Is this giant supernova a new type of cosmic object?

It’s a head-scratcher of astronomical proportions. A cosmic explosion 3.8 billion light-years away unleashed the energy of 570 billion Suns. And now, instead of dying, it’s brightening again, and has left astronomers scrambling to explain what’s powering one of the biggest bangs since, well, the Big Bang.

This mysterious “superluminous supernova” – twice as powerful as any cosmic explosion ever recorded – was first reported in Science by a team led by Peking University’s Subo Dong.

A supernova is the explosive death throes of a massive star collapsing in on itself, leaving a black hole in the centre of an expanding shell of gas. If the star is big enough – more than 30 times the mass of our Sun – the cataclysmic blast is dubbed a superluminous supernova.

Fewer than 50 of these big ones have ever been recorded and, like regular-sized supernova, they have started to dim within weeks, and kept fading. Not this time.

“We’re like, ‘OK, what the heck is this thing?’,” says Jeff Cooke of Melbourne’s Swinburne University of Technology, who is studying the blast but wasn’t involved with the paper. In work yet to be published, he and his colleagues will report how the afterglow faded for two months – then ramped up its brightness again.

The explosion was first spotted by the All-Sky Automated Survey for Supernovae network of telescopes in Hawaii and Chile in June 2015. At its peak, the flash was 200 times more powerful than run-of-the-mill supernovae, or 20 times brighter than the 100 billion stars in the Milky Way combined.

Exactly what powers superluminous supernovae is itself a mystery. One idea is that the explosion doesn’t form a black hole, but a magnetar – an extremely dense, rapidly spinning object, roughly the size of Berlin, with a powerful magnetic field that speeds the rate of release of energy from the initial explosion.

That explanation looks especially shaky for this superluminous supernova. For a start, the magnetar would need to spin 1,000 times a second, which for an object of this size would be at its rotational limit. Beyond that speed, simple physics dictates that it would be flung apart. It would also have to be 100% efficient at converting spin energy into light. “It makes me uncomfortable to find something that’s right on the edge of the energy limit,” says Ben Shappee, an astronomer at Carnegie Observatories in Pasadena, California and co-author of Dong's study.

Which is why later this year Dong and his team will use the Hubble telescope to help investigate alternative explanations.

One possibility is that the explosion is not a supernova at all, Shappee says, but instead is powered by the supermassive black hole in the centre of its host galaxy. If so, Hubble should find the blast hurtling from a supermassive black hole, and it will lack the tell-tale gas shell of a supernova.

If that turns out to be the case, it will be the first time such an explosion has been observed. But Shappee and Cooke both suspect the source of the blast may be something completely new.

“I think it’s very likely it’s something no one’s thought of before,” Shappee says. “It’s a great open question.”