Spectral and Fourier analyses of X-ray QPO in accreting black holes

We study energy dependencies of quasi-periodic oscillations 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 QPO in the 1-10 Hz range. We have constructed QPO root-mean-square energy spectra, which provide information about underlying physical processes leading to QPO generation. These spectra show an interesting anti-correlation with the time averaged spectra. The QPO r.m.s. 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.