### 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 ∼170-d quasi-period) in the folded and averaged light curves, and find it to be by a factor of ∼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 ≃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 language | English |
---|---|

Pages (from-to) | 1006-1016 |

Number of pages | 11 |

Journal | Monthly Notices of the Royal Astronomical Society |

Volume | 377 |

Issue number | 3 |

Early online date | 25 Apr 2007 |

DOIs | |

Publication status | Published - 21 May 2007 |

### Fingerprint

### Keywords

- Accretion, accretion discs
- Binaries: general
- Globular clusters: individual: NGC 6624
- Stars: individual: 4U 1820-303
- X-rays: binaries
- X-rays: stars

### Cite this

*Monthly Notices of the Royal Astronomical Society*,

*377*(3), 1006-1016. https://doi.org/10.1111/j.1365-2966.2007.11686.x

}

*Monthly Notices of the Royal Astronomical Society*, vol. 377, no. 3, pp. 1006-1016. https://doi.org/10.1111/j.1365-2966.2007.11686.x

**The superorbital variability and triple nature of the X-ray source 4U 1820-303.** / Zdziarski, Andrzej A.; Wen, Linqing; Gierliński, Marek.

Research output: Contribution to journal › Article

TY - JOUR

T1 - The superorbital variability and triple nature of the X-ray source 4U 1820-303

AU - Zdziarski, Andrzej A.

AU - Wen, Linqing

AU - Gierliński, Marek

PY - 2007/5/21

Y1 - 2007/5/21

N2 - 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 ∼170-d quasi-period) in the folded and averaged light curves, and find it to be by a factor of ∼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 ≃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.

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 ∼170-d quasi-period) in the folded and averaged light curves, and find it to be by a factor of ∼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 ≃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.

KW - Accretion, accretion discs

KW - Binaries: general

KW - Globular clusters: individual: NGC 6624

KW - Stars: individual: 4U 1820-303

KW - X-rays: binaries

KW - X-rays: stars

UR - http://www.scopus.com/inward/record.url?scp=34248217078&partnerID=8YFLogxK

U2 - 10.1111/j.1365-2966.2007.11686.x

DO - 10.1111/j.1365-2966.2007.11686.x

M3 - Article

VL - 377

SP - 1006

EP - 1016

JO - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

SN - 0035-8711

IS - 3

ER -