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

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

In 2018, astronomers discovered a new type of cosmic explosion that brightened faster than any other. The flash, called AT2018cow or “The Cow” for short, took just a few days to reach its peak brightness, rather than the weeks required for typical supernovae.

There was no initially obvious explanation for such an outburst, and in the years since the cow was discovered, we’ve only seen a handful of other explosions like these, collectively known as fast blue optical transients (FBOTs). Their origins are still a mystery.

Now Jialian Liu of Tsinghua University in China and his colleagues believe that a recent cosmic flash, the most luminous 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 pinned down by a black hole.

This explosion, called AT 2024wpp, or “The Whippet”, was first detected by the Zwicky Transient Observatory in late 2024, and quickly became about 10 times brighter than 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 the initial discovery to build up a complete picture of the different wavelengths of light it produced.

The light’s spectrum suggested that the blast responsible must have been more than six times hotter than the surface of the Sun and blasted out plasma at about one-fifth the speed of light. They also found that about a month after the first burst of light, there was another 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 that lacks an outer layer of gas. The researchers say that the Whippet is the result of such a star being swallowed 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 remaining material, orbiting the black hole, later fell back towards the black hole, producing the second X-ray burst. 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 come from a young galaxy, where short-lived extreme stars, such as Wolf-Rayet stars, are more common, Crimes says. “These are the kinds of environments that you would expect to see these kinds of events, and in addition, you see this bump at late times, which could be material falling back after a merger. That’s promising.”