The superorbital variability and triple nature of the X-ray source 4U 1820-303

A.A. Zdziarski, Linqing Wen, M. Gierlinski

    Research output: Contribution to journalArticle

    36 Citations (Scopus)

    Abstract

    We perform a comprehensive analysis of the superorbital modulation in the ultracompact X-ray source 4U 1820-303, consisting of a white dwarf accreting on to a neutron star. Based on RXTE data, we measure the fractional amplitude of the source superorbital variability (with a similar to 170-d quasi-period) in the folded and averaged light curves, and find it to be by a factor of similar to 2. As proposed before, the superorbital variability can be explained by oscillations of the binary eccentricity. We now present detailed calculations of the eccentricity-dependent flow through the inner Lagrangian point, and find a maximum of the eccentricity of similar or equal to 0.004 is sufficient to explain the observed fractional amplitude. We then study hierarchical triple models yielding the required quasi-periodic eccentricity oscillations through the Kozai process. We find the resulting theoretical light curves to match well the observed ones. We constrain the ratio of the semimajor axes of the outer and inner systems, the component masses and the inclination angle between the inner and outer orbits. Last but not least, we discover a remarkable and puzzling synchronization between the observed period of the superorbital variability (equal to the period of the eccentricity oscillations in our model) and the period of the general-relativistic periastron precession of the binary.
    Original languageEnglish
    Pages (from-to)1006-1016
    JournalMonthly Notices of the Royal Astronomical Society
    Volume377
    Issue number3
    DOIs
    Publication statusPublished - 2007

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    eccentricity
    oscillation
    x rays
    oscillations
    light curve
    X Ray Timing Explorer
    precession
    neutron stars
    inclination
    synchronism
    orbits
    modulation

    Cite this

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    title = "The superorbital variability and triple nature of the X-ray source 4U 1820-303",
    abstract = "We perform a comprehensive analysis of the superorbital modulation in the ultracompact X-ray source 4U 1820-303, consisting of a white dwarf accreting on to a neutron star. Based on RXTE data, we measure the fractional amplitude of the source superorbital variability (with a similar to 170-d quasi-period) in the folded and averaged light curves, and find it to be by a factor of similar to 2. As proposed before, the superorbital variability can be explained by oscillations of the binary eccentricity. We now present detailed calculations of the eccentricity-dependent flow through the inner Lagrangian point, and find a maximum of the eccentricity of similar or equal to 0.004 is sufficient to explain the observed fractional amplitude. We then study hierarchical triple models yielding the required quasi-periodic eccentricity oscillations through the Kozai process. We find the resulting theoretical light curves to match well the observed ones. We constrain the ratio of the semimajor axes of the outer and inner systems, the component masses and the inclination angle between the inner and outer orbits. Last but not least, we discover a remarkable and puzzling synchronization between the observed period of the superorbital variability (equal to the period of the eccentricity oscillations in our model) and the period of the general-relativistic periastron precession of the binary.",
    author = "A.A. Zdziarski and Linqing Wen and M. Gierlinski",
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    doi = "10.1111/j.1365-2966.2007.11686.x",
    language = "English",
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    pages = "1006--1016",
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    The superorbital variability and triple nature of the X-ray source 4U 1820-303. / Zdziarski, A.A.; Wen, Linqing; Gierlinski, M.

    In: Monthly Notices of the Royal Astronomical Society, Vol. 377, No. 3, 2007, p. 1006-1016.

    Research output: Contribution to journalArticle

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    AB - We perform a comprehensive analysis of the superorbital modulation in the ultracompact X-ray source 4U 1820-303, consisting of a white dwarf accreting on to a neutron star. Based on RXTE data, we measure the fractional amplitude of the source superorbital variability (with a similar to 170-d quasi-period) in the folded and averaged light curves, and find it to be by a factor of similar to 2. As proposed before, the superorbital variability can be explained by oscillations of the binary eccentricity. We now present detailed calculations of the eccentricity-dependent flow through the inner Lagrangian point, and find a maximum of the eccentricity of similar or equal to 0.004 is sufficient to explain the observed fractional amplitude. We then study hierarchical triple models yielding the required quasi-periodic eccentricity oscillations through the Kozai process. We find the resulting theoretical light curves to match well the observed ones. We constrain the ratio of the semimajor axes of the outer and inner systems, the component masses and the inclination angle between the inner and outer orbits. Last but not least, we discover a remarkable and puzzling synchronization between the observed period of the superorbital variability (equal to the period of the eccentricity oscillations in our model) and the period of the general-relativistic periastron precession of the binary.

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