Characterizing X-ray binary long-term variability

TL;DR

This study systematically analyzes long-term superorbital variability in 25 X-ray binaries using a dynamic power spectrum method, revealing diverse behaviors from stable to chaotic modulations over 15 years of X-ray data.

Contribution

It introduces a systematic approach to characterize superorbital periodicities and their variability over time in X-ray binaries.

Findings

Some sources show remarkably stable superorbital periods.

Others exhibit chaotic or variable long-term modulations.

The variability can depend on X-ray spectral states.

Abstract

Long-term ("superorbital") periods or modulations have been detected in a wide variety of both low and high-mass X-ray binaries at X-ray and optical wavelengths. A variety of mechanisms have been proposed to account for the variability properties, such as precessing and/or warped accretion discs, amongst others. The All Sky Monitor on board the Rossi X-ray Timing Explorer provides the most extensive (~15 years) and sensitive X-ray archive for studying such behaviour. It is also clear that such variations can be intermittent and/or a function of X-ray spectral state. Consequently, we use a time-dependent Dynamic Power Spectrum method to examine how these modulations vary with time in 25 X-ray binaries for which superorbital periodicities have been previously reported. Our aim is to characterize these periodicities in a completely systematic way. Some (such as Her X-1 and LMC X-4) are remarkably stable, but others show a range of properties, from even longer variability time-scales to quite chaotic behaviour.