  We discuss the ringdown behavior of the nonequilibrium Green's function in a
strongly coupled theory with the holographic dual with a focus on
quasinormal-mode equilibration. We study the time resolved spectral function
for a probe scalar in Vaidya-AdS spacetime in detail as a complement to the
preceding work arXiv:1603.06935 using further numerical results in very
nonadiabatic temperature changes. It is shown that the relaxation of the
nonequilibrium spectral function obtained through the Wigner transform is
governed by the lowest quasinormal mode frequency. The timescale of the
background temperature change is also observed in the frequency analysis. We
then consider a toy model motivated by the quasinormal mode behavior and
discuss these main features in numerical results are simply realized.
