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Recent gamma-ray burst was 70 times brighter than any cosmic explosion

On October 9, 2022, astronomers detected what is considered to be the brightest and most energetic gamma-ray burst ever observed.

As most gamma-ray bursts, the so-called GRB 221009A occurred when the core of a massive star collapsed, becoming a black hole – a type of event which regularly releases as much energy in just a few minutes as our Sun will release during its entire lifetime. 

However, analyses revealed that GRB 221009A was 70 times brighter than the previous record holder, for reasons not yet clearly understood. Now, observations from NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array) observatory have shed new light on this mystery.

Unique characteristics of GRB 221009A

By using NuSTAR observations, an international team of scientists has shown how the collapsing star ejected a jet of material not previously seen among gamma-ray bursts, along with other unique characteristics. Researchers believe that the origin of these differences may be related to the progenitor star, whose physical properties could influence the features of the jet.

“This event was so much brighter and more energetic than any gamma-ray burst we’ve seen before, it’s not even close,” said lead author Brendan O’Connor, an astronomer at George Washington University. “Then, when we analyzed the NuSTAR data, we realized that it also has this unique jet structure. And that was really exciting, because we have no way of studying the star that produced this event; it’s gone now. But we now have some data giving us clues about how it exploded.”  

Gamma rays are the most energetic form of light in the universe, yet they are invisible to the human eye. Although all known gamma-ray bursts were detected outside our galaxy, they are bright enough to be spotted from distances of billions of light-years. 

While some of these cosmic explosions are very brief, lasting less than two seconds, so-called long gamma-ray bursts usually radiate gamma rays for a minute or more, and other wavelengths even for weeks.

More exceptional properties of the gamma-ray burst

GRB 221009A was a long, extremely bright burst that has blinded most gamma-ray instruments in space. To determine its actual brightness, scientists reconstructed the event with data from NASA’s Fermi Gamma-ray Space Telescope. As other previously detected gamma-ray bursts, GRB 221009A had a jet which erupted from the collapsing star as if shot into space by a fire hose, with gamma rays violently radiating from the hot gas and particles at the jet’s core. 

However, while in other bursts, the jet remained remarkably compact and there was very little stray light or other materials outside its narrow beam, in GRB 221009A the jet had a narrow core with wider, sloping sides. 

Although some of the most energetic gamma-ray jets have exhibited similar properties, in this case, the energy of the material also varied. Thus, instead of all the material in the jet having the same energy, like a single bullet from a shot, the energy of the material appeared to change with distance from the jet’s core – a phenomenon never observed before in a long gamma-ray burst jet.

“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,” said co-author Eleonora Troja, a professor of Physics at the University of Rome, who led NuSTAR’s observations of the event. “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.”

Further research is needed 

By observing this burst with a variety of X-ray telescopes – including NASA’s Neil Gehrels Swift Observatory, the Neutron star Interior Composition Explorer (NICER), and ESA’s XMM-Newton telescope – the scientists have also found that, as the jet traveled into space, it collided with the interstellar medium (the sparse collection of atoms and particles that fills the space between stars), creating X-rays, or particles of slightly less energetic potential than gamma rays.

However, further research is needed to clarify the differences between this gamma-ray burst and previous ones and better understand the complex factors behind such extraordinary astronomical occurrences. Fortunately, the necessary technology for observing and interpreting such events is already well developed. 

“There are multiple X-ray telescopes operating in space, each with different strengths that can help astronomers understand these cosmic objects better,” concluded Daniel Stern, a NuSTAR project scientist.

The study is published in the journal Science Advances.

Image credit: NASA / Swift / Cruz deWilde

By Andrei Ionescu, Staff Writer

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