Rapid X-ray variability in galactic black hole candidates

The rapid X-ray variability of galactic accreting black-hole candidates is compared to that of accreting neutron stars with low magnetic fields. The power spectra of these objects can be described in terms of a small number of simple power spectral shapes: power law noise, band limited noise, and quasi-periodic oscillations (QPO). In a given source, the properties of these power spectral components seem to depend, to first order, only on mass flux. Similarities in the power spectral properties between source types strongly suggest that similar physical mechanisms underlie power spectral components seen in black-hole candidates and in neutron stars with various magnetic-field strengths. Two rapid (≳ 1 Hz) QPO and two band limited noise components appear to occur across all types of X-ray binaries; the situation with respect to the ubiquitous power law components, as well as with respect to slow (≲1 Hz) QPO is as yet unclear. One QPO and one band limited noise component appear to be magnetospheric as they are not seen in black-hole candidates and atoll sources (which are inferred to be neutron stars with a very low magnetic field). Another QPO and band limited noise component may be related to the presence of an inner, radiation pressure dominated accretion disk, as they do not occur in X-ray pulsars. It is discussed to what extent the relatively small quantitative differences between the rapid X-ray variability properties of neutron stars and black-hole candidates can be used to identify black holes, and whether there exist any qualitative differences (i.e., black hole signatures) in the rapid X-ray variability.