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The cosmic particle accelerator has reached its limit

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Observatory for Gamma Rays H.E.S.S. shows in unparalleled detail a cosmic particle acceleration mechanism.

Researchers have detected a cosmic particle accelerator for the first time, thanks to the use of specialized telescopes. Observations with the gamma ray observatory H.E.S.S. in Namibia reveal for the first time the trajectory of an acceleration process in a stellar activity known as a nova, which consists of intense eruptions on the surface of a white dwarf. A nova generates a shock wave that blasts through the surrounding material, attracting particles and accelerating them to extraordinary energies. Surprisingly, the nova “RS Ophiuchi” seems to force particles to accelerate at theoretical maximum speeds, matching to optimum circumstances. The findings were reported in the journal Science.

White dwarfs are ancient stars that have burnt out and collapsed in on themselves, resulting in highly compact objects. Novae occurrences occur, for example, when a white dwarf is in a binary system with a big star and, because to gravity, the white dwarf takes material from its more massive partner. When the accumulated material reaches a critical level, it causes a thermonuclear explosion on the white dwarf’s surface. Some novae have been seen to recur. One of these periodic novae is RS Ophiuchi, which explodes on its surface every 15 to 20 years. “The stars in the system are almost the same distance apart as the Earth and the Sun,” says Alison Mitchell, chief investigator of the H.E.S.S Nova program and researcher at Friedrich-Alexander-Universität Erlangen-Nürnberg. “When the nova detonated in August 2021, the H.E.S.S. telescopes enabled us for the first time to detect a galactic explosion in very-high-energy gamma rays,” she says.

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The researchers discovered that the particles were accelerated to energies hundreds of times greater than those previously recorded in novae. Furthermore, the energy generated by the explosion was converted exceptionally effectively into accelerated protons and heavy nuclei, causing particle acceleration to approach theoretical maximum speeds. According to Ruslan Konno, a doctorate candidate at DESY in Zeuthen and one of the study’s primary authors, “The discovery that the theoretical limit for particle acceleration may be attained in true cosmic shock waves has far-reaching consequences for astrophysics. It implies that the acceleration mechanism in their considerably more extreme cousins, supernovae, might be just as efficient.”

During the eruption of RS Ophiuchi, the researchers were able to track the evolution of the nova in real time for the first time, enabling them to view and analyse cosmic particle acceleration as if they were watching a movie. The scientists were able to detect high-energy gamma rays up to one month after the explosion. “This is the first time we’ve ever been able to conduct such observations, and it will help us to obtain even more accurate future insights into how cosmic explosions function,” says Dmitry Khangulyan, a theoretical astrophysicist at Rikkyo University in Tokyo, Japan. “We could learn, for example, that novae add to the ever-present sea of cosmic rays and so have a significant influence on the dynamics of their immediate environs.” Cosmic rays are massive showers of intense subatomic particles that originate from every direction in space at the same time and whose precise origin is unknown.

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These measurements need the use of specialized telescopes. The H.E.S.S. (High Energy Stereoscopic System) facility in Namibia is made up of five Cherenkov telescopes that are used to study gamma rays from space. The biggest telescope has received a new, very sensitive state-of-the-art camera called as FlashCam. The FlashCam concept is now being refined for the Cherenkov Telescope Array, the next generation gamma-ray observatory (CTA). “The new camera has been in operation since late 2019, and this measurement demonstrates exactly how much potential the current generation of cameras has,” says Simon Steinmaßl, a PhD candidate at the Max Planck Institute for Nuclear Physics in Heidelberg who was engaged in the camera data analysis.

After amateur astronomers initially reported the nova to the astrophysics community, the telescopes were directed in the direction of the nova on extremely short notice. The success of the observation was attributed in large part to the researchers’ and the larger astronomical community’s quick response, which paved the path for extended followup observations. “Over the following three years, study utilizing the CTA telescopes will indicate if this form of nova is exceptional,” says H.E.S.S. Director Stefan Wagner, a professor at the regional observatory in Heidelberg. Furthermore, researchers now have a better concept of what to look for. This opens up a slew of new opportunities for acquiring a better knowledge of and explaining novae-related occurrences. “This measurement is yet another triumph in gamma-ray astronomy, and it is an optimistic indicator that we will be able to investigate many more cosmic explosions with future H.E.S.S. and gamma-ray telescopes.”

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