Nova explosions -- The fascinating gamma-ray emitting recurrent nova RS Ophiuchi (2302.01276v1)

Abstract: Classical and recurrent nova explosions occur on top of white dwarfs accreting H-rich matter from a companion main sequence or red giant star, in a close binary system. In the recent years, since the launch of the Fermi gamma-ray satellite by NASA in 2008, several novae have been detected by Fermi/LAT (LAT: Large Area Telescope) in high-energy (HE) gamma rays, with energies larger than 100 MeV. This emission is known to be related to the acceleration of particles in the internal and/or external shocks occurring early after the thermonuclear nova explosion. However, very-high-energy (VHE) gamma-rays, with energies larger than 100 GeV, produced as a consequence of nova explosions have only been discovered very recently, in the recurrent nova RS Oph, that had an outburst in August 2021. These require the acceleration of protons, and not only of electrons; this was in fact predicted theoretically - based in observations at other wavelengths - in the previous eruption of RS Oph, in 2006, but has not been confirmed observationally until now. We review the origin of the different types of gamma-ray emission in novae and highlight the relevance of the recent VHE gamma-ray emission discoveries for the nova theory, mainly in the field of the mass ejection and the associated particle (electrons and protons) acceleration processes.