A decade of radio imaging the relativistic outflow in the peculiar X-ray binary Circinus X-1

Authors

  • V. Tudose,

    Corresponding author
    1. ‘Anton Pannekoek’ Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, the Netherlands
    2. Astronomical Institute of the Romanian Academy, Cutitul de Argint 5, RO-040557 Bucharest, Romania
    3. Research Center for Atomic Physics and Astrophysics, Atomistilor 405, RO-077125 Bucharest, Romania
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  • R. P. Fender,

    1. ‘Anton Pannekoek’ Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, the Netherlands
    2. School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ
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  • A. K. Tzioumis,

    1. Australia Telescope National Facility, CSIRO, PO Box 76, Epping, New South Wales 1710, Australia
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  • R. E. Spencer,

    1. Jodrell Bank Observatory, University of Manchester, Cheshire SK11 9DL
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  • M. van der Klis

    1. ‘Anton Pannekoek’ Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, the Netherlands
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E-mail: vtudose@science.uva.nl

ABSTRACT

We present observations of the neutron star X-ray binary and relativistic jet source Circinus X-1 made at 4.8 and 8.6 GHz with the Australia Telescope Compact Array during a time interval of almost 10 yr. The system shows significant variations in the morphology and brightness of the radio features on all time-scales from days to years. Using the time delay between the successive brightening of the different components of the radio emission we were able to provide further evidence for the relativistic nature of the arcsec-scale outflow, with an apparent velocity βapp≥ 12. No compelling evidence for an evolution of the orientation of the jet axis was found. We also place an upper limit on the proper motion of the system which is consistent with previous optical studies. Besides the previously reported radio flares close to the orbital phase 0.0 (interpreted as enhanced accretion at periastron passage), we identified outbursts with similar properties near the orbital phase 0.5. The global spectral index revealed a preferentially steep spectrum over the entire period of monitoring with a mean value and standard deviation α=−0.9 ± 0.6(Fν∝να), which became significantly flatter during the outbursts. Polarization was detected in one third of the epochs, and in one case Faraday rotation close to the core of the system was measured.

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