Accretion disks

blumfeld0

Light cannot escape from a black hole; however, x-rays have been detected from black holes. How is this possible?

is it because of the accretion disk? where does it come from?
is that all there is to the answer?

thanks

Kurdt

As the accretion disc falls into a black hole it releases massive amounts of gravitational potential energy which heats the disc up. The hot accretion disc then emits x-ray radiation. The x-rays do not come from the black hole and they can escape because they are nowhere near the event horizon.

Jorrie

Can one simply say that gravitational potential energy is converted into heat? Is it not rather the compression of dusty gas, with accompanying friction that does the heating? I do not recall reading that a solo object falling straight into a black hole releases energy; it needs to be in an accretion disk as far as I know.

Kurdt

The accretion disk rotates differentially which also causes the heating but the energy has to come from somwhere and that is the gravitational potential.

SpaceTiger

There are several processes that can produce X-rays in a quasar, including thermal, synchrotron and inverse compton emission. Sometimes the X-rays come from the disk, other times from a jet. A disk (or something disk-like) is thought to be present in every quasar, while jets only appear in some of them.

It's true that an object falling into a black hole does not necessarily need to emit radiation -- there are orbits that pass through the event horizon. In that case, the gravitational potential energy would be converted to kinetic energy rather than radiation. However, disks are thought to appear in all quasars, so some of the gravitational potential energy invariably gets converted into radiation. There is a massive pool of energy and, whatever form it takes in the short-run (infall, magnetic fields, jets, winds), there is some radiation mechanism that can occur and make the process observable.

tarbag

There is no signal which can escape from a black hole. It is for that it is called black hole. The emission of X-radiation can be produced by several phenomena like the synchrotron radiation and the inverse Compton effect. But the principal cause is always ignored. There are several models which try to explain that like the model of accretion disc, coalescence between a black hole and neutron star, and there is also several telescopes embarked on satellite which observe this phenomenon for a long time.
One quotes also the mystery of gamma ray bursts (GRBs) who lasts since more than 30 without real resolution.
Thanks