A unified description of the timing features of accreting X-ray binaries

We study an empirical model for a unified description of the power spectra of accreting neutron stars and black holes. This description is based on a superposition of multiple Lorentzians and offers the advantage that all quasi-periodic oscillation and noise components are dealt with in the same way, without the need of deciding in advance the nature of each component. This approach also allows us to compare frequencies of features with high and low coherences in a consistent manner and greatly facilitates comparison of power spectra across a wide range of source types and states. We apply the model to six sources: the low-luminosity X-ray bursters 1E 1724-3045, SLX 1735-269, and GS 1826-24; the high-latitude transient XTE J1118+480; the bright system Cir X-1; and the Z source GX 17+2. We find that it provides a good description of the observed spectra without the need for a scale-free (1/f) component. We update previously reported correlations between characteristic frequencies of timing features in the light of this new approach and discuss similarities between different types of systems that may point toward similar underlying physics.