Astrophysicists Strike Gold

BATON ROUGE – Since the Big Bang, the early universe had hydrogen, helium, and a scant amount of lithium. Later, some heavier elements, including iron, were forged in stars. But one of the biggest mysteries in astrophysics is: How did the first elements heavier than iron, such as gold, get created and distributed throughout the universe? A new answer has come from an unexpected place – magnetars.

Neutron stars are the collapsed cores of stars that have exploded. They are so dense that one teaspoon of neutron star material, on Earth, would weigh as much as a billion tons. A magnetar is a neutron star with an extremely powerful magnetic field.

On rare occasions, magnetars release an enormous amount of high-energy radiation when they undergo “starquakes,” which, like earthquakes, fracture the neutron star’s crust. Starquakes may also be associated with powerful bursts of radiation called magnetar giant flares, which can even affect Earth’s atmosphere. Only three magnetar giant flares have been observed in the Milky Way and the nearby Large Magellanic Cloud, and seven from other nearby galaxies.

Astrophysicist Eric Burns and his team of researchers at Louisiana State University in Baton Rouge study magnetars extensively through the observation of gamma-rays. These are the most energetic photons, most famous for turning Bruce Banner into the Incredible Hulk.

Burns joined with researchers at Columbia University and other institutions to see if we could use gamma-rays to understand if magnetar giant flares forge the heaviest elements, and unexpectedly found the smoking-gun signature in decades-old data. The study, led by Anirudh Patel, a doctoral student at Columbia University in New York, is published in The Astrophysical Journal Letters.

“It’s answering one of the questions of the century and solving a mystery using archival data that people had just forgotten about, demonstrating something that occurred when the Universe was younger,” said Burns. “Giant flares should occur just after the first stars died, meaning we have identified what could be the origin of the first gold in the universe.”

How could gold be made at a magnetar?

Patel and colleagues, including his advisor Brian Metzger, Professor at Columbia University and senior research scientist at the Flatiron Institute in New York, have been thinking about how radiation from giant flares could correspond to heavy elements forming there. This would happen through a “rapid process” of neutrons forging lighter atomic nuclei into heavier ones.

Protons define the element’s identity on the periodic table: hydrogen has 1 proton, helium has 2, lithium has 3, and so on. Atoms also have neutrons which do not affect identity, but do add mass. Sometimes when an atom captures an extra neutron the atom becomes unstable and a nuclear decay process happens that converts a neutron into a proton, moving the atom forward on the periodic table. This is how, for example, a gold atom could take on an extra neutron and then transform into mercury.

In the unique environment of a disrupted neutron star, in which the density of neutrons is extremely high, something even stranger happens: single atoms can rapidly capture so many neutrons that they undergo multiple decays, leading to the creation of a much heavier element like uranium.

When astronomers observed the collision of two neutron stars in 2017 using NASA telescopes and the gravitational wave observatory LIGO, they confirmed that this event could have created gold, platinum, and other heavy elements. “LIGO tells us there was a merger of compact objects, and Fermi tells us there was a short gamma-ray burst. Together, we know that what we observed was the merging of two neutron stars, dramatically confirming the relationship,” said Burns. But neutron star mergers happen too late in the universe’s history to explain the earliest gold and other heavy elements.

Finding secrets in old data

At first, Metzger and colleagues thought that the easiest signature to study from the creation and distribution of heavy elements at a magnetar would appear in the visible and ultraviolet light, and published their predictions. But Burns in Louisiana wondered if there could be a gamma ray signal bright enough to be detected, too. He asked Metzger and Patel to work out what that signal could look like.

Burns looked up the gamma ray data from the last giant flare that was observed, which was in December 2004. He realized that while scientists had explained the beginning of the outburst, they had also identified a smaller signal from the magnetar, in data from ESA (European Space Agency)’s INTEGRAL, a retired mission with NASA contributions. “It was noted at the time, but nobody had any conception of what it could be,” Burns said.

Metzger remembers that Burns thought he and Patel were “pulling his leg” because the prediction from their team’s model so closely matched the mystery signal in the 2004 data. In other words, the gamma ray signal detected over 20 years ago corresponded to what thought it should look like when heavy elements are created and then distributed in a magnetar giant flare.

"This is my favorite discovery I've contributed to,” said Burns. “My colleagues found this signal in the past, but nobody knew what it could be at the time. Once these models were ready, everything fit like a perfect puzzle, which is extremely rare in science."

Researchers supported their conclusion using data from two NASA heliophysics missions: the retired RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) and the ongoing NASA Wind satellite, which had also observed the magnetar giant flare. Other collaborators on the new study included Jared Goldberg at the Flatiron Institute.

Next steps in the magnetar gold rush

Patel’s study estimates that magnetar giant flares could contribute about 10% of the total abundance of elements heavier than iron in the galaxy. Since magnetars existed relatively early in the history of the universe, the first gold could have been created this way.

LSU PhD candidate Aaron Trigg, a NASA FINESST (Future Investigators in NASA Earth and Space Science and Technology) fellow, who works with Burns, is responsible for finding more magnetar giant flares to study. “These are gargantuan outbursts of energy from the strongest magnets in the Universe, which are powerful enough to affect Earth’s atmosphere,” said Burns. Trigg’s work will help us better understand these sources.”

NASA’s forthcoming COSI (Compton Spectrometer and Imager) mission can follow up on these results. COSI, a wide-field gamma ray telescope, is expected to launch in 2027 and will study energetic phenomena in the cosmos, such as magnetar giant flares. COSI will be able to identify individual elements created in these events, providing a new advancement in understanding the origin of the elements.

LSU is one of the lead science institutes for COSI. Burns and LSU Assistant Professor Michela Negro have key responsibilities in the mission, and Trigg is working through how best to study giant flares with COSI. These LSU astrophysicists will be growing their research group as they approach launch in 2027.

“I have so many questions about the cosmos and our place in it,” said Trigg. “This research allows me to explore those questions and share the answers with the world.”