1999ApJ...522..397R

We have investigated 52 Rossi X-Ray Timing Explorer pointed observations of GRO J1655-40 spanning the X-ray outburst that commenced on 1996 April 25 and lasted for 16 months. Our X-ray timing analyses reveal four types of quasi-periodic oscillations (QPOs): three with relatively stable central frequencies near 300, 9, and 0.1 Hz, and a fourth QPO that varied over the range 14-28 Hz. The 300 and 0.1 Hz QPOs appear only when the power-law component dominates the X-ray spectrum and the estimated unabsorbed X-ray luminosity is above ∼0.2L_Edd. At lower luminosity (L_X∼0.1L_Edd), the thermal component dominates the spectrum; the disk appears somewhat cooler (∼1.3 keV), and its inner radius appears larger. In this state only two of the QPOs are observed: the broad and spectrally ``soft'' 9 Hz QPO and the narrow, ``hard'' QPO that varies from 14 to 28 Hz as the hard flux decreases. At still lower luminosities (L_X<0.1L_Edd), the power-law component contributes less than 30% of the total luminosity, the inner disk appears both larger and cooler, the 9 Hz QPO vanishes, and only a very weak (rms 0.3%) and narrow QPO at 28 Hz remains. The 300 Hz QPO is likely to be analogous to the stationary QPO at 67 Hz seen in the microquasar GRS 1915+105. We discuss four models of these high-frequency QPOs that depend on effects due to general relativity. The models suggest that these rapid QPOs may eventually provide a measure of the mass and rotation of the accreting black hole. The 9 Hz QPO displays a spectrum consistent with a thermal origin, but this frequency is not uniquely consistent with any of the natural timescales associated with the disk. The mechanism for the 14-28 Hz QPOs appears to be linked to the power-law component, similar to the 1-10 Hz QPOs in GRS 1915+105. Thus these low-frequency QPOs have the potential to lead us to the origin of the energetic electrons that radiate the power-law spectral component. Finally, we show data for GRO J1655-40 and GRS 1915+105 as each source teeters between relative stability and a state of intense oscillations at 0.1 Hz. A comparison of the respective spectral parameters allows us to speculate that the black hole mass in GRS 1915+105 is very large, possibly ∼100 M_☉.