Rare Space Explosion Of A Millennium Just Occurred
In October 2022, scientists detected an unprecedented gamma-ray burst, now identified as a once-in-a-thousand-year event, named GRB 221009A. This gamma-ray burst emitted up to 18 teraelectronvolts of energy, making it the most powerful one ever recorded.
Scientists have eagerly awaited further analysis, and now, three papers on the subject have been submitted to The Astrophysical Journal Letters and are available on the preprint server arXiv.
The recent analyses of GRB 221009A reveal that this extraordinary gamma-ray burst is defying conventional wisdom. Its afterglow light curve is deviating from theoretical predictions, suggesting there is something exceptional about this event.
Gamma-ray bursts are the most intense explosions known to occur in the Universe, releasing more energy in seconds than the Sun would produce in 10 billion years. These bursts of electromagnetic radiation are caused by catastrophic events such as supernova or hypernova explosions at the end of massive stars’ lifetimes or collisions of binary systems involving at least one neutron star.
Initially, on 9 October 2022, GRB 221009A was thought to be a relatively weak flash of X-rays originating from a nearby source. However, further investigation revealed that the burst traveled from a distance of 2.4 billion light-years, making it one of the closest gamma-ray bursts ever detected and much more powerful than initially estimated.
Over the course of 73 days, astronomers closely monitored the burst, tracking the evolution of its light curve, which displays the intensity of light over time. Observations had to be halted around the 70-day mark when the afterglow moved behind the Sun but are expected to resume soon.
In a recently published paper led by Maia Williams of Pennsylvania State University, a team of astronomers reported that the X-ray afterglow of GRB 221009A was the brightest ever detected by the Swift observatory, surpassing all previous observations by an order of magnitude. Their simulation of randomly generated bursts showed that only one in 10,000 bursts were as powerful as GRB 221009A.
Once the distance was taken into account, the brightness of GRB 221009A was consistent with other gamma-ray bursts in the Swift catalog, with the apparent dimming being a result of distance. The team’s analysis revealed that it is the combination of GRB 221009A’s characteristics that makes it an exceptionally rare event.
The evolution of the afterglow of GRB 221009A is what truly sets it apart from other gamma-ray bursts, as it does not adhere to the standard theory. After a gamma-ray burst, electrons typically emit synchrotron radiation, a glow resulting from the shocks formed when the initial explosion collides with the interstellar medium.
The gamma-ray bursts themselves are thought to consist of energy concentrated in parallel rays that form highly collimated jets. Studying the ensuing 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 either more complicated than expected, or isn’t narrowly collimated. The latter scenario, they say, will have profound implications for the event’s energy budget.
In another paper led by Tanmoy Laskar from 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 afterglow of the gamma-ray burst, but the implications could also be more serious. The problem, they suggest, could be something fundamentally wrong with synchrotron afterglow theory.
And a third paper led by astronomer Manisha Shrestha from 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 find, could mean that most of the energy budget of GRB 221009A was spent on the jets, leaving very little behind to suggest an exploding star was responsible.
The afterglow is expected to re-emerge from behind the Sun this month, and is expected to still be very visible to our telescopes in multiple wavelengths. Whatever is going on with this peculiar explosion, astronomers are going to be working very hard to get to the bottom of it.