Kilonovae are even more varied and difficult to interpret than we thought
Among the most violent phenomena the universe permits, gamma-ray bursts have long forced humanity to reckon with the limits of its own cosmic comprehension. Now, researchers at Los Alamos National Laboratory have reexamined two such bursts — detected in 2021 and 2023 — and concluded that what appeared to be the fingerprints of colliding neutron stars may instead belong to collapsars: massive, rapidly spinning stars that implode into black holes. The finding does not merely reassign a label; it quietly unsettles our understanding of where the universe's heaviest elements are born, and reminds us that even the brightest signals in the sky can be misread.
- Two of the most scrutinized gamma-ray bursts in recent memory had been reinterpreted as neutron-star mergers — a conclusion that now appears to be wrong.
- The reddish infrared glow that astronomers read as evidence of gold and lead being forged turns out to require no such heavy metals at all, exposing a critical gap in how kilonovae are interpreted.
- Using NASA's Fermi data and simulations run on the Chicoma supercomputer, the Los Alamos team modeled elemental production in collapsars and found it matches observations without invoking a merger.
- The field is being pulled back toward its original framework — long-duration bursts from collapsing stars — but now carrying a harder-won awareness of how misleading the signatures can be.
- The definitive answer awaits gravitational-wave detectors capable of catching the spacetime ripples of these events in real time, turning multi-messenger astronomy into the arbiter of cosmic origin.
In the span of a few seconds, a gamma-ray burst releases more energy than our sun will produce in ten billion years. For decades, Los Alamos National Laboratory has been among the institutions trying to understand what ignites them.
Two bursts — GRB 211211A in 2021 and GRB 230307A in 2023 — became flashpoints in that effort. When astronomers first studied them, the data seemed to implicate neutron-star mergers: the collision of two impossibly dense stellar remnants, an event known to forge gold, lead, and uranium. The reddish infrared glow in particular looked like a classic kilonova signature. But a new analysis published in The Astrophysical Journal Letters argues the real culprit is a collapsar — a massive, fast-spinning star that collapses into a black hole, unleashing torrents of gamma rays in the process.
The Los Alamos team, working with Fermi Gamma-ray Burst Monitor data and simulations on the Chicoma supercomputer, modeled what elements a collapsar would actually produce — and found the match to observations was nearly perfect, without requiring any heavy metals like gold at all. Theoretical physicist Matthew Mumpower noted that the red infrared signature long assumed to signal gold production does not, in fact, require it. A single-component model, he suggested, is sufficient — meaning kilonovae are far more varied and harder to decode than the field had assumed.
Postdoctoral fellow Marko Ristić underscored what makes these events so difficult: the conditions involved — extreme densities, relativistic effects, colliding timescales — push physics to its limits and demand that interpretive models be constantly interrogated.
The conventional framework had always held that long-duration bursts come from collapsars, not mergers. These two events fit that category from the start, but their unusual chemical signatures had thrown the assumption into doubt. Now, with new modeling, the field may be returning to its original instinct — though with a more careful understanding of what these explosions can and cannot tell us.
What settles the question definitively will likely be gravitational waves. When spacetime ripples from a neutron-star merger or black-hole formation are detected alongside traditional light observations, astronomers will finally hold the full picture. Until then, the universe's most energetic explosions remain, in part, an open question.
In the span of a few seconds, a gamma-ray burst releases more energy than our sun will produce across ten billion years. These are among the most violent, most energetic events the universe has to offer—and for more than half a century, scientists at Los Alamos National Laboratory have been trying to understand what creates them.
Two recent bursts detected in 2021 and 2023 have become the center of a debate about cosmic origins. When astronomers first observed GRB 211211A and GRB 230307A, the data seemed to point toward a particular culprit: the collision of two neutron stars, those impossibly dense remnants left behind when massive stars die. The signatures in the light—especially the reddish infrared glow—looked like what you'd expect from such a merger. But a new analysis from Los Alamos researchers, published in The Astrophysical Journal Letters, challenges that interpretation. The team argues these bursts actually came from something different: a collapsar, a massive star spinning so fast that it collapses in on itself to form a black hole, unleashing torrents of gamma rays in the process.
The distinction matters because it reshapes how we understand where the universe's heaviest elements come from. When neutron stars collide, the theory goes, they forge gold, lead, uranium—all those elements heavier than iron that we find in the cosmos. When a collapsar forms, the story is supposed to be different. Yet the Los Alamos team found something unexpected. Using NASA's Fermi Gamma-ray Burst Monitor data and running simulations on the Laboratory's Chicoma supercomputer, they modeled the creation of kilonovae—the light that accompanies the formation of heavy elements—and traced what elements would actually be produced. What they discovered was that the composition of elements created in a collapsar could match the observations almost perfectly, without requiring the presence of those very heavy metals like gold and lead.
This finding upends a key assumption in the field. Matthew Mumpower, a theoretical physicist on the team, explained that the red infrared signature everyone had been reading as a sign of gold production doesn't necessarily mean gold is being made at all. "A simple explanation arises from this work, requiring only a single-component model," he said, suggesting that kilonovae are far more varied and harder to interpret than previously thought. The team's work points back to the original understanding of these two bursts—that they came from collapsars, not mergers—despite the unusual signals that had recently suggested otherwise.
Marko Ristić, a postdoctoral fellow at Los Alamos, emphasized the broader significance. These events represent some of the most extreme conditions in existence: densities and temperatures so high that the normal rules of physics bend, relativistic effects dominate, and multiple timescales collide. Understanding them requires rethinking the models that have guided interpretation of the data.
The conventional wisdom has long held that short gamma-ray bursts—those lasting less than two seconds—come from neutron star mergers, while long-duration bursts, stretching beyond two seconds, originate from collapsars. These two events fit the long-duration category, which should have pointed toward collapsars from the start. But the unusual chemical signatures threw that assumption into question. Now, with new modeling in hand, the field may be returning to its original framework, though with a more nuanced understanding of what these extreme events can produce.
What comes next will depend on a new generation of observations. Gravitational waves—ripples in spacetime itself—offer a way to directly detect the merger of neutron stars or the formation of black holes. When those detections are combined with traditional electromagnetic observations, astronomers will finally have the tools to definitively answer where these bursts come from and what elements they forge. Until then, the universe's most energetic explosions remain partly mysterious, their origins still being written.
Notable Quotes
Gamma-ray bursts are products of some of the most intense, exotic situations in the universe, with really high densities and temperatures, relativistic effects and different time scales coming together.— Marko Ristić, Los Alamos postdoctoral fellow
The type of kilonova represented with these long-duration gamma-ray bursts does not inherently imply the synthesis of gold, despite the signal showing a red component typically associated with lanthanide production.— Matthew Mumpower, Los Alamos theoretical physicist
The Hearth Conversation Another angle on the story
So these two bursts looked like they came from neutron stars colliding. What made people think that in the first place?
The light they gave off—especially that reddish infrared glow—matched what theory predicted you'd see from a merger. And mergers are supposed to be where you make the heaviest elements. That red color was read as a signature of gold being created.
But the Los Alamos team says that's not what's happening here.
Right. They modeled what a collapsar would produce—a massive star collapsing into a black hole—and found it could match the observations just as well, without needing to make gold at all. The red signature doesn't necessarily mean what everyone thought it meant.
So we've been misreading the data?
Not misreading exactly. The data is real. But the interpretation was too narrow. It turns out kilonovae are more varied than the field assumed. The same light signature can come from different sources.
Does this settle the question of where these bursts come from?
Not entirely. It shifts the weight of evidence back toward collapsars for these two events. But the real answer will come when we can detect gravitational waves alongside the light. That's when we'll know for certain what's actually happening out there.