A record-breaking gamma-ray burst detected in October 2022 has now been described as a once-in-a-thousand-year event.
It's called GRB 221009A, and with up to 18 teraelectronvolts of energy packed into its light emissions, it's considered the most powerful gamma-ray burst ever recorded.
We've been waiting to learn more about this incredible explosion, and now the analyses have begun to arrive on the preprint server arXiv, with a trio of papers submitted to The Astrophysical Journal Letters.
According to the analyses, this extraordinary burst breaks the rules: the light curve of its afterglow doesn't fit neatly into theoretical descriptions of how it should go, suggesting that there's something interesting and unique about GRB 221009A.
To recap, gamma-ray bursts are the most violent explosions in the universe, erupting in fire and fury so powerful that they release more energy than the sun would in 10 billion years. The bursts of electromagnetic radiation are caused by cataclysmic events: the supernova or hypernova explosions of massive stars at the end of their lives, or the collision of binary systems containing at least one neutron star.
GRB 221009A was first detected on October 9, 2022, and was initially thought to be a low-power X-ray flash from a relatively nearby source. However, follow-up observations revealed that the flash came from much farther away than first thought - 2.4 billion light-years (which still makes it one of the closest gamma-ray bursts ever detected) - meaning that it was also much more powerful than first thought.
For the first 73 days after the initial discovery, astronomers watched it eagerly, tracking the evolution of its light curve, the shape the intensity of the light makes on a graph over time. They had to stop after about 70 days as the afterglow moved behind the Sun, but it is expected to reappear around now.
 |
| Light echoes from the gamma-ray burst, produced by the light traveling through thick dust as it moves towards us, creating an "expanding ring" effect. (Williams et al., arXiv, 2023) |
In a paper led by Maia Williams of Pennsylvania State University, a team of astronomers found that the X-ray afterglow of GRB 221009A immediately after the burst was the brightest ever detected by the Swift observatory, by an order of magnitude. In a simulation of randomly generated bursts, only one in 10,000 was as powerful as GRB 221009A.
When distance was taken into account, the brightness of GRB 221009A was consistent with other gamma-ray bursts in the Swift catalog. Others simply appear dimmer because they're farther away. According to the team's calculations, it's the combined characteristics of GRB 221009A that make it very rare indeed.
"We estimate," they write, "that GRBs as energetic and nearby as GRB 221009A occur at a rate of ≲1 per 1000 yr - making this a truly remarkable opportunity that is unlikely to be repeated in our lifetimes."
What makes the GRB truly peculiar is the evolution of the afterglow, which doesn't fit the standard theory. Gamma-ray bursts are typically followed by a glow from electrons traveling at near-light speeds. This is called synchrotron emission, and it's the result of the shocks that form as the initial explosion slams into the interstellar medium.
The gamma-ray bursts themselves are thought to consist of energy concentrated in parallel beams that form highly collimated jets. Studying the resulting synchrotron emission can help astronomers figure out the shape of the explosion and the jets.
According to Williams and her team, the afterglow suggests that either the jet structure of GRB 221009A is more complicated than expected, or it's not tightly collimated. The latter scenario, they say, will have profound implications for the energy budget of the event.
In another paper, led by Tanmoy Laskar of the University of Utah, a team of astronomers suggests that the peculiar afterglow could mean that there's an additional source of synchrotron emission in the gamma-ray burst's afterglow, but the implications could be more serious. The problem, they suggest, may be something fundamentally wrong with the theory of synchrotron afterglows.
And a third paper, led by astronomer Manisha Shrestha of the University of Arizona, finds that the afterglow doesn't contain some of the features you'd expect to see in a supernova explosion. This, they note, could mean that most of GRB 221009A's energy budget was spent on the jets, leaving very little to suggest that an exploding star was responsible.
The afterglow is expected to re-emerge from behind the Sun this month, and should still be very visible to our telescopes at several wavelengths. Whatever is going on with this strange explosion, astronomers will be working very hard to get to the bottom of it.
The research papers have all been submitted to The Astrophysical Journal Letters, and are available on preprint server arXiv. They can be found here, here, and here.
