Characterizing the quasi-periodic oscillation behavior of the X-ray nova XTE J1550-564

For all 209 Rossi X-Ray T iming Explorer observations of the X-ray nova XTE J1550-564 during its major outburst of 1998-1999, we have analyzed the X-ray power spectra, phase lags, and coherence functions. These observations constitute one of the richest and most complete data sets obtained for any black hole X-ray nova. The phase lags and coherence measures of low-frequency quasi-periodic oscillations (QPOs; nu<20 Hz) are used to distinguish three types (one more than those reported by Wijnands, Homan, & van der Klis). For the most common type (C), the phase lag is correlated with both the QPO frequency and the amplitude. The physical significance of the QPO types is evident in the relationships between QPO properties and the apparent temperature and flux from the accretion disk. There is also a clear pattern in how the QPO types relate to the presence of high-frequency QPOs (ν>100 Hz). In general, the amplitude of low-frequency QPOs decreases when high-frequency oscillations appear, and when both low-frequency and high-frequency QPOs are present, their Q-values (nu/FWHM) are anticorrelated. We speculate that this opposition between low-frequency and high-frequency QPOs arises from competing structures in a perturbed accretion disk. However, we find that the frequencies of slow and fast QPOs are not correlated. In addition, we encounter systematic problems in attempting to reliably compare the QPO frequencies with broad features in the power continuum, since there are a variable number of features or spectral breaks in the power spectra. These results cast some doubt on the reported global relationship between QPOs from neutron stars and those from black hole systems.