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

A sudden, mysterious flash of bright light in the sky could be from a black hole devouring a massive, unusually naked star.

In 2018, astronomers noticed a new kind of cosmic explosion that became brighter faster than any other. Called AT2018cow, or “cow” for short, the flash took just a few days to reach its peak brightness, rather than the weeks required for typical supernovae.

There was no initial obvious explanation for such a burst, and in the years since the first discovery of the Cow, we have seen only a few other similar bursts, collectively called fast blue optical transitions (FBOTs). Their origin remains a mystery.

Now, Jialian Liu at Tsinghua University in China and his colleagues believe that the recent cosmic flash, the brightest of all FBOTs so far, must be the result of an exotic star more than 30 times the mass of our Sun that has lost its outer hydrogen layers and been devoured by a black hole.

Called AT 2024wpp, or “oha,” the explosion was first spotted by the Zwicky Transient Observatory in late 2024 and quickly became about 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 a complete picture of the different wavelengths of light it produced.

The spectrum of light indicated that the explosion responsible must have been more than six times hotter than the surface of the sun, ejecting plasma at about one-fifth the speed of light. They also found that about a month after the first flash of light, there was a new burst of X-rays that had never been seen in a previous FBOT.

The best explanation for these observations, say Liu and his team, is an unusual star called a Wolf-Rayet star, which has a bare stellar core without an outer layer of gas. Scientists say 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 to the black hole and produced the second burst of X-rays. That’s a compelling argument for what happened, he 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 extremely short stars like Wolf-Rayet stars are more common, Crimes says. “These are the kinds of environments where you would expect to see this kind of event, plus you’re going to see this bump at late times, which may be material that recedes after the merger. It’s promising.”

topics: