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Observations of V404 Cyg performed using the Westerbork Synthesis Radio Telescope at four frequencies, over the interval 1.4-8.4 GHz, have provided us with the first broad-band radio spectrum of a 'quiescent' stellar mass black hole. The measured mean flux density is of 0.35 mJy, with a spectral index α = +0.09 ±0.19 (such that Sv oc vα). Synchrotron emission from an inhomogeneous partially self-absorbed outflow of plasma accounts for the flat/inverted radio spectrum, in analogy with hard-state black hole X-ray binaries, indicating that a steady jet is being produced between a few 10-6 and a few per cent of the Eddington X-ray luminosity.

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Monthly Notices of the Royal Astronomical Society

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