Astronomers find bizarre 'zombie supernova' that just won't die

Astrophysicists have discovered one of the weirdest stars yet in the universe: one that refuses to die, exploding as a supernova multiple times over fifty years.

Normally when large stars reach the end of their lives they terminally explode as a supernova with a burst of bright light and matter and that is that. But the supernova, known as iPTF14hls, keeps on burning. More details can be found in a Nature paper published on Wednesday.

It was first observed in September 2014 by the Samuel Oschin Telescope in San Diego, California, and monitored at multiple different observatories. At first, it appeared as a normal supernova and was classified as type II-P, a star that had enough mass to erupt as a supernova on its own.

But unlike most supernovae, which normally stay bright for about 100 days, iPTF14hls, was still shining for more than 600 days later and the light intensity periodically increased and decreased at least five times. The event also seems to approximately ten time slower than typical type II-P supernovae.

"This supernova breaks everything we thought we knew about how they work," Iair Arcavi, lead author of the paper and a postdoctoral fellow at the Las Cumbres Observatory (LCO) and the University of California Santa Barbara. "It's the biggest puzzle I've encountered in almost a decade of studying stellar explosions."

Data from previous observations show that there was another possible outburst recorded at the same position back in 1954. Multiple energetic pre-supernova explosions have been known to occur, but only in much more massive stars with 95 to 130 solar masses - the star for iPTF14hls is estimated to be around 50 solar masses.

"It remains bright but it is fading, though it depends what one means by "bright". It's bright for a 3-year-old supernova, the fact that we still see it three years after is astonishing," Arcavi said.

"But indeed it is fading and is now about 20 times fainter than it was at its peak. As to whether it will explode again, all bets are off. We'll just have to wait and see."

The team reckon that PTF14hls may have been the most massive stellar explosion ever seen. The sheer size might be the reason that its behaviour seems to be so strange.

Nick Konidaris, co-author of the paper and an astronomer at the California Institute of Technology, called iPTF14hls a “zombie supernova”.

Konidaris was involved in building the SED Machine, a spectrograph that splits the light from incoming supernova, for scientists to study.

“Not too long ago it was faster to identify short-lived celestial phenomena than it was to classify them and determine what they could teach us. Which is why we built SED, but I never expected it would help us analyze an explosion as strange as this zombie star," he said.

Antimatter bomb?

There is one theory that might be able to explain the phenomena: Pulsational Pair Instability.

The idea was first suggested in the 1960s, and explains how a star can have multiple supernova bursts without being destroyed. Daniel Kasen, co-author of the paper and an associate professor at the University of California ,Berkeley said it describes a star “so massive and hot that it generated antimatter in its core. That would cause the star to go violently unstable, and undergo repeated bright eruptions over periods of years."

Arcavi explained to The Register that, "according to Einstein's famous equation, E=mc², energy can be turned into matter, but you need a lot of energy to make a little matter. In this case, the immense heat in the core of a very massive star could be enough to turn some of that energy into pairs of electrons and anti-electrons."

The presence of matter and antimatter together makes the star volatile, leading to repeated explosions that may even repeat over decades before one final colossal outburst. It would then collapse into a black hole.

Andy Howell, co-author of the study and the leader of the supernova group at LCO, said: "These explosions were only expected to be seen in the early universe and should be extinct today. This is like finding a dinosaur still alive today. If you found one, you would question whether it truly was a dinosaur."

But the star may not be an example of a pulsational pair instability as the energy it released is more than the theory predicts. The theory also dictates that hydrogen reserves should have run out, but the star observed by the astronomers shows that there was still hydrogen left in the 2014 explosion.

"So, if iPTF14hls is the first pulsational pair instability supernova then the theory needs to be revised. Otherwise, it's something completely new," Arcavi added.

Peter Nugent, co-author of the study and a senior scientist at the Lawrence Berkeley National Laboratory, California, said that “this is one of those head-scratcher type of events."

"At first we thought it was completely normal and boring. Then it just kept staying bright, and not changing, for month after month. Piecing it all together, from our observations at Palomar Transient Factory, Keck Observatory, [Las Cumbres Observatory], and even the images from 1954 in the Palomar Sky Survey, has started to shed light on what this could be. I would really like to find another one like this."

“More of these objects need to be found if we really want to understand them in any kind of detail. What's remarkable is that we don't know how many such zombie supernovae we've missed before."

The team might be in luck in the future as more detailed spectrograph instruments are installed in other observatories.

"More instruments like SED Machine would be really helpful in studying these objects. In fact, we're hoping to build more copies in the Southern Hemisphere,” Konidaris told The Register. ®