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Why is mystery object Cygnus X-3 so bright? Astronomers may now have the answer




A binary system containing a massive star and what is probably a black hole, and which together are a source of intense X-rays, has been shown to be a smaller-scale example of some of the most luminous quasars in the universe.

The new findings, from an international team that used NASA's Imaging X-ray Polarimetry Explorer spacecraft (IXPE), describe how an X-ray binary system located about 24,000 light-years away in our Milky Way galaxy is amplifying its X-ray emission in a funnel-shaped cavity that encircles the probable black hole.

The system, Cygnus X-3, was discovered in the early 1970s when radio telescopes spotted powerful jets radiating out from it at nearly the speed of light. The radio emission from these jets lasts for a few days, before switching off, only to turn back on again later.

The origin of the jets was, at the time, Mysterious. The system was described as an "astronomical puzzle," not helped by the fact that we cannot even see Cygnus X-3 in visible light; it's blocked by thick dust in the plane of our galaxy. During the 1970s, radio astronomers at observatories all around the world coordinated by telephone to try to catch Cygnus X-3 in the act of switching on or off.

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Over the years, further observations in radio, infrared and X-ray wavelengths allowed astronomers to figure out that Cygnus X-3 is an X-ray binary system involving the transferral of matter between a massive star and a compact object that orbit a common center of gravity. The compact object is either a neutron star or, more likely, a black hole with a mass about five times greater than the mass of our sun. The massive star is a Wolf-Rayet star — a rare phase that supergiant stars undergo, wherein they radiate powerful stellar winds that begin lifting large chunks of their outer envelope into space. It is the material blown on the wind from this Wolf-Rayet star that is feeding an accretion disk that spirals around the compact object.

However, Cygnus X-3's luminosity is scarcely believable. The flow of matter onto a compact object like a black hole is controlled by a property known as the Eddington Limit. If the rate of accretion is high enough, the accretion disk becomes a logjam — matter ends up backing up, the disk grows dense and so hot that the amount of radiation pouring out can stall the iNFLow of fresh material. In this way, black holes can regulate their own growth, and some of the material is spat back out in the radio-emitting jets.