Journal Article

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

V. Tudose, R. P. Fender, A. K. Tzioumis, R. E. Spencer and M. van der Klis

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 390, issue 1, pages 447-464
Published in print October 2008 | ISSN: 0035-8711
Published online October 2008 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.13788.x
A decade of radio imaging the relativistic outflow in the peculiar X-ray binary Circinus X-1

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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.

Keywords: accretion, accretion discs; radiation mechanisms: non-thermal; stars: individual: Circinus X-1; ISM: jets and outflows; radio continuum: stars; X-rays: stars

Journal Article.  8732 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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