The ABC of Low-Frequency Quasi-periodic Oscillations in Black Hole Candidates: Analogies with Z Sources

Three main types of low-frequency quasi-periodic oscillations (LFQPOs) have been observed in black hole candidates. We reanalyzed RXTE data of the bright systems XTE J1859+226, XTE J1550-564, and GX 339-4, which show all three types. We review the main properties of these LFQPOs and show that they follow a well-defined correlation in a fractional rms versus softness diagram. We show that the frequency behavior through this correlation presents clear analogies with that of horizontal-, normal-, and flaring-branch oscillations in Z sources, with the inverse of the fractional rms being the equivalent of the curvilinear coordinate S_z through the Z track.