  The successful joint observation of the gravitational wave event GW170817 and
its multi-wavelength electromagnetic counterparts first enables human to
witness a definite merger event of two neutron stars (NSs). This historical
event confirms the origin of short-duration gamma-ray bursts (GRBs), and in
particular, identifies the theoretically-predicted kilonova phenomenon that is
powered by radioactive decays of $r$-process heavy elements. However, whether a
long-lived remnant NS could be formed during this merger event remains unknown,
although such a central engine has been suggested by afterglow observations of
some short-duration GRBs. By invoking this long-lived remnant NS, we here
propose a model of hybrid energy sources for the kilonova AT2017gfo associated
with GW 170817. While the early emission of AT2017gfo is still powered
radioactively as usually suggested, its late emission is primarily caused by
delayed energy injection from the remnant NS. In our model, only one single
opacity is required and an intermediate value of $\kappa\simeq0.97\,\rm
cm^2g^{-1}$ is revealed, which could be naturally provided by lanthanide-rich
ejecta that is deeply ionized by the emission from a wind of the NS. These
self-consistent results indicate that a long-lived remnant NS, which must own a
very stiff equation of state, had been formed during the merger event of
GW170817. This provides a very stringent constraint on the strong interaction
in nuclear-quark matter. It is further implied that such GW events could
provide a probe of the early spin and magnetic evolutions of NSs, e.g., the
burying of surface magnetic fields.
