  Over the past three years, the fading non-thermal emission from the GW170817
remained generally consistent with the synchrotron afterglow from the forward
shock of a relativistic structured jet. Recent observations by Hajela et al.
(2021) indicate the emergence of a new component in the X-ray band. We show
that the new observations are compatible with a rebrightening due to
non-thermal emission from the fast tail of the dynamical ejecta of ab-initio
BNS merger simulations. This provides a new avenue to constrain binary
parameters. Specifically, we find that equal mass models with soft equation of
state (EOS) and high mass ratio models with stiff EOSs are disfavored as they
typically predict afterglows that peak too early to explain observations.
Moderate stiffness and mass ratio models, instead, tend to be in good overall
agreement with the data.
