The outburst of the recurrent nova  RS Ophiuchi (RS Oph) in 2021 confirms the predictions made in 2006 by researcher Margarita Hernanz, from the Institute of Space Sciences (ICE-CSIC) and the Institut d'Estudis Espacials de Catalunya (IEEC), about its emission of very high-energy gamma rays related to particle acceleration.

Located 5,000 light years away from Earth, RS Oph is an unstable binary system formed by a red giant star and a white dwarf, which regularly experiences thermonuclear explosions every 15 or 20 years. It has recorded outbreaks in 1898, 1933, 1958, 1967, 1985, 2006 and 2021. In fact, all novae are recurrent, but only very few of them have recurrence periods short enough (shorter than 100 years) so that they have more than one recorded outburst.

Its last eruption in 2021 was detected by the Large Area Telescope (LAT) of the Fermi mission, the Cherenkov Telescopes Array H.E.S.S. in Namibia and the MAGIC Cherenkov Telescopes in the Canary Islands (Spain). This was the first time an explosion like this was observed in the Milky Way in very high-energy gamma rays by ground-based telescopes.

Particle acceleration was not considered feasible in novae until the observation of RS Oph, so it has become a key object for studying the microphysics of particle acceleration in astrophysical shock waves.

The 2006 explosion was studied by the researcher Margarita Hernanz from the Institute of Space Sciences (ICE-CSIC) and the Institut d'Estudis Espacials de Catalunya (IEEC), in a study published in The Astrophysical Journal. In this paper, co-authored by Hernanz with V. Tatischeff, the authors analysed the observations of the outburst with radio, infrared, and X-ray telescopes, and studied the shock wave generated by the explosion that swept at thousands of kilometres per second the red giant star wind.

Hernanz and Tatischeff predicted an acceleration of protons in this shock wave to energies of several teraelectronvolts (TeV) and a potentially observable gamma emission for several days after the explosion. This was the first prediction ever of such a process in a nova explosion.

It was deduced from observations at all wavelengths in 2006 that particle acceleration had occurred in RS Oph as a consequence of strong collisions between the nova's ejecta and the red giant's wind. At that time, gamma rays could not be detected due to the absence of sensitive enough gamma instruments. Nonetheless, the observation of the RS Oph explosion in 2021 confirms the 2006 predictions very accurately.

Existing observations at different wavelengths of this nova explosion in 2006 were crucial to discover particle acceleration (protons and electrons) and contributed to the study of diffusive acceleration theory in novae, previously applied to supernova explosions only. "The fact that the process is much faster in novae than in supernovae -because less mass is involved in nova explosions- will allow us to study such processes in more detail", says Margarita Hernanz.