  In this work, we extend the recent study of entropy dynamics induced by an
external impulse in open quantum systems, where the entropy response follows
the Page curve. For small system-bath coupling $\kappa$, we expect that the
entropy first increases exponentially $\kappa^2 e^{\varkappa t}$ in the
early-time regime $t\lesssim |\log \kappa|$ due to quantum many-body chaos, and
then decreases as $~e^{-\lambda_0 t}$ with $\lambda_0 \propto \kappa^2$ due to
the energy relaxation. These results are confirmed through explicit
calculations using two methods: 1) generalized Boltzmann equation for systems
with quasi-particles; 2) scramblon effective theory in the early-time regime
and perturbation theory in the late-time regime for 0+1-d systems. We also
prove that in the second stage, the entropy of the system is equal to the
coarse-grained entropy.
