The universe’s brightest supernovae are being turbocharged by newborn magnetars

The universe’s brightest supernovae are being turbocharged by newborn magnetars

By Joseph Howlett edited by Lee Billings

Every star’s death is dramatic. Superluminous supernovae take theatrics to a new level.

In the early 2000s, scientists first saw these eye-catching catastrophes, which can shine much longer and be more than 10 times brighter than a normal supernova. And ever since, they have wondered what physical process explains such supernovae’s exceptional, lingering afterglow.

Now they know. In an article published today in the journal Nature, astrophysicists nailed down a superluminous supernova’s true source: radiation emanating from a newly formed, highly magnetized, fast-spinning ball of neutrons the size of a city, a so-called magnetar. Besides solving the mystery of superluminous supernovae, this also marks the first time scientists have witnessed the birth of a magnetar. And what gave it all away is a strange quirk of Einstein’s general theory of relativity.

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“It’s so far from anything we’ve ever thought of,” says Joseph Farah, a graduate student affiliated with Las Cumbres Observatory (LCO) and the University of California, Santa Barbara, who led the study. “We know so little about these things.”

What is what is known is that when a massive star exhausts its fuel, it collapses in on itself and explodes, leaving behind an expanding, slowly cooling cloud of radioactive gas and debris with a small stellar remnant at its center. When such a star was 10 to 25 times the mass of our Sun, the remnant is usually a neutron star. These are the strangest bits of matter in the cosmos – a teaspoon of their material weighs as much as Mount Everest – making neutron stars the sites of some of the most extreme physics in existence.

Neutron stars become particularly extreme when they spin rapidly, pulsing out beacon-like beams of radiation from their poles; astronomers call these objects pulsars. And magnetars are the most extreme of all: most of them are newborn pulsars that have magnetic fields up to 1000 times stronger than normal.

Although theorists already had a hunch that a magnetar’s stormy birth might help explain superluminous supernovae, getting the hang of it proved difficult. A potential breakthrough came in late 2024 with the eruption of a new superluminous supernova, SN 2024afav, about a billion light-years from Earth. Monitored over 200 days by astronomers at the LCO, SN 2024afav’s brightness dropped at regular intervals, oscillating back and forth, with the time between dips getting shorter and shorter over the course of the measurement.

Farah and his co-authors went to the blackboard in search of explanations for this specific pattern. They landed on only one who could explain it. As a magnetar spins on its axis at nearly the speed of light, its enormous magnetic field twists, coils and twists to pump out powerful radiation. Energy from this astrophysical engine causes the surrounding gas to glow, increasing the supernova’s brightness and lifetime.

But what caused these star embers to wax and wane? The answer boils down to how the spinning dead star left space and time in its wake.

The magnetar was originally surrounded by a swirling disk of matter, which funneled from its inner edge to the stellar remnant. The disc was slightly tilted from the magnetar’s spin axis, and the violent maelstrom of spacetime it created spun the disc around it. From afar, this consequence of general relativity, called “Lense-Thirring precession,” made the whole system look like a spinning top wobbling on a table.

From Earth’s vantage point—just along the distant magnetar’s equator—the wobbly disk acted like a movie projector’s shutter, periodically blocking our view of the dead star that supercharges SN 2024afav. As the days passed and the magnetar ate away at its disk, that torus of material shrank inward. This sped up the shutter effect, which made the blackouts more and more frequent until the disc was gone.

This fantastic origin story, the authors say, fits the data better than anything else they could come up with. That makes it the surest evidence yet of what really happens at the center of a superluminous supernova. “Other possible energy sources would not produce such a pattern,” said Daniel Kasen of the University of California, Berkeley, one of the astrophysicists who first proposed the magnetar explanation in 2010 and is credited for providing helpful discussion in the new paper. “A magnetar can act as a powerful engine that illuminates the supernova to extraordinary brightness.”

The confirmation opens up magnetars as yet another cosmic laboratory for testing general relativity. “Everything about the system is extreme,” says Adam Ingram, an astrophysicist at Newcastle University in England, who served as a peer reviewer for the study. “The gravitational field is strong enough that the most exotic predictions of general relativity can have large effects.”

During its lifetime, the newly operational Vera C. Rubin Observatory in Chile will see millions of supernovae, including many more of these rare events. And however general relativity is visible in the world, says Farah, there is an opportunity to understand it better – and perhaps even find new cracks in the edifice of Einstein’s greatest theory, from which new ideas can spring. “It means we can test one of our fundamental theories about reality in one of the most extreme environments in the universe,” he says.

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