As mysteries go, they don’t come much bigger than this. What was the strange ultrabright flash in a distant galaxy, spotted last year? Some astronomers thought it was the brilliant blast from a particularly massive exploding star, while others suspected it was an entirely new cosmic object altogether.
Now new observations pin the blame on a rapidly spinning supermassive black hole, lurking in the centre of the galaxy, tearing a star apart.
The work, reported in Nature Astronomy, offers a plausible solution to the enigmatic event, including why it brightened after dimming, says Jeff Cooke, an astronomer at the Swinburne University of Technology in Melbourne, Australia who was not involved with the study.
“It’s been a ‘problem child’ from the start,” he says. “But the story’s likely not over.”
The saga began on 14 June 2015, when the All-Sky Automated Survey for Supernovae (or ASASSN) in Chile picked up a dazzling burst of light from a big galaxy some 3.9 billion light-years from Earth.
The eruption – dubbed ASASSN-15lh – was 20 times brighter than the output of all 100 billion stars in the Milky Way galaxy.
In January this year, some astronomers thought it might be a so-called superluminous supernova – a particularly bright explosion that marks the end of a gigantic star’s life. What’s left is a dense, spinning neutron star called a magnetar, with an immensely powerful magnetic field and rotation that decay into extremely energetic light.
But ASASSN-15lh displayed some unusual traits.
Where a supernova’s initial blast is usually followed by fading, ASASSN-15lh started ramping up its brightness again in September.
And a magnetar would need to spin at 1,000 times per second – the very upper limit before it rips apart – to produce such epic amounts of radiation.
So to nut it out, Giorgos Leloudas from the University of Copenhagen in Denmark and colleagues analysed 10 months’ observations of ASASSN-15lh with an assortment of space- and ground-based telescopes, including the Hubble Space Telescope and the Very Large Telescope in Chile.
While the January paper that interpreted the event as a superluminous supernova observed it for around four months after peak brightness, “our extended monitoring also showed that the source remained bluer – and thus hotter – than previously observed superluminous supernovae for many months”, says study co-author James Miller-Jones from the International Centre for Radio Astronomy Research and Curtain University in Perth, Australia.
So they came to the conclusion that it wasn’t a supernova at all, but that the supermassive black hole in the centre of the galaxy was shredding a star that wandered a little too close.
While its name might signify otherwise, a black hole can produce phenomenal amounts of light and heat in its surroundings.
As its immense gravitational tug pulls matter near, a nearby star can “spaghettify” – warp, stretch then rip apart to form a disc of matter around the black hole.
Heat produced by the accreting material, as well as shockwaves generated through debris collisions, produced these bright flares. And ASASSN-15lh’s flashes originated near the centre of the galaxy – just where you’d expect if the supermassive black hole were involved.
Another clue came from the galaxy itself. Extraordinarily bright supernovae tend to crop up in blue galaxies, which vigorously pump out stars.
But ASASSN-15lh’s host is a “massive and passive red galaxy”, the team writes – not a superluminous supernova’s usual haunt.
Yet the simple existence of the supermassive black hole is not enough to explain ASASSN-15lh’s dazzling display, they add – the black hole must be rotating quickly too.
A star around the size of the sun, approaching a black hole, would simply be swallowed whole and be torn apart inside the event horizon – the limit where nothing, not even light, can escape. So we wouldn’t see the bright flare.
But spinning black holes stretch the fabric of spacetime nearby, so stars like the sun can orbit closer than if they didn’t rotate. This means the black hole can rip matter apart outside the event horizon – which is emitted as light and detectable.
The explanation “all sounds great”, Cooke says, but he’s not entirely convinced. The scenario works if everything is just so – but not if, for instance, the accretion disc isn’t spinning in the right way, if the star didn’t get dragged in at a certain angle or if the supermassive black hole isn’t spinning fast enough.
He and his colleagues are analysing the same Hubble data, which is publicly available, and postulate a slightly different theory. Exactly what remains to be seen – they’re keeping their cards close to their chest – but their explanation might not include an extreme superluminous supernova or magnetar either.