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 re-analyzed RXTE data of the bright systems XTE J1859+226, XTE J1550-564 and GX 339-4, which show all three of them. We review the main properties of these LFQPOs and show that they follow a well-defined correlation in a fractional rms vs. 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 Sz through the Z track. 
P. Casella (1 and 2), T. Belloni (1), L. Stella (2) ((1) INAF - Oss.   Brera, (2) INAF - Oss. Roma)