After the Big Bang, the early universe contained hydrogen, helium, and a trace amount of lithium. Later, some heavier elements, including iron, were forged in stars. But one of the biggest mysteries in astrophysics is how the first elements heavier than iron, such as gold, were created and distributed throughout the universe.
“This is a pretty fundamental question in terms of the origin of complex matter in the universe,” said Anirudh Patel, a doctoral student at Columbia University in New York. “It’s a fun puzzle that hasn’t really been solved.”
Patel led a study using 20-year-old archived data from NASA and ESA telescopes that finds evidence for a surprising source of large amounts of these heavy elements: flares from highly magnetized neutron stars called magnetars. The study is published in The Astrophysical Journal Letters.
The authors of the study calculated that outbursts of giant magnetars could account for up to 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 in this way.
“This is an answer to one of the questions of the century and a solution to a mystery using archival data that had been nearly forgotten,” said Eric Burns, a co-author of the study and an astrophysicist at Louisiana State University in Baton Rouge, according to a NASA press release.
A rupture in the crust of a highly magnetized neutron star, shown here in an artist’s rendering, could trigger high-energy eruptions. NASA’s Goddard Space Flight Center/S. Wissinger
Neutron stars are the collapsed cores of exploded stars. They are so dense that a single teaspoon of neutron star material on Earth would weigh a billion tons. A magnetar is a neutron star with an extremely powerful magnetic field.
In rare cases, magnetars emit huge amounts of high-energy radiation when they undergo “starquakes,” which, like earthquakes, rupture the crust of a neutron star. Starquakes can also be associated with powerful bursts of radiation called giant magnetar flares, which can even affect the Earth’s atmosphere. Only three giant magnetar flares have been observed in the Milky Way and the nearby Large Magellanic Cloud, and seven beyond.
Patel and his colleagues, including his advisor Brian Metzger, a professor at Columbia University and a senior fellow at the Flatiron Institute in New York, wondered how radiation from giant flares might match the heavy elements that form there. This could happen through a “fast process” of neutrons fusing lighter atomic nuclei into heavier ones.
Protons determine the element’s identity on the periodic table: hydrogen has one proton, helium has two, lithium has three, and so on. Atoms also have neutrons, which don’t affect identity but add mass. Sometimes, when an atom captures an extra neutron, the atom becomes unstable and undergoes a process called nuclear decay, which converts the neutron into a proton, moving the atom forward on the periodic table. This is how, for example, a gold atom can pick up an extra neutron and then become mercury.
In the unique environment of a collapsed neutron star, where the density of neutrons is extremely high, something even stranger happens: individual atoms can quickly capture so many neutrons that they undergo multiple decays, resulting in the formation of a much heavier element like uranium.
This artist’s concept depicts a magnetar — a type of neutron star with a strong magnetic field — losing material into space. Shown as thin green lines, the magnetic field lines affect the movement of charged material around the magnetar. NASA/JPL-Caltech
When astronomers observed the collision of two neutron stars in 2017 using NASA telescopes and the Laser Interferometer Gravitational-Wave Observatory (LIGO), as well as numerous telescopes on Earth and in space that monitored the initial discovery, they confirmed that the event could have created gold, platinum, and other heavy elements. But neutron star mergers occur too late in the history of the universe to explain the earliest gold and other heavy elements. Recent research by the new study’s co-authors — Jakub Cechula of Charles University in Prague, Todd Thompson of Ohio State University, and Metzger — has shown that magnetar outbursts can heat and eject material from a neutron star’s crust at high speeds, making them a potential source.
At first, Metzger and his colleagues thought that the signature from the creation and distribution of heavy elements in the magnetar would show up in visible and ultraviolet light, and they published their predictions. But Burns in Louisiana wondered whether there might be a gamma-ray signal bright enough to detect. He asked Metzger and Patel to check, and they found that there might be such a signature.
“At some point we said, ‘Okay, we should ask the observers if they saw anything,'” Metzger said.
Burns looked at gamma-ray data from the last giant flare, which occurred in December 2004. He realized that while scientists had explained the flare’s onset, they had also identified a smaller signal from a magnetar in data from the European Space Agency’s (ESA) International Gamma-Ray Astrophysics Laboratory (INTEGRAL), a now-defunct NASA mission. “It was noted at the time, but no one had any idea what it could be,” Burns said.
Metzger recalls that Burns thought he and Patel were “pulling a prank on him” because their team’s model’s prediction matched the mysterious signal in the 2004 data so closely. In other words, the gamma-ray signal detected more than 20 years ago matched what it should look like when heavy elements are created and then distributed in a giant magnetar outburst.
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Patel was so excited that “for the next week or two, I didn’t think about anything else. That was the only thing I thought about,” he said.
The researchers backed up their conclusion with data from two NASA heliophysics missions: the retired Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the current NASA Wind satellite, which also observed the giant magnetar flare. Other co-authors of the new study included Jared Goldberg of the Flatiron Institute.
NASA’s upcoming COSI (Compton Spectrometer and Imager) mission could continue these studies. The COSI wide-field gamma-ray telescope is expected to launch in 2027 and will study energetic events in space, such as outbursts from giant magnetars. COSI will be able to identify individual elements created during these events, providing a new step in understanding the origins of elements. It is one of many telescopes that could work together to search for “transient” changes in the universe.
The researchers will also study other archival data to see if there are any secrets hidden in observations of other giant magnetar outbursts.
“It’s really cool to think about how some of the things on my phone or laptop were created in this extreme explosion in the history of our galaxy,” Patel said.
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