Spectral properties of low-frequency quasi-periodic oscillations in GRS 1915+105

We report on the timing analysis of RXTE observations of the Galactic microquasar GRS 1915+105 performed in 2003. Out of a total of six similar to20 ks observations, we focus here only on the three during which GRS 1915+105 is found in a steady C-state (referred to as class chi), resulting in a total of similar to50 ks. During these observations, we detect low-frequency quasi-periodic oscillations (QPOs) with high (similar to14%) rms amplitude in the 2-40 keV energy range. Contrary to what is usually observed in GRS 1915+105, in most of our observations the QPO frequency presents no correlation with the RXTE PCA count rate, nor with the RXTE ASM count rate. We present, for the first time, high-resolution (22 spectral channels) 2-40 keV spectral fits of the energy dependence of the QPO amplitude ("QPO spectra''). The QPO spectra are well modeled with a cutoff power law except on one occasion in which a single power law gives a satisfactory fit (with no cutoff at least up to similar to40 keV). The cutoff energy evolves significantly from one observation to another, from a value of similar to21.8 to similar to30 keV in the other observations in which it is detected. We discuss the possible origin of this behavior and suggest that the compact jet detected in the radio contributes to the hard X-ray (greater than or equal to20 keV) mostly through synchrotron emission, whereas the X-rays emitted below 20 keV would originate through inverse Compton scattering. The dependence of the QPO amplitude on the energy can be understood if the modulation of the X-ray flux is contained in the Comptonized photons and not in the synchrotron ones.