  The fluctuation-dissipation theorem (FDT) plays a fundamental role in
understanding quantum many-body problems. However, its applicability is limited
to equilibrium systems and it does in general not hold in nonequilibrium
situations. This violation of the FDT is an important tool for studying
nonequilibrium physics. In this paper we present results for the violation of
the FDT in the Kondo model where the impurity spin is frozen for all negative
times, and set free to relax at positive times. We derive exact analytical
results at the Toulouse point, and results within a controlled approximation in
the Kondo limit, which allow us to study the FDT violation on all time scales.A
measure of the FDT violation is provided by the effective temperature, which
shows initial heating effects after switching on the perturbation, and then
exponential cooling to zero temperature as the Kondo system reaches
equilibrium.
