2005MNRAS.356.1017G

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 S_ν∝ν^α). 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.