  X-rays produced by compact flares co-rotating with a Keplerian accretion disc are modulated in time by Doppler effects. We improve on previous calculations of these effects by considering recent models of intrinsic X-ray variability, and compute the expected strength of the relativistic signal in current data of Seyfert galaxies and black hole binaries. Such signals could clearly be seen in, for example, recent XMM-Newton data from MCG-6-30-15, if indeed the X-rays were produced by co-rotating flares concentrated toward the inner disc edge around an extreme Kerr black hole. Lack of the signal in the data collected so far gives support to models, where the X-ray sources in active galaxies do not follow Keplerian orbits close to the black hole. 
On the influence of relativistic effects on X-ray variability of   accreting black holes