Spectral Properties of Low‐Frequency Quasi‐periodic Oscillations in GRS 1915+105

We report on the timing analysis of RXTE observations of the Galacticmicro-quasar GRS 1915+105 performed in 2003. Out of a total of six times ~20ks, we focus here only on the three observations during which GRS 1915+105 isfound in a steady C-state (referred to as class $\chi$) resulting in a total of\~50 ks. During these observations, we detect low frequency quasi-periodicoscillations with high (~14 %) rms amplitude in the 2-40 keV energy range.Contrary to what is usually observed in GRS 1915+105, in most of ourobservations the QPO frequency presents no correlation with the RXTE/PCA countrate, nor with the RXTE/ASM count rate. We present, for the first time, highresolution (22 spectral channels) 2-40 keV spectral fits of the energydependence of the QPO amplitude (``QPO spectra''). The QPO spectra are wellmodeled with a cut-off power law except on one occasion where a single powerlaw gives a satisfactory fit (with no cut-off at least up to ~40 keV). Thecut-off energy evolves significantly from one observation to the other, from avalue of ~21.8 keV to ~30 keV in the other observations where it is detected.We discuss the possible origin of this behavior and suggest that the compactjet detected in the radio contributes to the hard X-ray (> 20 keV) mostlythrough synchrotron emission, whereas the X-ray emitted below 20 keV wouldoriginate through inverse Compton scattering. The dependence of the QPOamplitude on the energy can be understood if the modulation of the X-ray fluxis contained in the Comptonized photons and not in the synchrotron ones.Comment: 8 pages, 4 figures, accepted for publication in Ap