Boffins baffled by record-smashing supernova that shouldn't exist

New type of stellar explosion or alien pressing the wrong button?

supernova
ASASSN-15lh as it would have been seen from 10,000 light years away

Supernovae are nothing new in the universe, but scientists have spotted the largest yet seen, some 200 times as bright as anything recorded previously.

The explosion – dubbed ASASSN-15lh after the All-Sky Automated Survey for Supernovae telescopes in Chile that first found the big bang in June last year – was 570 billion times brighter than our sun's output and around 20 times the entire output of the 100 billion stars comprising our Milky Way galaxy.

"ASASSN-15lh is the most powerful supernova discovered in human history," said Subo Dong, an astronomer at the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University.

"The explosion's mechanism and power source remain shrouded in mystery because all known theories meet serious challenges in explaining the immense amount of energy ASASSN-15lh has radiated."

According to a newly published paper in Science, the explosion occurred 3.8 billion light years away and may be an example of a superluminous supernovae, a class of stellar explosion only recently suggested by astronomers. It was spotted thanks to machine-learning techniques applied to the twin ASASSN telescopes.

"We developed an automated software system based on machine-learning algorithms to reliably separate real transients from bogus detections," said Przemek Wozniak, the principal investigator of the project that created the software.

"This is a golden age for studying changes in astronomical objects, thanks to rapid growth in imaging and computing technology. ASAS-SN is a leader in wide-area searches for supernovae, using small robotic telescopes that repeatedly observe the same areas of the sky looking for interesting changes."

As soon as news broke of the initial spotting, earth and space telescopes were focused in, and the results have been puzzling. Spectral outputs from the explosion revealed materials signatures that have never been spotted before in a supernova.

The supernova was much hotter and brighter than other examples, and was lacking in hydrogen and helium, the two most abundant elements in the universe. The Hubble space telescope is now being repositioned to investigate further.

One hypothesis is that ASASSN-15lh was caused by the collapse of a magnetar, a class of small and incredibly dense magnetized bodies created by the remains of a much larger star that had entered its fatal compression phase. But simulations suggest that the explosions might not even be possible with such a dense body.

"The honest answer is at this point that we do not know what could be the power source for ASASSN-15lh," said Dong. "ASASSN-15lh may lead to new thinking and new observations of the whole class of superluminous supernova, and we look forward to plenty more of both in the years ahead."

Then again, there could be a more fantastic explanation. In his book Greetings, carbon-based bipeds, the great science fiction writer Arthur C Clarke speculated that some supernovae could be the failed attempts by civilization to tap into the zero-point energy. We may never know if ASASSN-15lh was a natural event or just someone pressing the wrong button. ®

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