Mystery ‘whippet’ space explosion is the brightest of its kind

A sudden, mysterious burst of bright light in the sky could be from a black hole devouring a vast, unusually bare star.

In 2018, astronomers spotted a new kind of cosmic explosion that became brighter more quickly than any other. The flash, called AT2018cow or “the Cow” for short, took only a few days to reach its peak brightness, rather than the weeks that are required for typical supernovae.

There was no initial obvious explanation for such a burst, and in the years since the Cow was first discovered, we have seen only a handful of other explosions like these, which are collectively called fast blue optical transients (FBOTs). Their origin remains a mystery.

Now, Jialian Liu at Tsinghua University in China and his colleagues think that a recent cosmic flash, which is the brightest of any FBOT so far, must be the result of an exotic star, more than 30 times the mass of our sun, that has lost its outer layers of hydrogen, being feasted on by a black hole.

This explosion, called AT 2024wpp, or “the Whippet”, was first spotted by the Zwicky Transient Observatory at the end of 2024, and quickly became around 10 times brighter than the Cow. Liu and his team then observed the explosion with several different telescopes, including the Swift X-Ray Telescope and the Australia Telescope Compact Array, in the weeks after its initial discovery to build up a complete picture of the different wavelengths of light it produced.

The light’s spectrum suggested that the explosion responsible must have been more than six times hotter than the surface of the sun and blasted out plasma at around a fifth of the speed of light. They also found that around a month after the first burst of light, there was a fresh burst of X-rays, which had never been seen in a previous FBOT.

The best explanation for these observations, Liu and his team argue, is an unusual star called a Wolf-Rayet star, which has an exposed stellar core lacking an outer gas layer. The researchers say that the Whippet is the result of such a star being devoured by a black hole 15 times the mass of the sun.

The initial merger of the two would have produced the first burst of light, while some of the star’s leftover material, which was orbiting the black hole, later fell back towards the black hole, producing the second burst of X-rays. This is a convincing argument for what happened, says Ashley Crimes at the European Space Agency. “Of all the different explanations that have been put forward, this one probably has the least problems.”

One of the most compelling pieces of evidence for this scenario is the fact that the event appears to be coming from a young galaxy, where short-lived extreme stars, like Wolf-Rayet stars, are more common, says Crimes. “These are the kinds of environments that you’d expect to see this kind of event, and then, in addition, you see this bump at late times, which could be material falling back after a merger. It’s promising.”