The broadband power spectra of X-ray binaries

We analyze the rapid, aperiodic X-ray variability of different types of X-r ay binaries (black hole candidates, atoll sources, the recently discovered millisecond X-ray pulsar, and Z sources) at their lowest inferred mass accr etion rates. At these accretion rates, the power spectra of all sources are dominated by a strong band-limited noise component, which follows a power law with an index of roughly 1 at high frequencies and breaks at a frequenc y between 0.02 and 32 Hz, below which the spectrum is relatively flat. Supe rimposed on this, a broad bump (sometimes a quasi-periodic oscillation) is present, with a 0.2-67 Hz centroid frequency that varies in good correlatio n with the frequency of the break. The black hole candidates and the low-lu minosity neutron star systems (including the millisecond X-ray pulsar) have the same relation between the frequency of the bump and the frequency of t he break. These similar characteristics strongly suggest that in all those different types of sources, the band-limited noise and the bump are produce d by the same physical mechanism. This mechanism cannot then depend on the presence or absence of either a small magnetosphere or a solid surface, so that it is most likely related to an instability in the flow in the accreti on disk that modulates the accretion rate. The Z sources, which are more lu minous than the other sources discussed here, follow a similar, but slightl y shifted correlation between the break frequency and the frequency of the bump. The data suggest that the band-limited noise in Z sources is more com plex than that in the other sources.