|MadSci Network: Astronomy|
The term "fountain of anti-matter" is strictly correct, but is a little deceptive. There's evidence for what is called pair production in the area around the black hole. That means there are large numbers of electrons, and their antiparticle, positrons, being created there. By no means is a black hole a net source of antimatter; neither is it producing bulk antimatter.
Pair production is the inverse of particle-antiparticle annihilation. Most people have heard of annihilation: put a particle together with its antiparticle, and they annihilate, making a pair of high-energy photons (gamma rays). Any physical process can run in the opposite sense if the conditions are appropriate; in this case, you could have two photons collide, and they would "un-annihilate" to form a particle-antiparticle pair. (Either or both of those two might subsequently collide with another particle or antiparticle, annhilate, and give you the two high-energy photons again.)
There's an important proviso: the photons colliding must collectively have enough energy to match or exceed the total rest-mass energy of the particle-antiparticle pair. If that isn't the case, then the photon pair can't make a particle-antiparticle pair. The rest-mass energies of even the lightest particles (electrons and positrons) are high, in the gamma-ray regime. This means that pair production isn't a common event; it only happens where you have extremely high temperatures.
Those high temperatures aren't made by the black hole itself, though it causes them to come about. Material which is falling toward a black hole often doesn't get into the black hole directly, because the black hole is so small; often it will crash into other material also falling in, but from another direction. The high-speed traffic jam of material on its way into the black hole makes something called an accretion disk, which is generally a lot larger than the black hole it is feeding. The inner parts of the disk, where the material is pinching together to get into the black hole, get extremely hot. It is there that the high-energy gamma rays are produced that go on to make particle-antiparticle pairs.
This process isn't Hawking radiation; Hawking radiation is produced around a black hole by the black hole itself. It turns out that Hawking radiation is unimportant (in terms of what we can observe) for anything but a really microscopic black hole. The big one (millions of solar masses, with a diameter roughly the same as the Sun) in the center of the Galaxy doesn't produce Hawking radiation at a level that we can hope to detect.
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