We present a multiwavelength study of the black hole X‐ray binary V404 Cyg in quiescence, focusing upon the spectral energy distribution (SED). Radio, optical, ultraviolet (UV) and X‐ray coverage is simultaneous. We supplement the SED with additional non‐simultaneous data in the optical through infrared where necessary. The compiled SED is the most complete available for this, the X‐ray and radio brightest quiescent black hole system. We find no need for a substantial contribution from accretion light from the near‐UV to the near‐IR, and in particular the weak UV emission constrains published spectral models for V404 Cyg. We confirm that no plausible companion spectrum and interstellar extinction can fully explain the mid‐IR, however, and an infrared (IR) excess from a jet or cool disc appears to be required. The X‐ray spectrum is consistent with a Γ∼ 2 power law as found by all other studies to date. There is no evidence for any variation in the hardness over a range of a factor of 10 in luminosity. The radio flux is consistent with a flat spectrum (in fν). The break frequency between a flat and optically thin spectrum most likely occurs in the mid or far‐IR, but is not strongly constrained by these data. We find the radio to be substantially variable but with no clear correlation with X‐ray variability.

We present XMM-Newton observations of the black hole X-ray nova V404 Cyg in quiescence. Its quiescent spectrum can be best fitted by a simple power-law with slope Γ ∼ 2. The spectra are consistent with that expected for the advection-dominated accretion flow (ADAF). V404 Cyg was roughly equal in luminosity compared to the previous observation of Chandra. We see variability of a factor of 4 during the observation. We find no evidence for the presence of fluorescent or H-like/Helike iron emission, with upper limits of 52 eV and 110 eV respectively. The limit on the fluorescent emission is improved by a factor of 15 over the previous estimate, and the restriction on H-like/He-like emission is lower than predicted from models by a factor of roughly 2.

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