Scientists Detect Rare Cosmic Explosion That Defies Current Physical Theory

An observatory in Namibia has recorded the most energetic radiation and the longest gamma-ray glow from a gamma-ray burst (GRB).

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Observations made with the High Energy Stereoscopic System (HESS) challenge the established idea of ​​how gamma rays are produced in these colossal stellar explosions which are the birth cry of black holes, according to the international team of researchers in the journal ‘Science’.

“Gamma ray bursts are bright flashes of X and gamma rays observed in the sky, emitted by distant extragalactic sources”, explains in a statement the scientist of the German Synchrotron of Electrons (DESY) Sylvia Zhu, one of the authors of the article.

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“They are the biggest explosions in the universe and they are associated with the collapse of a massive star that rotates rapidly until it becomes a black hole – he continues -. A fraction of the released gravitational energy fuels the production of an ultrarelativistic shock wave. Its emission is divided into two distinct phases: an initial phase of chaotic impulse that lasts tens of seconds, followed by a phase of long-lasting glow and soft fading ”.

On August 29, 2019, the Fermi and Swift satellites detected a gamma-ray burst in the constellation Eridanus. The event, listed as GRB 190829A according to its date of occurrence, turned out to be one of the closest gamma ray bursts observed so far, with a distance of about a billion light years. For comparison, the typical gamma-ray burst is about 20 billion light-years away.

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“We were really sitting in the front row when this gamma ray burst happened,” celebrates DESY co-author Andrew Taylor. The team caught the post-explosion glow immediately when it became visible to the HESS telescopes. “We were able to observe the afterglow for several days at unprecedented gamma ray energies,” añade Taylor.

The comparatively short distance to this gamma ray burst allowed detailed measurements of the glow spectrum, which is the distribution of ‘colors’ or energies of the radiation photons, in the very high energy range.

“We were able to determine the spectrum of the GRB 190829A down to an energy of 3.3 tera-electron volts, that is, a trillion times more energetic than photons in visible light -explains co-author Edna Ruiz-Velasco, from the Max Planck Institute for Nuclear Physics in Heidelberg, in Germany-. Here’s what’s unique about this gamma-ray burst: it happened in our cosmic backyard, where very high-energy photons were not absorbed in collisions with backlight on their way to Earth, as they do at greater distances in the cosmos. ”.

The team was able to follow the afterglow for up to three days after the initial explosion. The result was a surprise: “Our observations revealed curious similarities between the X-ray emission and the very high energy gamma ray emission from the post-blast glow.”, informa Zhu.

Established theories assume that the two components of the emission must be produced by separate mechanisms: the X-ray component originates from ultra-fast electrons that are deflected in the strong magnetic fields around the explosion. This ‘synchrotron’ process is quite similar to the way that Earth’s particle accelerators produce bright X-rays for scientific investigations.

However, based on existing theories, it seemed highly unlikely that even the most powerful explosions in the universe could accelerate the electrons enough as to directly produce the observed very high energy gamma rays.

This is due to the ‘burn limit’, which is determined by the balance between acceleration and cooling of the particles within an accelerator. The production of very high energy gamma rays requires electrons with energies well above the burn limit. Instead, current theories assume that in a gamma-ray burst, fast electrons collide with synchrotron photons and thus drive them to gamma-ray energies in a process called an autocompton synchrotron.

But the afterglow observations of GRB 190829A now show that both components, the X-rays and the gamma rays, faded in a synchronized fashion. Furthermore, the gamma-ray spectrum was clearly consistent with an extrapolation of the X-ray spectrum. Taken together, these results are a strong indication that the X-rays and the very high-energy gamma rays from this glow were produced by the same mechanism.

“It is quite unexpected to observe such similar spectral and temporal characteristics in the energy bands of X-rays and very high-energy gamma rays, if the emission in these two energy ranges had different origins,” says co-author Dmitry Khangulyan of Rikkyo University in Tokyo. This poses a challenge for the auto-compton synchrotron origin of the emission of very high energy gamma rays ”.

The great implication of this possibility highlights the need for further studies on the post-shock emission of very high energy GRBs. GRB 190829A is only the fourth gamma-ray burst detected from the ground.

However, the previous explosions detected occurred much further out in the cosmos and their afterglow could only be observed for a few hours each and not up to energies above 1 tera-electron volt (TeV).

“Looking to the future, the prospects for detecting gamma ray bursts using new generation instruments such as the Cherenkov Telescope Array currently being built in the Chilean Andes and on the Canary Island of La Palma appear promising”says HESS spokesperson Stefan Wagner of Landessternwarte Heidelberg.

“The general abundance of gamma-ray bursts leads us to expect that regular detections in the very high-energy band will become quite common, helping us to fully understand their physics.” concludes.



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