2020MNRAS.496.5262V

We measure the centroid frequency ratios of simultaneous quasi-periodic oscillations (QPOs) that occur at low frequency (0.1-30 Hz) in a selection of accreting black hole (BH) low-mass X-ray binaries. We use all data in the RXTE archive on GX 339-4, GRO J1655-40, 4U 1630-47, XTE J1550-564, and H 1743-322. We select the power spectra that show at least two simultaneous QPOs, and empirically divide them into four main categories, whose occurrence correlates systematically to X-ray spectral state. In the hard/hard-intermediate state, all sources show sets of QPO peaks with near-harmonic frequency relations, which we measure as precisely as possible using an improved analysis method. We find small but significant offsets from purely harmonic frequency relations that in most cases can be explained by the fit function not describing the QPOs accurately; for some QPO pairs, however, the 'sub-harmonic' is at a higher frequency than expected. In the intermediate and ultraluminous states, in all sources we find non-harmonic QPO pairs, some previously reported. We distinguish several different types of non-harmonic QPO pairs that occur across sources. We discuss these findings in the framework of classification schemes and models proposed for black hole low-frequency QPOs. We conclude that the phenomenology of the frequency ratios indicates that in addition to the physical mechanism (possibly precession) explaining the common harmonically related sets of (Type B and C) QPO peaks, at least one additional mechanism is required to explain the occurrence of pairs of QPOs in other states that are not only not harmonically related, but also stand out by the absence of harmonics to either of them.