Exploring the Power of Magnetic Fields Around Black Holes

[abhiroop_k]

recently i came to know that the jets of gases from black holes are due to magnetic fields...so in effect does this field overpower the effect of the gravitational pull of the BH?

 

[Ken G]

In a way yes, but remember that the fields are anchored in the accretion disk, not the event horizon, so it's not the "one-way" gravity inside the black hole that is being overpowered, but more mundane (yet strong) gravity. The accretion disk is the matter that has not yet fallen across the event horizon, because it had too much angular momentum so is orbiting outside the event horizon. It is called an "accretion" disk because it gradually spirals into the black hole. Such a disk is also the reason that stellar-mass black holes are often a source of hard X-ray emission, and supermassive black holes are often spectacular sources of visible light, in contrast with their name.

 

[zhermes]

recently i came to know that the jets of gases from black holes are due to magnetic fields...so in effect does this field overpower the effect of the gravitational pull of the BH?

Not really. The Magnetic fields primarily 'collimate' the ejected material into jets. The ejection energy arrises from the high densities, pressures and temperatures during the accretion process (and especially from shock phenomena). In many situations, however, magnetic fields are required to achieve this situation.

In the absence of a strong (or ordered) magnetic field, there can still be strong outflows---but they'll be much more diffuse and isotropic.

 

[abhiroop_k]

so does the matter in these jets eventually go into the BH?...or can it escape completely?

 

[Ken G]

The matter in the jets escapes completely, usually at relativistic speed.

 

[abhiroop_k]

i.e it is free of the gravitational pull of the BH?...are these jets then visible to a distant observer?

 

[Ken G]

Yes, on both counts-- that is in fact why we know they are there, we are the distant observer.

 

[abhiroop_k]

thanks