A supernova that has exploded multiple times is puzzling astronomers with its drawn-out death throes, which challenge what they know about such catastrophic events.
Some stars like to make a dramatic exit by exploding in a supernova, one of the brightest events in the universe.
Astronomers used to think that once a star went supernova, that was it – the supernova would fade in about one hundred days, never to be seen again. Now one supernova refuses to die, raising difficult questions and potentially opening the door for new physics.
The Palomar Transient Factory, an international astronomical survey, first discovered the supernova in September 2014. Dubbed iPTF14hls, scientists initially categorised it as a type IIP supernova – here type II refers to the presence of hydrogen and the P refers to a “plateau” in its brightness.
However, while most supernovae fade after about three months, iPTF14hls was observed brightening and dimming for almost two years.
That in itself is unusual but not completely unexpected; some supernovae can interact with the gas and dust surrounding them, allowing them to shine brightly for four years or more.
When astronomers analyse data from these supernovae, they see telltale signs of the interactions. iPTF14hls showed none of these signs; instead, it shows all of the characteristics of a normal type IIP supernova, but over a far longer time period.
This supernova is more than just long-lived, however; it might have come back from the dead. Researchers combed the archives and found that in 1954, a supernova exploded in the same location. Could a star explode, survive, and somehow manage to explode again?
“This supernova breaks everything we thought we knew about how they work,” said Iair Arcavi of the Las Cumbres Observatory (LCO), the lead author of a study published today in Nature. “It’s the biggest puzzle I’ve encountered in almost a decade of studying stellar explosions.”
The star which produced iPTF14hls makes most others seem tiny: Arcavi’s study calculated that it weighed fifty solar masses, possibly more. Its high mass might account for why the supernova behaves so oddly.
One possibility is that iPTF14hls is a pulsational pair-instability supernova. Here, a star’s core becomes so hot that it triggers pair production – the formation of electrons and their antimatter twins, known as positrons.
This reduces the pressure holding up the star against gravitational collapse, so it contracts in on itself and the star begins to burn material until it expands, cooling down in the process. The core produces more electrons and positrons and the cycle continues in fits and starts.
Because this process can last anywhere between a few hours and ten thousand years, it could explain why iPTF14hls seems to have exploded twice.
However, there are issues with categorising iPTF14hls as a pulsational pair-instability supernova.
“These explosions were only expected to be seen in the early universe and should be extinct today,” explained Andy Howell, a co-author on the paper and the LCO supernova group’s leader.
“This is like finding a dinosaur still alive today. If you found one, you would question whether it truly was a dinosaur.”
In addition, iPTF14hls released more energy than models of pulsational pair-instability supernovae predict. This could be a sign that some supernovae might be triggered by a completely different mechanism.
For now, LCO will keep observing iPTF14hls, which still hasn’t faded three years after its discovery. By tracking the supernova’s evolution, scientists can gain a better understanding of its behaviour and hopefully find out just why it’s survived for 60 years.
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