  Aging in spin glasses is analyzed via the Probability Density Function (PDF)
of the heat transfer between system and bath over a small time $\delta t$. The
PDF contains a Gaussian part, describing reversible fluctuations, and an
exponential tail caused by intermittent events. We find that the relative
weight of these two parts depends, for fixed $\delta t$, on the ratio of the
total sampling time to the age $t_w$. Fixing this ratio, the intensity of the
intermittent events is proportional to $\delta t/t_w$ and independent of the
temperature. The Gaussian part has a variance with the same temperature
dependence as the variance of the equilibrium energy in a system with an
exponential density of states. All these observations are explained assuming
that, for any $t_w$, intermittent events are triggered by local energy
fluctuations exceeding those previously occurred.
