Energy dependence of variability in low mass X-ray binaries

Black hole and neutron star X-ray binaries show variability on time-scales ranging from milliseconds to years. In the last two decades a detailed phenomenological picture of short-term variability in low-mass X-ray binaries has emerged mainly based on RXTE observations that cover energies above 3 keV. This picture comprises periodic or quasi-periodic variability, seen as spikes or humps in power density spectra, that are superposed on broad noise components. The overall shape of the noise components as well as the occurrence of quasi-periodic oscillations is known to vary with the state of the X-ray binary. We are accomplishing a comprehensive study of archival XMM-Newton observations in timing or burst mode of more than ten black hole and more than thirty neutron star low-mass X-ray binaries to investigate the variability properties of these sources at softer energies where the thermal disk component starts to emerge.Here we present the results of a comprehensive variability study of a sample of black hole X-ray binaries, including a discussion of the energy dependence of the overall power spectral shape and of the characteristic frequency and amplitude of the band-limited noise component. Furthermore, we present the results of our recently conducted thorough study of covariance spectra on different time scales for the sample of black hole X-ray binaries. We will discuss the implications of these findings for the picture of the accretion geometry in black hole X-ray binaries.