Geometrical constraints on the origin of timing signals from black holes

As it has been known for a long time, inclination strongly affect the observed properties of Active Galactic Nuclei. Over the last years it has become more and more clear that the same is true for galactic accreting BH binaries: above all, the disk and outflows properties largely depend on the inclination to the light of sight of the sources. We have systematically studied the effect of the orbital inclination on the fast time- variability properties of black-hole transients. We have considered the black-hole binaries observed by the RossiXTE satellite that have gone through all the canonical spectral states and we found that the amplitude of low-frequency quasi periodic oscillations (QPOs) strongly depends on the orbital inclination. Type-C QPOs are stronger for nearly edge-on systems, while type-B QPOs are stronger for nearly face-on ones. Furthermore, we found that the broad band noise associated to both types of QPOs is inclination-independent. I will discuss these results in the context of recently proposed theoretical models for low-frequency QPOs, showing how QPOs can be used as effective tools to study accretion close to a compact object, where the effects of General Relativity are expected to be strongest.