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Students in right place, right time witness first-ever detected neutron star collision


Credit: University of Notre Dame

In August, Kaitlin Rasmussen and Devin Whitten, third-year physics graduate students at the University of Notre Dame, were settling into their observation at the Las Campanas Observatory atop a rocky mountain in Las Campanas, Chile, when they saw something unexpected.

Before they arrived, a brief flash of gamma rays was detected 143 million light-years away in a galaxy located in the constellation of Hydra. Accompanying the bright burst were gravitational waves detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO).

All data pointed to a never-before-seen event: the merger of two neutron stars. The light emitted from the collision peaked while Rasmussen and Whitten were on-site.

New research published in Science details perhaps one of the biggest discoveries so far in the field of astrophysics. Rasmussen and Whitten, along with Timothy Beers, chair of astrophysics at Notre Dame, and Vinicius Placco, research assistant professor, contributed to studies published on the collision.

"It's hard to describe the feeling of seeing something with your own eyes that is completely new to science," Rasmussen said. "It's incredible to know that you are one of the few people on the planet, in history, to ever witness a new type of astronomical event. And being included on the paper was a tremendous honor."

Neutron star mergers have proven to be responsible, either in whole or in part, for the formation of nearly half the metals heavier than iron in our solar system including gold, platinum and uranium.

The students had been scouring for a type of star enhanced by a set of reactions called the rapid-neutron capture process, or r-process, on the 2.5-meter Irénée du Pont Telescope owned by the Carnegie Institution for Science.

The concept of the r-process was first suggested in 1957. Astrophysicists theorized that the universe's heaviest elements are formed after a set of reactions, and later suggested this could possibly occur when two neutron stars collide.

Neutron stars — the densest stars astronomers can visually observe — result when a supernova collapses and its electrons and protons melt into a neutron core only a few miles wide, yet weighing more than two suns. When two such stars collide, neutron-heavy metals spray outward throughout the universe. The metals become incorporated into the gas clouds of newly minted stars, which is what happened when our sun formed billions of years ago. The same cloud eventually forms planets, like Earth, where the metals are also found.

Light from the collision peaked and then cooled quickly, but continued to radiate for about three weeks. "The best analogy to this collision is fireworks," Beers said. "Initially nothing happens, but then it reaches a critical temperature at which the element that gives off the particular color ignites. So it very rapidly brightens, then falls off."

Placco said that for Whitten and Rasmussen, it was a matter of being in the right place at the right time.

"They're assigned time on the telescope on a semester-by-semester basis, and on a given semester, they could have been there any two nights," he said. "But they were there exactly those two nights."

Now that scientists know neutrons can create the heavy elements through the r-process, they'll redouble efforts to determine whether these neutron star collisions are the only source of those elements, or if another astrophysical event, like a particularly energetic type of supernova explosion, also has a hand in the process, according to Beers.

And there's always that next step in science, when a new discovery unearths even more questions to be explored and answered. Rasmussen and Whitten will move on to answer other astronomical questions and search for even more stars enhanced with r-process elements. But they'll always have the experience of watching a major astronomical event many astrophysicists waited decades to witness.

"Gravitational wave observations represent a new era of astronomy," Whitten said. "We're detecting gravitational events beyond our Milky Way; that's simply amazing. The potential for this new mode of observation to inform us about the galaxy we live in, as well as our local universe, promises much, much more to come."

The research was funded by a grant through the Luksburg Foundation, which encourages Notre Dame partnerships with the Pontifica Universidad Católica de Chile.


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