Long-term properties of accretion discs in X-ray binaries - II. Stability of radiation-driven warping

A significant number of X-ray binaries (XRBs) are now known to exhibit long-term 'superorbital' periodicities on time-scales of similar to10-100 d. Several physical mechanisms have been proposed that give rise to such periodicities, in particular warping and/or precession of the accretion disc. Recent theoretical work predicts the stability to disc warping of XRBs as a function of the mass ratio, binary radius, viscosity and accretion efficiency, and here we examine the constraints that can be placed on such models by current observations. In Paper I we used a dynamic power spectrum (DPS) analysis of long-term X-ray data sets (Compton Gamma Ray Observatory , Rossi X-Ray Timing Explorer ), focusing on the remarkable, smooth variations in the superorbital period exhibited by SMC X-1. Here we use a similar DPS analysis to investigate the stability of the superorbital periodicities in the neutron star XRBs Cyg X-2, LMC X-4 and Her X-1, and thereby confront stability predictions with observation. We find that the period and nature of superorbital variations in these sources are consistent with the predictions of warping theory. We also use a dynamic light-curve analysis to examine the behaviour of Her X-1 as it enters and leaves the 1999 anomalous low state (ALS). This reveals a significant phase shift some 15 cycles before the ALS, which indicates a change in the disc structure or profile leading into the ALS.