A transient relativistic radio jet from Cygnus X-1

R P Fender, A M Stirling, R E Spencer, I Brown, G G Pooley, T W B Muxlow, J C A Miller-Jones

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    We report the first observation of a transient relativistic jet from the canonical black hole candidate, Cygnus X-1, obtained with the Multi-Element Radio-Linked Interferometer Network (MERLIN). The jet was observed in only one of six epochs of MERLIN imaging of the source during a phase of repeated X-ray spectral transitions in 2004 Jan-Feb, and this epoch corresponded to the softest 1.5-12keV X-ray spectrum. With only a single epoch revealing the jet, we cannot formally constrain its velocity. Nevertheless, several lines of reasoning suggest that the jet was probably launched 0.5-4.0d before this brightening, corresponding to projected velocities of 0.2c app ~0.3c. We also report the occurrence of a major radio flare from Cyg X-1, reaching a flux density of ~120mJy at 15GHz, and yet not associated with any resolvable radio emission, despite a concerted effort with MERLIN. We discuss the resolved jet in terms of the recently proposed `unified model' for the disc-jet coupling in black hole X-ray binaries, and tentatively identify the `jet line' for Cyg X-1. The source is consistent with the model in the sense that a steady jet appears to persist initially when the X-ray spectrum starts softening, and that once the spectral softening is complete the core radio emission is suppressed and transient ejecta/shock observed. However, there are some anomalies, and Cyg X-1 clearly does not behave like a normal black hole transient in progressing to the canonical soft/thermal state once the ejection event has happened.
    Original languageEnglish
    Pages (from-to)603-607
    Number of pages5
    JournalMonthly Notices of the Royal Astronomical Society
    Publication statusPublished - 2006


    • accretion
    • accretion discs: black hole physics: ISM: jets and outflows: X-rays: binaries
    • accretion discs
    • black hole physics
    • ISM: jets and outflows
    • X-rays: binaries


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