Can a dead star continue to explode?

This is an artist’s impression of AT2022tsd, an explosion in a distant galaxy. The image shows a possible explanation for the strange object. It could be a black hole accumulating matter from a disk and propelling a jet. Photo credit: Robert L. Hurt/Caltech/IPAC.

In September 2022, an automated sky survey spotted what looked like a supernova explosion about a billion light-years away.

The Zwicky Transient Facility (ZTF) discovered it and named it AT2022tsd.

But something was different about this supernova. Supernovae explode and shine brightly for months, while AT2022tsd exploded brightly and then faded within a few days.

That’s when astronomers realized it wasn’t a normal supernova at all. It was a luminous fast blue optical transient (LFBOT), a relatively new class of object discovered only a few years ago and still enigmatic.

Then something even stranger happened. AT2022tsd, nicknamed the “Tasmanian Devil,” kept brightening.

This was both a surprise and a mystery, two things that always attract astronomers’ attention.

After its initial discovery, follow-up X-ray observations showed that the Tasmanian Devil was the brightest LFBOT ever discovered. What’s more, it rivaled the brightest gamma-ray bursts ever discovered.

These facts, along with the object’s repeated brightening, made it one of the strangest transients ever discovered.

A team of researchers led by Anna Ho, assistant professor of astronomy at Cornell University, collected all available data to find out what was behind the Tasmanian devil’s unusual behavior. Their results are published in a new paper titled “Minutes-duration optical flares with supernova luminosities.” The article was published in the journal Nature.

“This ends years of debate about what drives this type of explosion and reveals an unusually direct method for studying stellar corpse activity.” Anna Ho, Cornell University

“In recent years, certain luminous extragalactic optical transients have been observed to last only a few days,” the authors write in their paper. “Their short observed duration implies a different energy mechanism than the most common luminous extragalactic transients (supernovae), whose time scale is weeks.”

“Here we report observations of minute-long optical bursts following an AT2018cow-like transient, AT2022tsd.”

The Tasmanian Devil’s repeated flashes lasted only a few minutes. They were also just as powerful as the first explosion 100 days earlier. These repeated flashes only deepened the mystery surrounding LFBOTs. 15 telescopes around the world observed the flashes, and the research team combed through the observations to figure out what type of object was behind this remarkable behavior.

In December, Ho routinely monitored the fading first explosion and took five new images of the object, each lasting a few minutes. The first image showed nothing, but the middle image showed a strong brightening that quickly disappeared. Then nothing again.

Ho shared the pictures with employees and they were speechless.

“We think these flares could have come from one of these newly formed corpses,” says Anna Ho of Cornell University

“Nobody really knew what to say,” Ho recalled. “We had never seen anything like this before in a supernova or FBOT – something as fast and as bright as the original explosion months later. We’ve never seen this before in astronomy.”

The team’s analysis found 14 separate brightness peaks over a 120-day period. According to Ho, that’s probably just a fraction of the total.

“Surprisingly, instead of fading steadily as expected, the source briefly brightened again – over and over again,” she said. “LFBOTs are already kind of a strange, exotic event, so this was even stranger.”

There are some events and objects out there in the cosmos that flash repeatedly, and researchers have been working to either confirm or eliminate them. The Devil wasn’t a supernova, but could it have been a Tidal Disruption Event (TDE), where a black hole feeds on matter and flares brightly? Do its light and frequency match a different type of astronomical transient?

The team detected a single optical and multiple X-ray flashes from the Tasmanian devil. But the optical flare didn’t match any of the X-ray flashes. There was also no apparent periodicity of flaring. Also puzzling was the fact that the team couldn’t detect any optical scattering when examining other LFBOTs.

“To the best of our knowledge, there is no precedent in the literature for this phenomenon – tiny optical flares with supernova-like luminosity and amplitude fluctuations on the order of magnitude lasting over 100 days,” the researchers write in their paper.

AT2022tsd is still a mystery for now. It could be a supergiant star that collapses as a supernova and then forms an accretion ring. It could be a black hole that sends jets of material into space. Or it could be a different LFBOT, but we could be looking at it from a strange angle. If so, that could explain the Tasmanian devil’s X-rays.

“The lack of detected flares in other LFBOTs could be due to the viewing angle: AT2018cow is thought to have been observed close to the plane of the circumburst “disk” rather than head-on, and AT2022tsd having a more axial viewing angle could also help explain this “contribute to the much more luminous X-ray emission,” the authors explain in their study.

The researchers came to a conclusion that is, however, incomplete.

The Tasmanian Devil must be a dead star, either a black hole or a neutron star. “We don’t think anything else can produce these types of flares,” said Anna YQ Ho, assistant professor of astronomy in the College of Arts and Sciences. “This ends years of debate about what drives this type of explosion and reveals an unusually direct method for studying star corpse activity.”

If the Tasmanian Devil is some kind of dead star, it doesn’t behave like the others.

“We may be seeing a completely different channel for cosmic catastrophes,” Ho said.

Something unprecedented could happen to the devil. As a dead star, the light emitted from it could signal its transition into some kind of stellar afterlife. It could be a new type of star corpse.

“Because the corpse isn’t just lying around, it’s active and doing things that we can detect,” Ho said. “We think these flares could have come from one of these newly formed corpses, which gives us the opportunity to study their properties investigate when they have just emerged.”

Written by Evan Gough/Universe today

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