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We present simultaneous X-ray and radio observations of the black hole X-raybinary V404 Cygni at the end of its 2015 outburst. From 2015 July 11-August 5we monitored V404 Cygni with Chandra, Swift, and NuSTAR in the X-ray, and withthe Karl G. Jansky Very Large Array and the Very Long Baseline Array in theradio, spanning a range of luminosities that were poorly covered during itsprevious outburst in 1989 (our 2015 campaign covers 2e33<Lx<1e34 erg/s). Duringour 2015 campaign, the X-ray spectrum evolved rapidly from a hard photon indexof Gamma~1.6 (at Lx~1e34 erg/s) to a softer Gamma~2 (at Lx~3e33 erg/s). Weargue that V404 Cygni reaching Gamma~2 marks the beginning of the quiescentspectral state, which occurs at a factor of ~3-4 higher X-ray luminosity thanthe average pre-outburst luminosity of ~8e32 erg/s. V404 Cygni falls along thesame radio/X-ray luminosity correlation that it followed during its previousoutburst in 1989, implying a robust disk-jet coupling. We exclude thepossibility that a synchrotron cooled jet dominates the X-ray emission inquiescence, leaving synchrotron self-Compton from either a hot accretion flowor from a radiatively cooled jet as the most likely sources of X-ray radiation,and/or particle acceleration along the jet becoming less efficient inquiescence. Finally, we present the first indications of correlated radio andX-ray variability on minute timescales in quiescence, tentatively measuring theradio emission to lag the X-ray by 15+/-4 min, suggestive of X-ray variationspropagating down a jet of length <3.0 AU.

The 2015 Decay of the Black Hole X-ray Binary V404 Cygni: Robust Disk-Jet Coupling and a Sharp Transition into Quiescence