A Dead Star in a Nearby Galaxy Just Did Something Wild
A massive star in M82 lived fast, died young, and left an extremely magnetic corpse.
Astronomers just caught a dead star in a distant galaxy throwing a massive temper tantrum.
A magnetar in galaxy M82 recently sent a huge burst of gamma rays and other radiation blasting out into space. Magnetars are the burned-out, collapsed cores of massive stars, wrapped in the strongest magnetic fields in the universe. And sometimes, the restless shifting of those immensely powerful magnetic fields releases a huge blast of energy: a giant flare, bright enough to be seen tens of millions of light-years away.
In the last 50 years, astronomers have seen one of these dramatic flares just three times, and never in another galaxy – until recently. Astrophysicist Sandro Mereghetti and his colleagues published their work in the journal Nature.
Astronomers expect magnetars to be more common in galaxies like M82, where massive stars are being born, and dying, at a tremendous rate.
It Came From a Starburst Galaxy
After 20 years of being woken in the middle of the night by news of a gamma ray burst (the brief, bright flash of gamma rays released when two neutron stars or black holes collide) these cosmic cataclysms are starting to seem routine, Mereghetti tells Inverse.
But the one ESA’s orbiting International Gamma Ray Astrophysics Laboratory (Integral) detected on the night of November 15, 2023 was different. For one thing, it came from a bright galaxy full of massive stars, just 12 million light years away — not the sort of place you usually find neutron star collisions; those tend to come from galaxies full of aging stars, billions of light years in the distance.
“I immediately noticed that the position was not a random position in the sky, but right in the center of a very bright galaxy, M82,” says Mereghetti. "This clearly was very exciting, and in a sense, unexpected. It opened the possibility that this could be something else, not a normal, boring GRB – but something more interesting for me, because my field is the study of magnetars.”
When a massive star burns up the last of fuel, it collapses onto its burned-out core. The force of that collapse triggers a cosmic explosion called a supernova, but it leaves behind a dense ball of matter, packed so tightly that there’s no room for atoms, only subatomic particles called neutrons. And sometimes, for reasons physicists don’t fully understand yet, one of these neutron stars forms with a tremendously powerful magnetic field, and a magnetar is born.
Exactly how and why that happens — and what comes next — are questions Mereghetti and his colleagues hope the recent magnetar flare in the M82 galaxy can help them answer.
Pinpointing a Giant Flare
Astrophysicists like Mereghetti already know that magnetars occasionally flare up dramatically, and they think it happens thanks to huge movements in the magnetar’s constantly shifting magnetic field. But these are rare events; just 3 have happened nearby in the last 50 years: twice in our galaxy and once in the nearby Large Magellanic Cloud (which is basically a suburb of our galaxy). Astronomers had never seen a giant flare from a magnetar in another galaxy. That’s mostly because it’s hard to tell the difference between an “ordinary” gamma ray burst and a flaring magnetar, unless you can very precisely pinpoint the source.
Integral’s high-resolution images allowed Mereghetti and his colleagues to do just that, and M82 — a bright spiral galaxy bursting with new star formation — looked like exactly the kind of place likely to host a restless magnetar.
The mid-November gamma ray burst also looked a little different from the kind that had long since become routine to Mereghetti. After the kind of cosmic collision that triggers a normal gamma-ray burst, the sky glows with x-rays and visible light for hours or days afterward, but this event left no afterglow in the sky. Instead, the astronomers just saw a short spike of bright gamma rays, lasting less than a second. That suggested the source wasn’t a neutron star collision, but an erupting magnetar.
And there was something missing: gravitational waves. When two dense, massive objects — like neutron stars or black holes — collide, they stir up ripples in the fabric of spacetime, which observatories like LIGO can measure when they eventually pass through our planet. But whatever caused the strange-looking gamma ray burst that November night, it didn’t stir up any gravitational waves: another clue pointing to a giant magnetar flare.
Magnetars have magnetic fields a thousand times stronger than an ordinary neutron star, and physicists aren’t sure why (according to NASA, that’s equivalent to several trillion fridge magnets).
The Wild Afterlives of Dead Stars
Now that they’ve seen a magnetar flaring in a (relatively) nearby galaxy, Mereghetti and his colleagues hope this will give them a chance to learn more about these bizarre objects and their even more bizarre behavior.
Magnetars don’t flare very often, and that means astronomers seldom get a chance to watch them in action.
“If you want to study the giant flares from our galaxy, we've seen just two of them. That means I have to wait another 50 years to study two more,” says Mereghetti. “Knowing that we can see them also in external galaxies will allow us to increase the number of flares that we observe. We cannot study them so well because they are further away, but we will have a larger number of them.”
Mereghetti hopes that more data from magnetar flares in other galaxies will help answer questions about exactly how and why these giant flares happen — and how magnetars form and eventually die. By understanding the weird behavior of these super-magnetic stellar corpses, astrophysicists hope to learn why some supernovae leave behind ordinary neutron stars, while others spawn magnetars, and still others leave pulsars: fast-spinning neutron stars that emit beams of x-rays from their poles, like cosmic lighthouses.
They also hope to learn what happens when a magnetar loses all the energy that powers its magnetic field, which happens, in part, through giant flares like this one.
“There are a few neutron stars in our galaxy that we see only in x-rays; they do not show flares, but they are particularly hot. And we believe that those are the descendant of magnetars; they are all the magnetars that lost all their magnetic energy,” says Mereghetti.
Mereghetti and his colleagues hope to link all these different kinds of neutron stars into an evolutionary chart of the afterlives of massive stars.
