Accretion Disks

Because angular momentum must be conserved and the two stars of a binary are revolving around their common center of mass, accretion from one star in a binary to the other often involves the accreting matter going into orbit around the other star. This is indicated schematially in the adjacent figure. This matter in orbit forms an accretion disk surrounding the second star. The animation below left illustrates schematically the orbit of a close binary system and the streaming of matter from the larger star into an accretion disk around the compact companion.

Coriolis Effects in the Revolving System
The deflection of the mass transfer stream so that it flows into orbit around the second star instead of directly into it is caused by the orbital revolution of the binary system. It is analogous to the Coriolis effects that deflect wind and ocean currents on the rotating Earth which were discussed in Chapter 8.

Heating of the Accretion Disk
The matter in the accretion disk undergoes collisions and interactions. This causes it to be heated and it loses energy by radiating light, with the wavelength of the emitted light depending on the temperature of the disk. This causes the matter to spiral from the accretion disk onto the second star. The details of how this happens are not very well understood. However, we have strong reason to believe that it does happen. For one thing, we see evidence that matter accumulates onto the second star (see the later discussion of novae and type Ia supernovae). For another, as we describe below, hot accretion disks are sources of strong X-ray emission that can be detected on Earth.

X-Ray Sources
Accreting binary systems are often strong sources of X-rays if the matter accretes onto a neutron star or a black hole. Within our own galaxy, there are many X-ray sources that are thought to be X-ray binaries where matter from one star is accreting onto a compact object, either a neutron star or a black hole.

X-Ray Sources in the Andromeda Galaxy
The adjacent ROSAT X-ray image shows the nearby Andromeda Galaxy (M31). Many strong localized X-ray sources may be seen. It is thought that most of these are probably accreting binary systems in M31, with the X-ray emission associated with heating of matter in an accretion ring around either a neutron star or a black hole.