The Mid-infrared-emitting Jet in the Black Hole V404 Cygni in Quiescence

Observations of some quiescent black hole X-ray binaries have revealed an excess of mid-infrared (MIR) emission above that expected from their donor stars. In one system, V404 Cygni, this excess has been variously suggested to arise from the accretion disk, circumbinary material, or a compact relativistic jet. Here we present simultaneous James Webb Space Telescope (JWST), Atacama Large Millimeter/submillimeter Array, and complementary multiwavelength observations undertaken to resolve this uncertainty. We observed large-amplitude 21 μ m variability on short timescales with JWST, particularly a dramatic flare, which swiftly rose to ≈2.4 mJy, over 10 times the lowest observed MIR flux density. Similar variability was simultaneously observed from radio to X-ray wavelengths with other facilities throughout the campaign. This variability and the flat radio/millimeter/MIR spectral index ( α = 0.04 ± 0.01) suggest that the MIR excess at and longward of 21 μ m in V404 Cyg does not arise from the accretion disk or circumbinary material. Instead, the emission at 21 μ m is dominated by synchrotron radiation from a jet, which persists into quiescence. This result reinforces the ubiquity of the disk–jet connection in accreting black holes across a range of masses and accretion rates.