Baffling ‘space cow’ explosion was probably a failed supernova

Cow in space

Illustration of a cow in space

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An explosion that baffled astronomers in 2018 may finally have an explanation. Observations of X-rays from the blast have revealed that it was probably a massive star that only partially blew up, leaving behind either a dense neutron star or a small black hole – something we have long suspected happens, but never seen until now.

When astronomers spotted AT2018cow – nicknamed “the Cow” because of the chance order of the letters in its official designation – it took only days to reach its maximum brightness. Most supernovae take weeks to months to reach their peaks. It was also 10 to 100 times brighter than typical supernovae.

That made it difficult to explain. “This sudden blip out in space was really unusual, and we don’t see many of these. This is the nearest by and the best studied,” says Kate Maguire at Trinity College Dublin in Ireland, part of the team that discovered it. “It’s such an extreme object, so bright, so fast-evolving, that it gives us a challenge in terms of traditional models.”

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It was so unusual that astronomers around the world immediately started watching it. “We watch stars explode all the time – supernovae are very common now, and there are lots of what are ultimately fairly minor differences – but it’s really rare that you look at something and just say, ‘I have no idea what this is’,” says Daniel Perley at Liverpool John Moores University in the UK. “There were so many things that made it weird.”

Luckily, because the Cow is relatively nearby, at about 195 million light years away, it was possible to use various telescopes to observe it in different wavelengths of light. Dheeraj Pasham at the Massachusetts Institute of Technology and his colleagues used the Neutron Star Interior Composition Explorer (NICER) space telescope to capture the X-rays coming from the explosion’s aftermath over a period of about two months.

They found that the strength of this radiation oscillated up and down every 4.4 milliseconds. As a general rule, the speed of oscillation of light coming from an object in space is proportional to the size of that object, so the researchers calculated that the object at the centre of the Cow must be no more than 1300 kilometres across.

That means it must be a neutron star or a relatively small black hole, referred to as compact objects. “People have been suspecting that these kind of extreme explosions could be the birth of black holes or neutron stars, but this is a final piece of evidence that I think really settles the case,” says Pasham.

The object at the centre of the Cow could be either, but it is harder to find a neutron star model that fits as the oscillation of light is steady. “In most neutron star models you wouldn’t expect it to be steady,” says Perley. “Other observations about this object also point towards it being a black hole.” Combining this work with other data from the Cow suggests that it probably formed after a failed supernova in which some of the material exploded and some fell back in on itself.

Regardless of which type of compact object lies at the heart of the Cow, the fact that we spotted it days after the explosion is a big deal, says Pasham. “When you go to school, you learn that when stars explode, black holes and neutron stars are born, but nobody has actually detected a compact object immediately after a supernova,” he says. “This confirms something we’ve thought was true for a long time, which is that right after a supernova, compact objects can be born.”

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-021-01524-8

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