Variable-Frequency Quasi-periodic Oscillations from the Galactic Microquasar GRS 1915+105

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Abstract
We show that the galactic microquasar GRS 1915+105 exhibits quasi-periodic oscillations (QPOs) whose frequency varies continuously from 1 to 15 Hz during spectrally hard dips when the source is in a flaring state. We report here 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 optically thick accretion disk plus power-law model, while the power density spectrum 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 intense and themselves have an energy spectrum consistent with the power-law component (with rms amplitudes as high as 10%). The frequency of the oscillations, however, is most strikingly correlated with the parameters of the thermal 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.
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