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The optical and X-ray light curves of Hercules X-1

Authors

  • E. Jurua,

    Corresponding author
    1. University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
    2. Mbarara University of Science and Technology, PO Box 1410, Mbarara, Uganda
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  • P. A. Charles,

    1. South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
    2. University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
    3. School of Physics & Astronomy, University of Southampton, Southampton SO17 1BJ
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  • M. Still,

    1. NASA Ames Research Center, M/S 244-30, Moffett Field, CA 94035, USA
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  • P. J. Meintjes

    Corresponding author
    1. University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
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E-mail: ejurua@gmail.com (EJ); MeintjPJ@ufs.ac.za (PJM)

ABSTRACT

The Galactic neutron star X-ray binary Her X-1 displays a well-known 35-day superorbital modulation in its X-ray and optical light curves. Detected across a broad energy range, the modulation is prevalent in X-rays, cycling between low and high states. The 35-day modulation is believed to be the result of the periodic occultation of the neutron star by a warped precessing accretion disc. Using optical observations of Her X-1 during both the anomalous low state (ALS) and the normal high state, it is shown that the orbital light curve of Her X-1 varies systematically over the 35-day precession cycle. The 35-day precessional profile is remarkably consistent between the ALS and normal high state of Her X-1, suggesting only a very slight change in the form of the disc warp between the two states. Comparison of optical and X-ray light curves suggests that a significant component of the X-ray flux during the ALS originates from the companion star.

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