We analyze a set of three RXTE Target of Opportunity observations
of the Galactic microquasar GRS 1915+105, observed on April
2000, during a multi-wavelength campaign.
During the three observations, a strong, variable low frequency
(2-9 Hz) quasi periodic oscillation (hereafter QPO), often referred to
as the ubiquitous QPO, is detected together with its first
harmonic.
We study the spectral properties of both features, and show that :
1) their frequency variations are better correlated with the soft X-ray flux (2-5 keV), favoring thus the location of the QPO in the
accretion disk;
2) the QPO affects more the hard X-rays, usually taken as the
signature of an inverse compton scattering of the soft
photons in a corona;
3) the fundamental and its harmonic do not behave in the same
manner: the fundamental sees its power increase with the energy
up to 40 keV, whereas the harmonic increases up to \\sim 10 keV.
The results presented here could find an explanation in the context
of the Accretion-Ejection Instability, which could appear as a
rotating spiral or hot point located in the disk, between its innermost
edge and the corotation radius.
The presence of the harmonic could then be a signature of the non-linear behavior of the instability.
The high-energy (>40 keV) decrease of the fundamental would
favor an interpretation where most or all of the quasi-periodic
modulation at high energies comes, not from the comptonized
corona as usually assumed, but from a hot point in the optically
thick disk. |
