An unusual burst of star formation is observed – with huge acceleration of particles

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On August 8, 2021, astronomers recorded a thermonuclear explosion on the star RS Ophiuchi, located at a distance of 7500 light years. Even with the naked eye, the star could be seen as a so-called nova in the constellation of Ophiuchus. Using the HESS telescope system in Namibia and the Fermi Satellite Observatory, an international team of researchers has for the first time been able to detect high-energy gamma radiation during the explosion of such a star. Apparently, the nova eruption has led to a huge acceleration of electrically charged particles in the area, scientists report in the journal Science.

RS Ophiuchi is a binary star consisting of a white dwarf and a red giant, orbiting about one and a half times farther than the distance from Earth to the Sun. When stars like our Sun have used up their nuclear energy, they first swell into a red giant and then collapse into a white dwarf, a compact star about the size of Earth that slowly cools down.

In a binary system such as RS Ophiuchi, matter – primarily hydrogen – constantly flows from the red giant’s atmosphere to the white dwarf. If enough hydrogen has accumulated there, a thermonuclear explosion occurs, similar to a hydrogen bomb, in which hydrogen is converted into helium. Astronomers first observed such a nova outburst in 1898 on RS Ophiuchi. Last August’s outburst was the ninth star explosion recorded by scientists.

For the first time high-energy radiation of the New

Since astronomers had been expecting a new outburst of a new star on RS Ophiuchus for a long time, they were ready: just a day later, they directed the antennas of the HESS telescope system to the corresponding part of the sky. With this system, researchers can capture radiation caused by high-energy gamma rays in the Earth’s atmosphere. The team has been successful: this is the first time such high-energy radiation from a new star has been detected.

For a whole month, astronomers watched the gamma radiation, which first steadily increased, reached a maximum, and finally decreased again. The Fermi satellite, equipped with gamma-ray detectors, provided additional data from the American space agency NASA. Then, based on data on the course and energy distribution of gamma radiation, the researchers drew conclusions about the physical processes in the vicinity of the nova.

“The most likely scenario is the strong acceleration of protons and atomic nuclei by the expanding shock wave of the explosion,” says Brian Reville of the Institute of Nuclear Physics. Max Planck in Heidelberg. “These particles then collide with compressed stellar wind material that is blown into space by the red giant. And that releases gamma rays.” Reville was instrumental in the modeling required to interpret the data.

According to the astrophysicist, this means that such bursts of novae are effective space accelerators that accelerate particles to energies corresponding to the theoretical maximum, and thus shower their surroundings with a large number of high-energy particles. Thus, they make a significant contribution to cosmic radiation in the vicinity of the new star. And this came as a surprise to astronomers: until now, only supernovae, much stronger explosions of stars, were considered such effective particle accelerators.

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