi Gamma-Ray Space Telescope. The signal also was picked up by the Virgo interferometer in Pisa, Italy.
The astronomers gave this gravity-wave phenomenon the name GW170817.
Then came — 1.7 seconds later — a burst of high-energy gamma rays from the same vicinity.
Were the two related? And what was the source?
The answer required an unprecedented collaboration between scientists around the globe, and telescopes in space, atop Haleakala and on three continents. Their conclusion — that this was the first-ever merger of orbiting neutron stars detected by gravity waves — is detailed in articles published today in Science, Nature and the Astrophysical Journal.
At the UH Institute for Astronomy, the research involved a small group called the One-Meter-Two-Hemisphere (1M2H) discovery team. A collaboration with the Carnegie Observatories and UC Santa Cruz, the team used the Carnegie Institute’s Swope and Magellan telescopes to look at galaxies that might have been the source of a neutron star merger.
Less than 11 hours after the LIGO alert, the 1M2H team found a new bright object, Swope Supernova
Survey 17a (SSS17a) in NGC4993, an elliptical galaxy 130 million light-years away in the constellation Hydra.
“It is amazing that with the multitude of teams searching for this source, our small team of mostly young astronomers was the first to image, discover and report it,” said UH astronomer Benjamin Shappee in a statement.
On Aug. 18, according to Nature.com, astronomer J. Craig Wheeler of the University of Texas at Austin tweeted, “New LIGO. Source with optical counterpart. Blow your sox off!”
Based on the coordinates provided by the UH team, astronomers and observatories worldwide quickly turned their attention to SSS17a.
One of the key observatories involved was Pan-STARRS, the UH sky survey facility atop Haleakala, which picked up the baton once dark fell here. Pan-STARRS watched SSS17a for six hours after the 1M2H discovery.
Pan-STARRS, which has surveyed most of the sky, was able to use its previous images along with the new data to determine that SSS17a had significantly faded in the few hours since its discovery.
That was a clear signature of what astronomers call a “kilonova.”
When the death-spiraling stars finally merged, they sent out a small amount of neutron-rich material very quickly — at about 47,000 miles per second, or a quarter of the speed of light.
The neutron-rich material in turn produced highly radioactive atomic nuclei, which decayed quickly in a ghostly iridescence, or kilonova.
“A new astronomical object fading this fast is unheard of, and the Pan-STARRS Team alerted the worldwide community to the unique nature of SSS17a,” said Ken Chambers, director of the Pan-STARRS Observatory.
The scientists concluded that colliding neutron stars are a source of some of the elements heavier than iron, such as silver. This upends decades of theory that those elements came only from supernova, the cataclysmic explosions of massive stars.
That conclusion followed a massive effort unprecedented in the history of astronomy, scientists say. The collaboration involved the Fermi Space Telescope for gamma rays, the Chandra Observatory for X-rays, the Swift Observatories for X-rays and ultraviolet light, the Hubble Space Telescope and radio wave arrays.
An important clue came from chemical signatures analyzed by the UH team and their collaborators on a variety of telescopes.
The neutron-rich material ejected from the collision turns out to be the perfect place for nuclear reactions that build larger nuclei from smaller ones.
“We see fingerprints of key elements that are heavier than iron,” said Chambers.
The result, the scientist said, is a “fundamental change” in our understanding of the genesis of some of the heavier elements, many of which are common on Earth.
“Now there is a new blossoming field of study in astronomy,” said Shappee. “This will be the focus of hundreds of astronomers, from around the world, for months and years to come.”
He added, “It is exciting to play a part in a story that will make it into the textbooks.”
