Relativistic Jets in Black Holes

Relativistic jets do not come out of the black hole. They originate from near the black hole but not, of course, from inside the event horizon.

The relativistic jets consist of streams of charged particles that are accelerated close to the black hole and travel, often in both directions, along the presumed rotation axis of the black hole.

The launching mechanism for these jets is still a substantial topic of research. The most likely ideas are that ionised, accreting material becomes entrained in a strong magnetic field that becomes compressed near the black hole. The field becomes toroidally twisted due to the rapid rotation around the black hole and this sets up a very strong magnetic field gradient along the rotation axis, which accelerates the charged particles in that direction - an example mechanism is the Blandford-Znajek effect. The ultimate energy source here is the rotational energy of the black hole.

Jets would be expected to be launched in both directions along the rotation axis. However, it is more usually the case that the jet in one direction is more prominent. This is due the Doppler boosting phenomenon - radiation emitted by particles with a component of their velocity towards the observer will be significantly boosted. Conversely if a jet is moving away from us in the line of sight then the radiation seen by us will be considerably weakened.

An excellent popular account of the mechanism can be found here.