Variable-frequency quasi-periodic oscillations from the galactic microquasar GRS 1915+105

We show that the galactic microquasar GRS 1915+105 exhibits quasi-periodic oscillations (QPOs) whose frequency varies continuously from I to 15 Hz dur ing spectrally hard dips when the source is in a flaring state. We report h ere analyses of simultaneous energy spectra and power density spectra at 4 s intervals. The energy spectrum is well fit at each time step by an optica lly thick accretion disk plus power-law model, while the power density spec trum consists of a varying red noise component plus the variable-frequency QPO. The features of both spectra are strongly correlated with one another. The 1-15 Hz QPOs appear when the power-law component becomes hard and inte nse and themselves have an energy spectrum consistent with the power-law co mponent (with rms amplitudes as high as 10%). The frequency of the oscillat ions, however, is most strikingly correlated with the parameters of the the rmal disk component. The tightest correlation is between QPO frequency and disk X-ray flux. This fact indicates that the properties of the QPO are not determined solely by a disk or solely by a corona.