Characterizing X-ray binary long-term variability

Authors

  • M. M. Kotze,

    Corresponding author
    1. South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
    2. Astrophysics, Cosmology and Gravity Centre (ACGC), Astronomy Department, University of Cape Town, Rondebosch 7701, South Africa
      E-mail: marissa@saao.ac.za
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  • P. A. Charles

    1. South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
    2. Astrophysics, Cosmology and Gravity Centre (ACGC), Astronomy Department, University of Cape Town, Rondebosch 7701, South Africa
    3. School of Physics & Astronomy, University of Southampton, Southampton SO17 1BJ
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E-mail: marissa@saao.ac.za

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 onboard 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.

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