2023MNRAS.518.1243F

We present analysis of comprehensive radio observations of the black hole V404 Cyg during its 2015 outburst. These data represent the best ever coverage of jet production and particle acceleration from any black hole. We report for the first time a clear and near-linear flux-rms correlation in the radio flux densities. Investigation of individual flares reveals in nearly all cases the peak corresponds to the transition from optically thick to thin to synchrotron emission, but an extended phase of particle acceleration is required in contrast to simple impulsive injection models. The largest radio flare is preceded by a phase of optical oscillations and followed one day later by a smaller but optically thin flare, likely due to ejecta interacting with the interstellar medium. Comparing the radio emission to contemporaneous X-ray and optical data, we find that the X-ray and radio measurements are correlated on all time-scales from seconds to one day. Correlation with the optical flux densities is weak at short time-scales, but becomes significant on time-scales greater than a few hours. We evaluate the physical conditions (size, magnetic field, and internal energy) associated with 86 individual radio flares, which in turn allows us to place a lower limit on the kinetic feedback over the 15 d of intense activity. If this energy was deposited locally to the source, as implied by the failure to detect jets on angular scales larger than milliarcsec, then we predict that a nova-like shell could have been formed.