We may have seen a ‘dirty fireball’ star explosion for the first time

Astronomers think they have seen a type of explosion produced by a dying star called a dirty fireball for the first time, and it could help us understand how massive stars die.

When a massive star runs out of fuel, it can collapse and explode in several ways. If a black hole is produced in the collapse, an extremely powerful jet of radiation can burst through the star, producing a flash of high-energy light called a gamma ray burst.

These bursts are among the most powerful explosions in the universe and can emit energy equivalent to the total lifetime output of smaller stars, such as the sun, in a single beam. But astronomers still don’t know exactly how this process works or how variations between different massive stars affect the jet.

Physicists have hypothesised that we might see something different if the jet somehow gets contaminated with heavier matter from the star, such as protons and neutrons. These particles would act as a sponge, slowing the jet down and causing it to emit X-rays, rather than gamma rays. But until now, this “dirty fireball” scenario hadn’t been observed.

Xiang-Yu Wang at Nanjing University in China and his colleagues have now picked up a flash of X-rays called EP241113a that fits the picture of a dirty fireball, using a new space telescope called the Einstein Probe.

Wang and his team detected a flash of light from a galaxy around 9 billion light years away, containing as much energy as a gamma ray burst, but in X-ray frequencies instead. The initial explosion faded to a glow that lasted several hours, before dying out gradually, similar to a typical gamma ray burst.

“It’s a very exciting prospect,” says Rhaana Starling at the University of Leicester, UK. “[Dirty fireballs] have been theorised to exist since the 90s, but there hasn’t really been any compelling evidence for them.”

While we know of thousands of gamma ray bursts, the event producing this blast is likely to be different from others, says Starling. It might be a black hole or neutron star that is interacting with the jet in an interesting physical way, for example. “If it’s a black hole, then we are able to then get a more complete picture of black hole formation across the universe,” she says.

It also shows us that the gamma ray bursts we typically see could be an observational bias, and there could be many more like this or weaker, says Gavin Lamb at Liverpool John Moores University, UK. “There could well be a continuum that goes right the way down to no jets.”

However, we can’t yet be certain that this is a dirty fireball, says Om Sharan Salafia at Brera Astronomical Observatory in Italy. First, we have to establish whether the explosion really did come from a galaxy as distant as Wang and his team claim. “If all of this holds, then indeed, this transient is a bit puzzling,” he says.

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