Cygnus X-3: a powerful galactic source of hard radiation

A review is given of experimental and theoretical research on the galactic source Cyg X-3, whose electromagnetic spectrum extends from radio frequencies to ultrahigh-energy (E$_\gamma\sim$ 10$^{16}$ eV) $\gamma$-rays. Cyg X-3 also has a high x-ray luminosity (10$^{38}$ erg/sec) and exhibits diversified sporadic and periodic variations, most notably occasional radio outbursts and a 4h.8 infrared, x-ray, and $\gamma$-ray cycle. Analysis of the observations indicates that Cyg X-3 is a close binary system comprising a compact relativistic object (neutron star, black hole) and a dwarf companion losing mass. Particles are accelerated to 10$^{16}$ eV within the system.