In 1967, an X-ray sounding rocket discovered a fantastically bright source of X-ray emission coming from the direction of the constellation of Cygnus. Named Cyg X-3, it was soon identified as coincident with a variable source known to be bright as well at radio wavelengths.
Astronomers have since been able to conclude that this object is really a binary star (that is, two objects orbiting around each other) in our galaxy, that it lies about 25,000 light-years away, and that one of the objects is a hot, massive star (soon to become a supernova) with strong winds. But what is the companion object, and why does it cause the pair to emit such intense X-ray emission? Astronomers have been trying to solve this question ever since.
There are two likely possibilities. The first is that the companion is a neutron star - the ultra-dense ash left behind after a supernova explosion. The second possibility is that the companion was originally so massive - more than about eight solar-masses - that its supernova explosion left behind a black hole. As material blowing from the hot star encounters the region around either dense companion, it will be heated to millions of degrees and emit X-rays.
SAO astronomer Michale McCollugh, together with two colleagues, has published an analysis of archival multi-wavelength data of Cyg X-3. Their analysis tries, for the first time, to account for the full behavior of the time-varying source -- its strong, weak, and intermediate phases of flaring emission, as well as the time-varying spectral character of the radiation. When the team compared its results with models and observations of known black hole and neutron binaries, they found that the Cyg-X-3 emission closely corresponds with that seem in black hole systems, both at X-ray and radio wavelengths, and that it differs from the emission seen in neutron star binaries. Although some additional analysis remains to be done, the results appear to have resolved at last one of the important lingering mysteries from the early days of X-ray astronomy.