V404

V404 Cygni has something of an identity crisis. The "V" in its name indicates that it is a variable star, so it gets brighter and fainter. It's also known as a nova, because at least three times in the 20th century it produced a bright outburst of energy. Finally, it's known as a soft X-ray transient because it periodically emits short bursts of X-rays.
Together, these identities tell astronomers that V404 Cygni is a binary system that consists of a black hole and a "normal" companion star, and that the black hole is stealing hot gas from the companion. The flow of gas between stars isn't even, though, so the system produces occasional "flickers." And when enough gas builds up in a disk around the black hole, there is a much larger outburst that can cause the system to shine hundreds of times brighter than normal.
V404 Cygni's black hole is one of the most solid cases of a stellar-mass black hole in the galaxy. Several careful studies of how the two stars orbit each other show that the "dark" member of the system is probably around 12 times as massive as the Sun. Only a black hole could be that small and heavy.
The companion star is about two-thirds as massive as the Sun. Its surface is cooler, so it shines yellow-orange.
The two stars orbit each other once every 6.5 days, which indicates that they are quite close together. At that range, the black hole's powerful gravity causes the companion to bulge toward it, so the star is egg-shaped. Gas flows from the "point" of the egg toward the black hole.
Most of the time, this flow is fairly thin but steady, although it's clumpy enough that the entire system can appear to flicker a little, which is one reason why it's designated as a variable star.
Gas slowly creates a disk around the black hole, known as an accretion disk. The gas in this disk produces energy, too, adding to the total brightness of the system. However, the disk isn't as bright as a disk around a neutron star. That's because in a neutron-star system, the gas remains visible as it spirals onto the surface of the star. It heats up as it gets closer to the neutron star's surface, so it shines brighter. In a black hole system, though, the gas is swallowed as it crosses the black hole's event horizon, so the system can't shine as brightly.


The process of transferring gas from one star to the other isn't completely smooth, though. There can be "lumps" in the stream, or in the disk around the black hole. These lumps create flares of X-rays that are easily detected by X-ray satellites in space -- hence the label "soft X-ray transient" ("soft" refers to the frequency of the X-rays). The X-rays also light up the gas between the two stars, making the system shine even brighter.
The gas continues to build up around the black hole until it reaches a critical density. The gas then quickly heats up, making the accretion disk shine brighter. Gas in the inner portion of the disk suddenly plunges into the black hole; in the moment before it crosses the event horizon, it shines brightest of all. This "big gulp" produces an outburst of energy not just in X-rays, but in all wavelengths. The system can shine hundreds or thousands of times brighter than normal -- hence the label "nova."
V404 Cygni first jumped to prominence during a nova eruption in 1938. It produced eruption in 1989, which was discovered by an orbiting X-ray satellite and confirmed by telescopes on the ground. The system grew about 200 times brighter in visible wavelengths, and much more than that in X-rays. The system is likely building toward another eruption, although so far, astronomers don't know when to expect it.