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Bright gamma-ray flash from close-up surprises astronomers

A gamma-ray burst erupted in August 2019. With a distance of “only” a billion light-years, it is easily observable from Earth – and researchers are surprised. Because the gamma-ray flash of GRB 190829A can disprove popular theories, scientists now write in the journal Science.

It’s surprisingly different: the converging and particularly bright gamma-ray burst baffles astronomers. Because its light curve does not fit the current theory. According to this, especially high-energy gamma rays for such an explosion must be generated through a different process than X-rays. But this does not appear to be the case with the gamma-ray flash, which is about a billion light-years away, the team reported in the specialized journal Science.

Gamma ray bursts are among the brightest and most energetic phenomena in the universe. Its high-energy radiation can be detected over billions of light years, and some of these explosions were visible to the naked eye in the sky. While the origin of particularly short gamma-ray bursts is still disputed, gamma-ray bursts with a long flare are, according to current theory, caused by supernovae – the explosion and collapse of massive stars into a black hole.

Gamma-ray flashes from “only” a billion light-years away

But now astronomers have caught an unusual gamma-ray burst in several ways. NASA’s Fermi and Swift satellites were the first to detect the sudden flash in the X-ray and gamma range on August 29, 2019. After a few minutes, several ground-based telescopes were turned on to follow the further development and subsequent glow of this volcanic eruption, GRB 190829A, at different wavelengths.

The first data already showed that the gamma-ray burst was one of the following observed events of this type: the source of the intense radiation was only a billion light-years away – this is relatively close to the gamma-ray burst. . “We really saw the front row gamma-ray burst,” says co-author Andrew Taylor of the German DESY synchrotron.

First look at the active aurora

This proximity enabled the team to more fully capture the energetic parts of the afterglow than previously possible. Because despite energies of several TeV, gamma rays are usually engulfed at greater cosmic distances. However, with GRB 190829A, astronomers were able to perform the gamma afterglow for the first time over the course of several days using HESS Observatory telescopes. in Namibia Continue.

“This is what makes this gamma-ray burst so extraordinary – it happened in our immediate cosmic neighborhood such that its very high-energy photons were not absorbed by background light collisions,” explains co-author Edna Ruiz-Velasco of Max Planck. Institute of Nuclear Physics in Heidelberg. “We were able to measure the spectrum of GRB 190829A up to an energy of 3.3 TeV, which is about a trillion times more energy than visible light.”

Radiation patterns contradict the theory

Observations revealed that the light curve of gamma radiation in the aurora was surprisingly similar to that of parallel X-ray light. Both components faded out almost simultaneously and the gamma spectrum showed properties similar to those of X-rays – thus the gamma spectrum was just an extension of the X-ray spectrum into regions of higher energy.

Surprisingly, these observations contradict the current theory of the origin of these two forms of radiation. Then, the X-ray radiation of the afterglow is generated in a similar way to a synchrotron: the shock wave from the explosion accelerates electrons to almost the speed of light, and then emits X-ray light under the influence of strong magnetic fields.

The component of gamma rays differs in the later aurora: according to the popular assumption, it appears only in a second step, called Synchrotron Self Compton (SSC). In this case, the strongly accelerated electrons collide with the synchrotron photons generated by their peers and raise them to higher energies. Gamma rays have a shorter wavelength than X-rays.

Looking for explanations

But this theoretical scenario does not match the one observed in GRB 190829A. Because according to him, X-rays and gamma rays should show different spectra and different light curves. But this is not the case. Instead, the significant similarities between the two radiation components indicate that they resulted from the same process. The new observations put the classic two-step model into question.

However, this raises the question of how this is possible and what physical processes are behind it. So far, researchers can only speculate about it. One possibility is that the jet of particles from the star’s explosion accelerated more strongly than previously assumed. However, it will be difficult to reconcile this with dynamic models of the eruption, according to the team. It is also possible to imagine that the particles have a different energy distribution – but this would require harsh conditions in the surrounding cosmic medium.

Hope for more gamma ray bursts nearby

Now astronomers hope that more gamma-ray bursts will be seen by astronomers. However, only close gamma-ray bursts provide such detailed data – and these are extremely rare: GRB 190829A are only the fourth gamma-ray bursts that are close enough to show sufficient gamma radiation in the later aurora.

“Next generation instruments such as the Cherenkov Telescope Array currently in Chile HESS spokesperson Stefan Wagner from the State Observatory in Heidelberg says (Science, 2021; doi: 10.1126/science.abe8560)

Source: German Electron Synchrotron DESY

This article was written by Nadja Podbregar

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