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Like many other X-ray sources, Cygnus X-3 seems to be a binary system that includes a compact object, but it is unusual in being enshrouded in gas, in displaying occasional huge outbursts and in emitting much more energy above 1011eV than in the keV X-ray region. If the compact object is a pulsar, the ultra-high-energy photons can be products of an accelerated beam of protons (or nuclei) striking the companion's atmosphere1,2. It is noted here that a roughly monoenergetic spray of ∼1017eV protons at ∼1039erg s−1, generating an electron–photon cascade in the environs of the companion star, best accounts for all photons above a few GeV, and might generate the gas shroud, as beam energy deposition inside the star could lift off a thick layer of matter to generate a fountain under the pulsar. This evidence on the nature of the accelerated beam is particularly significant as Cygnus X-3 seems likely to be responsible for most of the cosmic ray particles around 1017eV which are at present being injected into the Galaxy.