| We present a spectral and temporal analysis of the Cathedral QPO detected in the power density
spectra of the black hole binary and microquasar XTE J1859+226 obtained with RXTE. This peculiar
type of QPO has been seen on two occasions (on MJDs 51574.43 and 51575.43) during the 1998 outburst of this source. It manifests as two peaks with similar amplitudes (~3% and ~5% RMS)
and harmonically related centroid frequencies (~3 and ~6 Hz). The temporal properties of these two peaks are different: the amplitude of the ~3 Hz feature varies, in anticorrelation with the count rate, by about ~50%. The ~6 Hz feature, on the other hand, shows a slight increase (~7%) of its amplitude with count rate.
The RMS-spectra of the two peaks are also quite different. The ~3 Hz feature is softer than the other one, and although its RMS amplitude increases with energy it shows a cut-off at an energy of ~6 keV.
The RMS of the 6 Hz increases up to at least 20 keV. We also study the bicoherence, b^2(mu, nu) of both
observations. At the diagonal position of the peaks the values b^2(~3,~3) and b^2(~6,~6) are rather high and similar to what has been reported in the case of 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 than of the other peak, would tend to indicate that the ~3 Hz is the fundamental QPO, and the other its first harmonic. The value of b2(~3,~6) is, however, low and therefore tends to indicate a very weak or
no coupling between the two features, which is clearly different than the harmonic behavior seen in GRS
1915+105. We discuss the implications of these differences in the context of an harmonic relationship
of the peaks, and suggest that, rather than pure harmonics, we may see different modes of the same
underlying phenomenon competing to produce QPOs at different frequencies. |
