Black Hole Accretion

As we shall discuss later, one possible outcome of the death of a star is the formation of a black hole, which is a strange object having such strong gravity that it can trap even light in its gravitational clutches. It is thought that in some binary systems one of the stars is a black hole.

Although a black hole in a binary cannot be seen directly, it can signal its presence indirectly if matter accretes from the other star onto the black hole. The matter falling into the black hole is likely to form first an accretion disk. The matter in the accretion disk becomes very hot through collisions and emits X-rays as it spirals downward into the black hole.

Continuous X-Ray Sources
The top right figure is a ROSAT X-ray image of LMC X-1, a binary system in a nearby galaxy called the Large Magellanic Cloud. In this binary system, one star is a more normal star and one is unseen but estimated to have a mass of 5 solar masses or more and therefore is likely to be a black hole. The diffuse glow is X-ray emission in the vicinity of the binary (which isn't seen in the image). X-rays from the accretion disk of the binary knock electrons off atoms in a volume of space that may be light years in diameter. These atoms emit X-rays when the electrons re-combine, causing the observed glow. (We shall also see later that X-ray emission directly from the accretion disk can lead to a flickering in the X-ray intensity.) Accretion onto neutron stars in binaries can produce X-rays also, so the mere observation of X-rays is not sufficient to confirm the presence of a black hole. That requires a careful analysis of details.

Bipolar Mass Ejection

Some of the material accreting onto a black hole may get ejected at very high velocities along the directions defined by the black hole rotation axis; this is called bipolar flow. The adjacent image shows a nebula produced by possible bipolar flow from a binary system. Such mass ejection might also be produced by accretion onto neutron stars. Thus additional information, such as an estimate of the mass of the unseen compact object, is generally required to show that the mass ejection is associated with a black hole.

Accretion Disks and Jets

This animation illustrates schematically the transfer of matter from a star to an accretion disk around a black hole, subsequent heating of the accretion disk and emission of X-rays from the hot disk, spiraling of matter from the accretion disk into the black hole, and bipolar mass ejection in jets oriented on the polar axis of the spinning black hole. We shall discuss such phenomena in more detail in Chapters 22 and 25.