A strange explosion in space is starting to reveal its secrets. In 2018, astronomers spotted an extraordinarily fast and bright explosion unlike anything we had ever seen before, and now they are starting to narrow down what could have caused it.
The explosion was given the official designation AT2018cow – a listing based on the alphabetical order of objects reported in the Astronomer’s Telegram – and nicknamed “the Cow”. It took just a few days to reach its peak brightness after it began to explode, whereas most supernovae take weeks or longer, and it was between 10 and 100 times as bright as normal supernovae.
Astronomers came up with two ideas that could potentially explain the strange features of the Cow: either it came from a massive star exploding, or from a smaller star being eaten by a black hole and causing the black hole to blast out jets of material.
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Joseph Lyman at the University of Warwick, UK, and his colleagues examined the galaxy where the Cow exploded using the Very Large Telescope (VLT) in Chile. The VLT is one of the world’s most powerful telescopes, so they were able to take more detailed observations of the galaxy than anyone had before.
When they searched for signs of a black hole in the region surrounding the Cow, they didn’t find any. Instead, they found a stellar nursery. “We see a bright blob right at the explosion site of the Cow, and that blob is a region of star formation,” says Lyman. “That’s exactly where we’d expect a young, massive star to explode.”
The idea that the Cow was an exploding star is also supported by the fact that it remained hot for longer after the initial explosion than most supernovae do. That means that after the star exploded, it is likely to have left behind some sort of energetic object that could continue heating up the area.
“Something was born in the Cow,” says Anna Ho at the California Institute of Technology. “There was a newborn object – a black hole or a neutron star – that was active after the star died.” If the object was a black hole, it could also explain observations that showed the explosion didn’t actually contain very much material – the extra material could have been immediately swallowed up by the newly formed black hole.
“Weird explosions like this represent a gap in our understanding of how stars ultimately die,” says Lyman. The more strange explosions we find, the better we can understand the life cycles of stars.
Since the first observations of the Cow, we have spotted two similar events. One is called ZTF18abvkwla but nicknamed the Koala, and the other is called CSS161010. These discoveries confirm that the Cow isn’t just a weird one-off, as well as allowing astronomers to calculate how rare Cow-like explosions actually are in the universe.
“Cow-like things are intrinsically very rare: they represent below 1 per cent of all the supernovae,” says Raffaella Margutti at Northwestern University in Illinois. “It’s a very special way to die.” Finding similar explosions is difficult because of their speed, but as we spot more of them, we will be able to figure out what they are with more precision.
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