  We present the afterglow light curves produced by the deceleration of the
non-relativistic ejecta mass in a stratified circumstellar medium with a
density profile $n(r)\propto r^{-k}$ with $k=0$, $1$, $1.5$, $2$ and $2.5$.
Once the ejecta mass is launched with equivalent kinetic energy parametrized by
$E(>\beta)\propto \beta^{-\alpha}$ (where beta is the ejecta velocity) and
propagates into the surrounding circumstellar medium, it first moves with
constant velocity (the free-coasting phase), and later it decelerates (the
Sedov-Taylor expansion). We present the predicted synchrotron and
synchrotron-self Compton light curves during the free-coasting phase, and the
subsequent Sedov-Taylor expansion. In particular cases, we show the
corresponding light curves generated by the deceleration of several ejecta
masses with different velocities launched during the coalescence of binary
compact objects and the core-collapse of dying massive stars which will
contribute at distinct timescales, frequencies, and intensities. Finally, using
the multi-wavelength observations and upper limits collected by a large
campaign of orbiting satellites and ground telescopes, we constrain the
parameter space of both the KN afterglow in GW170817 and the possibly generated
KN afterglow in S190814bv. Further observations on timescales of years
post-merger are needed to derive tighter constraints.
