Stellar collapse and explosions distribute gold throughout the universe

For decades, astronomers only had theories about where some of the heaviest elements in nature, like gold, uranium and platinum, come from. But by taking a fresh look at old archival data, researchers now estimate that up to 10% of these heavy elements in the Milky Way are derived from the ejections of highly magnetized neutron stars, called magnetars.

Until recently, astronomers had unwittingly overlooked the role that magnetars, essentially dead remnants of supernovae, might play in early galaxy formation, said Todd Thompson, co-author of the study and a professor of astronomy at The Ohio State University.

"Neutron stars are very exotic, very dense objects that are famous for having really big, very strong magnetic fields," said Thompson. "They're close to being black holes, but are not."

While the origins of heavy elements had long been a quiet mystery, scientists knew that they could only form in special conditions through a method called the r-process (or rapid-neutron capture process), a set of unique and complex nuclear reactions, said Thompson.

Scientists saw this process in action when they detected the collision of two super-dense neutron stars in 2017. This event, captured using NASA telescopes, the Laser Interferometer Gravitational wave Observatory (LIGO) and other instruments, provided the first direct evidence that heavy metals were being created by celestial forces.

But further evidence showed that other mechanisms might be needed to account for all these elements, as neutron star collisions might not produce heavy elements fast enough in the early universe. According to this new study, building on these clues helped Thompson and his collaborators recognize that powerful magnetar flares could indeed serve as a potential ejectors of heavy elements, a finding confirmed by 20-year-old observations of SGR 1806-20, a magnetar flare so bright that some measurements of the event could only be made by studying its reflection off the moon.

By analyzing this magnetar flare event, researchers determined that the radioactive decay of the newly created elements matched up with their theoretical predictions about the timing and types of energies released by a magnetar flare after it ejected heavy r-process elements. The researchers also theorized that magnetar flares produce heavy cosmic rays, extremely high-velocity particles whose physical origin remains unknown.

"I love new ideas about how systems work, how new discoveries work, how the universe works," Thompson said. "That's why results like this are really exciting."

The study was recently published in The Astrophysical Journal Letters.

Magnetars may provide unique insights into galactic chemical evolution, including the formation of exoplanetary systems and their habitability.

Not only do magnetars produce valuable metals like gold and silver that end up on Earth, the supernova explosions that cause them also produce elements like oxygen, carbon and iron that are vital for many other, more complex celestial processes.

"All of that material they eject gets mixed into the next generation of planets and stars," said Thompson. "Billions of years later, those atoms are incorporated into what could potentially amount to life."

Altogether, these findings have deep implications for astrophysics, particularly for scientists studying the origin of both heavy elements and fast radio bursts -- brief shivers of electromagnetic radio waves from faraway galaxies. Understanding how matter ejects from magnetars could help scientists learn more about them.

Due to their rarity and short duration, magnetar flares can be difficult to observe,

and current space-based telescopes like the James Webb Space Telescope and Hubble don't have the dedicated abilities needed to detect and study their emission signals. Even more specialized observatories like NASA's Fermi Gamma-ray Space Telescope can only see the brightest part of gamma-ray flashes from nearby galaxies.

Instead, one proposed NASA mission, the Compton Spectrometer and Imager (COSI), could bolster the team's work by surveying the Milky Way for energetic events like giant magnetar flares. Though another event like SGR 1806-20 might not occur this century, if a magnetar flare did detonate in our backyard, COSI could be used to better identify the individual elements created from its eruption and allow this team of researchers to confirm their theory about where heavy elements in the universe come from.

"We're generating a bunch of new ideas about this field, and ongoing observations will lead to even more great connections," said Thompson.

The study was supported by the National Science Foundation, NASA, the Charles University Grant Agency and the Simons Foundation. Co-authors include Anirudh Patel and Brian D. Metzger from Columbia University, Jakub Cehula from Charles University in Prague, Eric Burns from Louisiana State University and Jared A. Goldberg from the Flatiron Institute.