A team of astronomers spent nearly an entire year completely confused by a flash of light they caught erupting from the corpse of a dead star.
“These white dwarfs showed bursts where the brightness…increased by a factor of 30 in less than an hour…and they disappeared in about 10 hours,” said astrophysicist Simone Scaringi, co-author of the study. THAT IS TO SAY.
After months of puzzlement 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 by a “shell” of hydrogen which burns intensely for weeks or months. Researchers believe that a micronova occurs when the white dwarf’s strong magnetic field traps hydrogen near its poles.
The finding is described in a paper published Wednesday in the peer-reviewed journal Nature.
“For the first time, we have now seen that hydrogen fusion can also occur in a localized fashion,” says astronomer Paul Groot, another co-author. “Hydrogen fuel may be contained at the base of the magnetic poles of some white dwarfs, so fusion only occurs at those magnetic poles.”
The explosion happened millennia ago
Here’s what researchers think happened. For billions of years, two stars not dissimilar to the Sun have 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 to fuse together and “died”, turning into a dense object called a white dwarf.
“A white dwarf is what the Sun will become once it has burned all its fuel,” Scaringi explains. (Don’t worry, it won’t be for several billion years.) “What you’ll be left with is the inert core, which is a very dense object. White dwarfs are typically about the size of Earth with a mass about that of the Sun.
The white dwarf and star continued to orbit each other as before. Then, around 4,000 years ago – as early civilization stirred in Greece – the white dwarf got its hands on 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 some of the companion star’s atmosphere, pulls it in and accretes to its surface,” Scaringi explains.
The white dwarf fused hydrogen into heavier elements in a huge, fast thermonuclear explosion. Researchers believe the amount of hydrogen fueling the explosion was around 3.5 billion times the mass of the Great Pyramids of Giza. It’s big by Earth standards, but it’s micro in cosmic terms. An ordinary nova is about a million times larger.
The nuclear explosion sent light – both visible and ultraviolet – radiating out into space. After millennia of motion, a very, very small percentage of this light landed on the detector of a NASA satellite as electromagnetic waves passed through our corner of the Milky Way.
“They are really hard to find because you have to look in the right place at the right time. If you miss that little window, it would be like 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 spent years using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) to keep tabs on around 100 white dwarfs. He studies the disks of matter that accumulate around stellar remnants.
“TESS stares at the same object for at least a month and sometimes up to a year, so we get these incredibly long and precise variations in brightness most of the time,” he says. “It was only because we had so much data on an object that we were able to see that 10-hour eruption. Otherwise it would have been lost.
“After finding the first one, we were just puzzled. And so for about a year we tried to explain these really fast variations in brightness [but] in vain,” he said. This may seem like an unusual situation, but with dozens of satellites and ground-based telescopes constantly collecting terabytes of data, this kind of confusion is normal.”
“That’s kind of how we do science, especially in our time when we’re overwhelmed 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 white dwarf theories the researchers had previously proposed, but a bright flash that lasted for such a short time didn’t fit any existing theory. Then they decided to share their knowledge with the scientific community.
“We started to write a draft [that] basically showed the discovery and listed all the [explanations for the data] we thought and how none of them would really work,” he says.
More data helped researchers understand what they had seen
“Then eventually we found two more objects that showed remarkably similar profiles,” he says. Adding more cases to the pile of evidence made all the difference.
“These other objects were also suspected to be magnetic white dwarfs. This helped us refine our idea of what was going on,” he says.
Researchers began building models based on what they knew for sure about larger nova explosions.
“We started asking questions like ‘What if the magnetic field was strong enough to keep the hydrogen localized to the magnetic poles?’ “How much mass would that burn?” and ‘how quickly would these events last?’
The additional 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 remnants of much larger stars that collapse into tiny, incredibly dense balls of protons and neutrons — create similar outbursts.
“For several decades we’ve known that neutron stars show these X-ray bursts that are the result of thermonuclear explosions, if you will,” he says. “The main differences are that neutron stars are for a small town and X-ray bursts only last about a minute or two,” he adds.
These differences are important, but they do not change an underlying pattern. “If you look at the light curve profiles, neutron stars [and white dwarfs] essentially look like carbon copies of each other,” he says.
The combination of models that made sense and the analogous process of neutron stars convinced the researchers that they had found an explanation for the mysterious explosions.
But that’s not the end of the story.
“But the fact that we were able to find three in such a short time probably means that there are many other systems showing micronovas,” Scaringi says.
“The search is now underway.”