Broadband Variability in USA and RXTE Lightcurves of Accreting X-Ray Sources

Uttley & McHardy (MNRAS, 323, L26, 2001) demonstrated a simple new diagnostic of variability in accreting sources. It involves the relation of the RMS (absolute, not fractional) variability of the source to its luminosity. They observed a linear relation in a black hole candidate and a neutron star, and observed behavior consistent with such a relation in an active galactic nucleus. We explore and expand on this diagnostic. The diagnostic is applied to data collected with ARGOS/USA and RXTE/PCA data for Cygnus X-1 and other accretion powered X-ray sources. We find that a two-dimensional distribution is necessary to describe the data. We apply the diagnostic to model light curves to constrain models of the time variability of the sources.