The puzzling harmonic behavior of the Cathedral QPO in XTE J1859+226

Abridged: We present a spectral and temporal analysis of the Cathedral QPO detected in the power density spectra of the microquasar XTE J1859+226 obtained with RXTE. This type of QPO manifests as two peaks with similar amplitudes and harmonically related centroid frequencies (~3 and ~6 Hz). The amplitude of the ~3 Hz feature varies in anticorrelation with the count rate, by about ~50. The ~6 Hz feature shows a slight increase (~7%) of its amplitude with count rate. The RMS-spectra of the two peaks are quite different. The ~3 Hz feature is softer than the other one, and shows a cut-off at an energy of ~6 keV while the RMS of the 6 Hz increases up to at least 20 keV. We also study the bicoherence, $b^2(\mu,\nu)$. The values b^2(~3,~3) and b^2(~6,~6) are rather high and similar to the type C QPOs of GRS 1915+105. By comparison with the latter source the fact that the bicoherence of the ~3 Hz feature is higher than that of the other peak, would tend to indicate that the ~3 Hz is the fundamental QPO. The value of b^2(~3,~6)$ is, however, low and therefore indicates a behavior that is different than that seen in GRS 1915+105. We suggest that, rather than pure harmonics, we may see different modes of the same underlying phenomenon competing to produce QPOs at different frequencies.