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The Question

(Submitted September 23, 1999)

I teach a 10th grade astronomy class and am unclear on how the strong rotating magnetic field of a black hole creates the jets of accelerated matter that is emitted from the polar regions. How is it that this matter/energy escapes the event horizon? Thank you for your time.

The Answer

This is a very good question, which the scientists are still trying to work out.

However, it is not that jets escape from inside the event horizon. Nothing can escape from an event horizon, by definition! Instead, it is thought that jets originate in the accretion disk that surrounds the event horizon.

A fluid falling onto a small object usually cannot fall directly onto it --- most of the matter will miss the object initially, rotate around it, and only gradually be able to hit the central object. Think of draining your bathtub: Accretion disks are the celestial equivalent of this phenomenon, and can be found around black holes, neutron stars, white dwarfs, or around ordinary stars (planets are believed to form from an accretion disk around a protostar).

Accretion disks around a variety of objects seem to be able to produce jets (protostars certainly do, in addition to accreting black holes). It is just that the ones from an accreting black hole tend to be the fastest and the most spectacular. A general rule of thumb is that the speed of a jet is about the same as the escape velocity of the central object --- so the jets from accreting black holes are at near the speed of light, while protostar jets are much more leisurely.

Although accretion disks have sufficient energy to eject a small fraction of the infalling material as jets, it is not clear exactly how. The accretion disks are thought to generate tangled-up magnetic field, which is probably what collimates the jets. However, astrophysicists are still working out the details of how material is lifted up from the accretion disk in the first place, and how exactly it is that the accelerating force (probably radiation pressure) can overcome the gravity of the central object.

Some theorists do think that there is something special about jets from black holes, and that an intrinsic property of black holes helps create such powerful jets. Other theorists disagree.

Hope this helps.

Koji Mukai and Maggie Masetti
for Ask an Astrophysicist

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