Spectral and Fourier analyses of X‐ray quasi‐periodic oscillations in accreting black holes

We study energy dependencies of quasi‐periodic oscillations (QPOs) from a number of black hole X‐ray binaries. The selected sources were observed by RXTE at time periods close to state transitions and showed QPOs in the 1–10 Hz range. We have constructed QPO rms energy spectra, which provide information about underlying physical process leading to QPO generation. These spectra show an interesting anticorrelation with the time‐averaged spectra. The QPO rms spectra are harder than the time‐averaged spectra when the latter are soft, while they are softer than the time‐averaged spectra when the latter are hard. We then discuss these observational results in the context of simple spectral variability models. Hard QPO spectra can be produced by quasi‐periodic modulations of the heating rate of the Comptonizing plasma, while soft QPO spectra result from modulations of the cooling rate by soft photons.