The Broadband Power Spectra of X‐Ray Binaries

We analyzed the rapid aperiodic X-ray variability of different types of X-raybinaries (black hole candidates, atoll sources, the recently discoveredmillisecond X-ray pulsar, and Z sources) at their lowest inferred massaccretion rates. At these accretion rates, the power spectra of all sources aredominated by a strong band-limited noise component, which follows a power lawwith an index roughly 1 at high frequencies and breaks at a frequency between0.02 and 32 Hz below which the spectrum is relatively flat. Superimposed onthis, a broad bump (sometimes a quasi-periodic oscillation) is present with a0.2-67 Hz centroid frequency that varies in good correlation with the frequencyof the break. The black hole candidates and the low-luminosity neutron starsystems (including the millisecond X-ray pulsar) have the same relation betweenthe frequency of the bump and the frequency of the break. These similarcharacteristics strongly suggest that in all those different types of sourcesthe band-limited noise and the bump are produced by the same physicalmechanism. This mechanism cannot then depend on the presence or absence ofeither a small magnetosphere or a solid surface, so that it is most likelyrelated to an instability in the flow in the accretion disk that modulates theaccretion rate. The Z sources, which are more luminous than the other sources discussed here,follow a similar, but slightly shifted correlation between the break frequencyand the frequency of the bump. The data suggest that the band-limited noise inZ sources is more complex than that in the other sources.Comment: 10 pages, 3 figures (of which one a color figure). Accepted for publication in Astrophysical Journal Part