  Galactic sites of acceleration of cosmic rays to energies of order 10^15 eV
and higher, dubbed PeVatrons, reveal themselves by recently discovered gamma
radiation of energies above 100 TeV. However, joint gamma-ray and neutrino
production, which marks unambiguously cosmic-ray interactions with ambient
matter and radiation, was not observed until now. In November 2020, the IceCube
neutrino observatory reported an ~150 TeV neutrino event from the direction of
one of the most promising Galactic PeVatrons, the Cygnus Cocoon. Here we report
on the observation of a 3.1-sigma (post trial) excess of atmospheric air
showers from the same direction, observed by the Carpet-2 experiment and
consistent with a few-months flare in photons above 300 TeV, in temporal
coincidence with the neutrino event. The fluence of the gamma-ray flare is of
the same order as that expected from the neutrino observation, assuming the
standard mechanism of neutrino production. This is the first evidence for the
joint production of high-energy neutrinos and gamma rays in a Galactic source.
