Satellites spot dead stars using magnetism to contain thermonuclear explosions
A team of astronomers spent almost an entire year utterly confused by a flash of light they caught bursting from the corpse of a dead star.
“These white dwarfs were showing bursts where the brightness… would increase by up to a factor of 30 in less than an hour… and they would fade in about 10 hours,” astrophysicist Simone Scaringi, a co-author on the study, tells IE.
After months of puzzling over the source of the sudden thermonuclear explosion, they realized the phenomenon — they call it a “micronova” — was new to science. In most other stellar explosions involving white dwarfs, the star is completely surrounded in a "shell" of hydrogen that burns brightly for weeks or months. The researchers think a micronova occurs when the white dwarf’s strong magnetic field corrals the hydrogen near its poles.
The discovery is described in a paper published on April 20 in the peer-reviewed journal Nature.
“For the first time, we have now seen that hydrogen fusion can also happen in a localized way,” says astronomer Paul Groot, another co-author. “The hydrogen fuel can be contained at the base of the magnetic poles of some white dwarfs, so that fusion only happens at these magnetic poles.”
The explosion happened millennia ago
Here’s what the researchers think happened. For billions of years, two stars not terribly different from the Sun orbited each other in a common arrangement that astronomers call a binary system. At some point, one of the stars had converted all of its fuel into atoms too heavy for it to fuse and “died,” transforming into a dense object called a white dwarf.
“A white dwarf is what the Sun will become once it's burned through all of its fuel,” Scaringi says. (Don’t worry — that won’t be for several billion years.) “What you will be left with is the inert core, which is an object that is very dense. White dwarfs are usually about the size of the Earth with a mass about that of the Sun.”
The white dwarf and star continued orbiting each other like before. Then, roughly 4,000 years ago — as the first civilization was stirring in Greece — the white dwarf got its hands on some hydrogen from its companion star.
“Because these two objects are so close to each other, the white dwarf actually pulls material from the companion. It literally steals part of the atmosphere of the companion star, pulls it, and accretes it onto its surface,” Scaringi says.
The white dwarf fused the hydrogen into heavier elements in a huge and fast thermonuclear explosion. The researchers think the amount of hydrogen fueling the explosion was roughly 3.5 billion times the mass of Great Pyramids of Giza. That’s big by Earthly standards, but it’s micro in cosmic terms. A regular nova is roughly a million times bigger.
The nuclear explosion sent light — both visible and ultraviolet — radiating out into space. After millennia on the move, an very, very small percentage of that light landed on the detector of a NASA satellite as the electromagnetic waves passed by our corner of the Milky Way.
“They're really hard to find because you have to look at the right place at the right time. If you miss that small window, it would be as if nothing ever happened,” Scaringi says.
"We were just puzzled"
Of course, Scaringi and his colleagues weren’t looking for micronovae when they found the first one. He’s spent years using data from the NASA probe, the Transiting Exoplanet Survey Satellite (TESS), to keep tabs on roughly one hundred white dwarfs. He studies the disks of matter that collect around the stellar remains.
“TESS stares at the same object for at least one month and sometimes up to a year, so we get these incredibly long and precise brightness variations most of the time,” he says. “It's only because we had so much data on one object that we were able to see this 10-hour flare. Otherwise, it would have been lost.”
“After finding the first one, we just were just puzzled. And so for about a year, we tried to explain these really rapid brightness variations [but] to no avail,” he says. It might sound like an unusual situation, but with dozens of satellites and ground-based telescopes collecting terabytes of data all the time, that kind of confusion is par for the course."
“That's kind of how we do science, especially in this day and age where we are swamped with data. Sometimes you're just going to find something that you can't immediately explain,” Scaringi says.
First, they compared the mysterious data with theories about white dwarfs that researchers had already proposed, but a bright flash that lasted for such a short amount of time didn’t square with any existing theory. Then they decided to share their connundrum with the scientific community.
“We started writing a draft [that] was basically showing the discovery and listing out all the [explanations for the data] we thought about and how none of them would really work,” he says.
More data helped the researchers understand what they'd seen
“Then, finally, we found another two objects that showed remarkably similar profiles,” he says. Adding extra cases to the pile of evidence made all the difference.
"These other objects were also suspected to be magnetic white dwarfs. This helped us to then refine our idea about what was going on," he says.
The researchers started building models based on what they knew for sure about larger nova explosions.
"We started asking questions like 'What would happen if the magnetic field is strong enough to keep the hydrogen localized at the magnetic poles?' 'How much mass would it burn?' and 'how quickly would these events last?'”
The extra data also made it easier for the researchers to compare the phenomenon they were studying with another well-understood astronomical event. It turns out that neutron stars — the remains of much larger stars that collapse into tiny, incredibly dense balls of protons and neutrons — create similar explosions.
"For several decades, we've known that neutron stars show these X-ray bursts that are the result of thermonuclear explosions, if you want," he says. "The key differences are that neutron stars are about a small city and the X-ray bursts only last about a minute or two," he adds.
Those differences are important, but they don't change an underlying pattern. "If you look at the light curve profiles, neutron stars [and white dwarfs] essentially look like carbon copies of one another," he says.
The combination of models that made sense and the analogous process from neutron stars convinced the researchers that they'd found an explanation for the mysterious explosions.
But that's hardly the end of the story.
"But the fact that we were able to find three in such a short space of time probably means that many more systems are out there showing micronova," Scaringi says.
"The search is now on."
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