Cygnus X-3

Cygnus X-3 is a high-mass X-ray binary and one of the brightest binary X-ray sources in the sky. Often classified as a microquasar, it is thought to be a compact object that draws gas from an ordinary stellar companion. It is the only known high-mass X-ray binary in the Milky Way that includes a Wolf–Rayet star. Though invisible to the eye, it can be detected at radio, infrared, X-ray, and gamma-ray wavelengths.

The source shows strong X-ray emission, with both soft and hard X-rays varying in intensity. Periods when hard X-rays are weakest are called soft states. It lies less than half a degree from a gamma-ray pulsar but is itself a faint gamma-ray source, and it produces periodic gamma-ray flares that always seem to occur during the soft state.

Cygnus X-3 cannot be seen at visual wavelengths because of heavy extinction in the galactic plane, but an infrared point source marks its location. It is a microquasar detected in high-energy gamma rays above 100 MeV, along with others like SS 433. Because its emission varies across wavelengths, it has been given the variable star designation V1521 Cygni.

Flares

Cygnus X-3 is known for intense X-ray emission, as well as for gamma-ray and radio flares that can make it the brightest radio source in the Milky Way. The gamma-ray flares appear during a quiet radio period just before a major radio flare. During giant radio flares, a relativistic jet has been resolved, aimed within about 14 degrees of directly toward Earth.

Binary system

The system shows consistent variations across all wavelengths with a 4.8-hour period. The infrared spectrum and X-ray emission suggest a binary system containing a Wolf–Rayet star and a compact object. The 4.8-hour variations have been interpreted as eclipses, but this is considered unlikely because there are no well-defined periodic dips in brightness. The orbit is not well known beyond the period, so the masses of the components are uncertain.

Orbital analysis suggests the compact object has a mass less than 5 solar masses, likely around 2 solar masses. Its nature is uncertain; it could be a neutron star or a black hole, with no consensus favoring either. Although the combination of a Wolf–Rayet star and a compact object would be unique, the Wolf–Rayet star will almost certainly become a black hole very quickly on astronomical timescales—likely within a million years, via a supernova or direct collapse.