With the help of specialised telescopes, astronomers have discovered a kind of natural particle accelerator in the depths of space, many of whose parameters are very, very close to the maximum possible theoretical values. Observations carried out with the gamma-ray observatory H.E.S.S. in Namibia have allowed scientists to study all the subtleties of the particle acceleration process during a cosmological event called the nova blast, which is a powerful explosion on the surface of a white dwarf star.
A nova explosion creates a shock wave that passes through the environment surrounding the explosion, trapping particles and accelerating them to extremely high energies. And in the case of a nova explosion called RS Ophiuchi, the particles are accelerated to speeds nearly reaching the theoretical limit of just over 100 GeV, possible only with the perfect coincidence of many conditions.
White dwarf stars are the remnants of stars of a certain type and mass, which, at the end of their life cycle, neither exploded in a supernova nor turned into a black hole. All the matter of a dead star collapsed under its own gravity to form a compact, hot and extremely dense object. New explosions occur when a white dwarf is located in a binary system and its gravity rips the upper layers of matter from its partner star. When the amount of 'stolen' matter crosses a critical limit, a massive thermonuclear explosion occurs on the surface of the white dwarf.
In some binary systems, such explosions are repeated with a certain periodicity, in particular, in the RS Ophiuchi system, in which the stars are located at a distance approximately equal to the distance from the Earth to the Sun, explosions follow after 15-20 years. "When another explosion occurred in the RS Ophiuchi system in 2021, we were ready and able to look at all the details in the high-energy part of the X-ray range," the researchers wrote.
Particles capable of emitting such high-energy X-rays must have been accelerated to thousands of times the acceleration rate observed in other new explosions. Moreover, some of the conditions around RS Ophiuchi are such that the explosion energy is used quite effectively to accelerate protons and heavier element nuclei, contributing to the acceleration to a speed whose maximum limit has been calculated by specialised mathematical models.
All the subtleties of the most powerful natural particle accelerator have been studied using the capabilities of the H.E.S.S. (High Energy Stereoscopic System) gamma-ray observatory. This observatory, located in Namibia, includes five gamma-ray telescopes on the Cherenkov effect, which visualize gamma rays coming from the depths of space. The H.E.S.S. observatory's largest telescope recently installed a new FlashCam, which has the highest sensitivity. The FlashCam was originally designed for use with the next-generation Cherenkov Telescope Array (CTA) gamma-ray observatory, and the H.E.S.S. Observatory tested it.
In a few years' time, once the CTA telescopes are operational, scientists will try to catch cases of other new explosions. By comparing the available and new data, they will be able to answer the question, is the RS Ophiuchi system common, or is it some kind of special system, only in which such exotic high-energy phenomena are possible?
