Noise components from black-hole binaries in our galaxy
  Accreting binaries containing a black hole are stellar systems composed of a normal star and a black hole. Because of the strong gravitational pull of the black hole, matter is removed from the companion star and falls into the compact ob ject. In falling, it forms an accretion disk of gas that spirals towards the center, heating up and emitting in X rays. The physics of such a structure is extremely complex and can be studied through observations with X-ray satellites. The time series derived from X-ray observations of bright black-hole binaries in the Galaxy show a complex phenomenology. Broad noise components with a variability of up to 40% are observed, as well as quasi-periodic features on time scales from 100 seconds down to a few milliseconds. The characteristic frequencies of the different components can change on very short time scales. However, some of these signals are elusive as they are very weak and are drowned in intrinsic and instrumental noise. The physical nature of these signals is still largely unknown, but it is clear that they originate from gas orbiting a few kilometers from the central black hole and accreting onto it. In addition of being important for the study of the accretion of matter onto a black hole, these observational properties constitute a unique probe for testing General Relativity in the strong field regime. I review the current observational status as well as the techniques used to study these signals. 
Tomaso M. Belloni (INAF - Osservatorio Astronomico di Brera)