A recently detected gamma-ray burst GRB 221009A was so intense that it temporarily blinded instruments and disturbed Earth’s atmosphere.
An unprecedentedly bright gamma-ray burst (GRB) lit up the gamma-ray sky on October 9th. The burst, cataloged as GRB 221009A, exploded in a galaxy about 2 billion light-years away — close for a GRB. Its brightness and proximity spurred astronomers worldwide to monitor the event from both ground and space.
This event likely belongs to the class of long GRBs, the end-of-life phase of an extremely rare set of massive stars. Once a massive star reaches a point in which the nuclear reactions in its core can no longer produce enough energy to support it, it collapses. If that star is rotating rapidly, the explosion releases material into space in the form of two opposite and narrow jets moving just shy of the speed of light. And if one of those jets points toward us, we see a burst of gamma rays, typically lasting for several minutes. A longer-lived fading afterglow, observable across the electromagnetic spectrum, often follows the initial burst.
Bright Gamma-ray Flash
GRB 221009A was so intense that it temporarily blinded multiple sensitive gamma-ray detectors in space. It’s the brightest GRB detected so far. The energy released by the burst, which researchers were able to estimate thanks to the measured distance to the burst, also puts it among the most energetic GRBs known.
Chinese observatory LHAASO reported the detection of several photons, presumably originating from the GRB, with extremely high energies (technically speaking, in the tera-electron volt range). Among them is also the most energetic photon from any GRB detected to date.
“We have astrophysical sources emitting very high-energy photons, for example, in our galaxy, but it is very difficult to detect such photons from distant objects,” says Paolo D’Avanzo (INAF-Osservatorio Astronomico di Brera, Italy), who observes GRBs and their afterglows with the Neil Gehrels Swift Observatory. High-energy photons from distant sources usually don’t survive the long trek toward Earth because they’re absorbed by material along the way. But when they do, they offer a unique glimpse into the physical processes that occur in extreme environments that we can’t replicate on Earth.
The intense burst even disturbed Earth’s upper atmosphere. The burst’s high-energy photons ionized the ionosphere and modified its radio-propagation properties. While GRBs are not common sources of these so-called sudden ionospheric disturbances — the culprits are typically solar flares — some energetic GRBs in the past have left similar imprints.
The burst’s afterglow is also off the scale. At X-ray energies, the afterglow is around 1,000 times brighter than typical GRBs. Various observatories have also detected the afterglow across the electromagnetic spectrum, all the way down to radio waves.
The observations also have implications closer to home. Early X-ray images revealed a series of prominent rings around the location of the GRB 221009A, but these features aren’t physically related to the explosion. The rings appear because the X-rays are scattering off of dust grains in interstellar clouds within our own galaxy. Such rings have been observed around several bright sources, including a few GRBs.
The rings around GRB 221009A are especially prominent thanks to its brightness and its location in the sky, which puts it close to the dust-rich galactic plane. By analyzing the rings and their evolution with time, scientists can study the size, composition, and spatial distribution of dust grains in detail.
The afterglow of this GRB will likely shine for months to come, and astronomers will keep their eyes on it. The exquisite data set will allow scientists to test various ideas concerning the origin and evolution of the gamma-ray burst explosions. D’Avanzo wonders whether the data will enable us to understand what powers the explosion: a black hole or a neutron star. “Literature predicts what signatures a neutron star engine would leave on the emission. I think it would be worth spending time on data to see if the proposed signatures are there.”
A supernova often accompanies gamma-ray bursts of this type. It usually takes about two to three weeks for such a supernova to peak in brightness, at which point it can outshine the afterglow. “Assuming we will detect it, and I think we will, we want to know whether the supernova is like a typical supernova associated with a gamma-ray burst, or if it is a peculiar one,” says D’Avanzo.
Gamma-ray astronomers are in for some exciting months to come.