Priceless Flares: Magnetars Can Make Gold and Platinum

An eruption from a magnetized neutron star produced enough heavy elements — such as gold, platinum, and uranium — to fill an entire planet the size of Mars. This surprising result, published by a team of U.S. and Czech astronomers, promises a major breakthrough: “This is really just the second time we've ever directly seen proof of where these elements form,” says Brian Metzger (Columbia University).

Most elements heavier than hydrogen and helium are forged in the cores of stars. But stars cannot fuse elements to create anything heavier than iron; many of these heavy elements form outside stars instead, in a mechanism dubbed r-process, in which atoms rapidly capture additional neutrons. That mechanism can only occur in extremely energetic events.

One of these events is a neutron star collision. The first evidence of this came in 2017, when LIGO and Virgo picked up gravitational waves from a pair of merging neutron stars. This event released electromagnetic radiation that contained spectral lines produced by heavy nuclei, including gold.

But other promising events — such as the supernovae of massive stars — have failed to deliver: When a team led by Peter Blanchard (then at Northwestern University) searched the remnants of gamma ray burst GRB 221009A (famously named the “brightest of all time” for its extraordinary luminosity) and its subsequent supernova, they only found the likes of carbon and oxygen. It looked like even very energetic GRBs don’t produce much of the lower rows of the periodic table.

That’s a problem, because neutron star mergers are too few and far between to be the only source.

That’s why, in 2024, Jakub Cehula (Charles University, Prague), Todd Thompson (Ohio State University), and Metzger were looking for alternatives. They turned to a class of neutron stars, known as magnetars, that harbor some of the strongest magnetic fields known in the universe. Similar to magnetic eruptions on our sun, but much more powerful, these fields can unleash outbursts of energy. These violent explosions, according to the calculations, can sling neutron star matter into space, briefly creating suitable conditions for heavy elements to form in the r-process mechanism.

For the r-process to occur, medium-size nuclei must be bombarded by neutrons in order to grow quickly into massive, neutron-rich, and unstable isotopes. These isotopes decay into lighter, but still massive nuclei that are stable, including elements such as gold, platinum, or rare-earth metals. In the aftermath of a magnetar flare, the team suggests, this decay happens within a few minutes, during which the decaying nuclei emit electromagnetic radiation — a telltale sign that may be observed.

Anirudh Patel, a doctoral candidate at Columbia University and lead author of a study published in the Astrophysical Journal Letters, recalls, “After we calculated the gamma-ray emission, our colleague and coauthor Eric Burns looked through some old literature published 20 years ago, and found that such a signal had already been observed, but was unexplained at the time.”

On December 27, 2004, the European INTEGRAL gamma-ray observatory as well as other satellites had spotted a giant flare from a magnetar named SGR 1806-20, one of only three such events in the Milky Way within the past 50 years. Although 30,000 light-years away, the energetic blast was strong enough to affect the upper layers of Earth’s atmosphere. For a few minutes after the burst, INTEGRAL had also registered a faint afterglow that astronomers at the time were unable to explain. No one thought of radioactive decay as a cause for this signal until 2024: “Our theoretical model matched the data perfectly,” says Patel.   

Blanchard, who wasn’t involved in the study, calls the evidence “compelling.”

“This would go a long way toward solving at least part of the r-process origin mystery, [provided that] future observations of other events confirm this picture,” he adds

With only one such flare properly documented, it’s too early for that, Patel acknowledges, but the team’s preparing to detect the next one. They hope that NASA's Compton Spectrometer and Imager, currently set to launch in 2027, will detect decay signals at gamma-ray energies from individual isotopes in future magnetar blasts. That data would allow them to identify the actual nuclei created — something that INTEGRAL couldn’t do. So far, the presence of gold, platinum, and other elements can only be deduced based on the overall spectrum emitted.

There may also be other ways to study future events. “We also predicted that heavy element production in giant flares should produce a UV/optical signal,” Patel adds. “This signal would be detectable by future telescopes such as ULTRASAT and UVEX.” Both telescopes are ultraviolet observatories, planned for launch in 2026 by the Israeli Space Agency and 2030 by NASA, respectively. Even if \ future observations confirm the findings, there’s probably more to the story. The case of SGR 1806-20 suggests that magnetar flares can only create up to 10% of all r-process elements,  leaving a gap that’s still a bit large for neutron star mergers alone to fill. “It’s a substantial leap in our understanding of heavy-element production,” Metzger says. At the same time, he acknowledges, “we can't exclude that there could be third or fourth sites out there that we just haven’t seen yet.”