Classical novae are cataclysmic binary star systems in which the matter of a
companion star is accreted on a white dwarf (WD). Accumulation of the matter in
a layer eventually causes a thermonuclear explosion on the surface of the WD,
brightening the WD to ~ 10 5 solar luminosities and triggering ejection of the
accumulated matter. They provide extreme conditions required to accelerate
particles, electrons or protons, to high energies. Here we present the
detection of gamma rays by the MAGIC telescopes from the 2021 outburst of RS
Ophiuchi (RS Oph), a recurrent symbiotic nova, that allowed us, for the first
time, to accurately characterize the emission from a nova in the 60 GeV to 250
GeV energy range. The theoretical interpretation of the combined Fermi -LAT and
MAGIC data suggests that protons are accelerated to hundreds of GeV in the nova
shock. Such protons should create bubbles of enhanced Cosmic Ray density up to
about 13 pc from the recurrent novae.
