Beams from brightest gamma ray burst ever seen were pointed directly at Earth

Plus: Boffins puzzle over why GRB 221009A's shape is unlike any other astronomical electromagnetic jets

The brightest gamma-ray burst ever detected yet, codenamed GRB 221009A, has a strange structure that astronomers have never seen before.

On 9 October 2022, high-energy monitoring space satellites detected an extremely bright gamma-ray signal that suddenly exploded from the constellation of Sagitta; it lasted hundreds of seconds. The burst was later determined to be the most energetic flare observed by gamma-ray observatories, and is over a magnitude brighter than previous record breakers.

Astronomers nicknamed GRB 221009A BOAT (the brightest of all time), and have been observing its afterglow ever since. 

"It is 70 times brighter than any other gamma-ray burst discovered in over 50 years of observation," Brendan O'Connor, an astronomer at the University of Maryland, told The Register. O'Connor is first author of the study published in Science Advances, and was at the time a PhD student at the University of George Washington,

O'Connor and his colleagues found that the jets emitted from the event – they estimated humans' chance to observe a similar event would come around only once in 1,000 years – have an odd shape with no clear edge, unlike other gamma-ray bursts. GRB 221009A is described as having a narrow core with wide sloping wings, whereas other similar bursts are shaped more like ice cream cones.

Part of the reason why the explosion is so bright is because the beams were directly pointed at Earth, according to Hendrik Van Eerten, co-author of the paper and a researcher at the University of Bath. Van Eerten described it as "much like a garden hose angled to spray straight at you." But that alone isn't enough to explain why it's so bright, and its properties don't seem to fit observations for most predictive models.

"The combination of extreme gamma-ray brightness and never-ending  afterglow is fatal for most models because it creates an energy crisis," Eleonora Troja, lead author of the paper and an associate professor at the University of Rome, explained to us. "Using standard theories we were faced with a serious problem: this burst was a hundred times more powerful than what we could possibly explain."

The energy and structure of GRB 221009A stem from the star that produced it, leaving behind a black hole estimated to be 30 times the mass of the Sun. "The only way to produce a different jet structure and vary the energy is to vary some property of the star that exploded, like its size, mass, density or magnetic field. That's because the jet has to basically force its way out of the star, so, for example, the amount of resistance it meets would potentially influence the features of the jet," O'Connor said.

It's possible that the burst mixed with the star's material and produced waves of shock-heated gas that astronomers are still detecting in the afterglow. They hope to continue studying GRB 221009A and believe it will still be observable for a few more years. As it starts to fade, the team will be looking out for any changes and possible interactions with objects in its environment to figure out more information about the galaxy and star it was birthed from.

"Gamma-ray bursts trace the deaths of massive stars and can allow us to trace the star formation history of the universe and also specifically the star formation in the galaxies in which they form. They are explosive phenomena exhibiting the most extreme states of matter and magnetic fields, allowing us to observe physical processes that are not reproducible on Earth," O'Connor concluded. ®

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