  We investigate the dynamics and electromagnetic (EM) signatures of neutron
star-neutron star (NS-NS) or neutron star-black hole (NS-BH) merger ejecta that
occurs in the accretion disk of an active galactic nucleus (AGN). We find that
the interaction between ejecta and disk gas leads to important effects on the
dynamics and radiation. We show five stages of the ejecta dynamics:
gravitational slowing down, coasting, Sedov-Taylor deceleration in the disk,
re-acceleration after the breakout from the disk surface, and
momentum-conserved snowplow phase. Meanwhile, the radiation from the ejecta is
so bright that its typical peak luminosity reaches a few times
$10^{43}-10^{44}~\rm erg~s^{-1}$. Since most of the radiation energy has
converted from the kinetic energy of merger ejecta, we call such an explosive
phenomenon an interacting kilonova (IKN). It should be emphasized that IKNe are
very promising, bright EM counterparts to NS-NS/BH-NS merger events in AGN
disks. The bright peak luminosity and long rising time (i.e., ten to twenty
days in UV bands, thirty to fifty days in optical bands, and one hundred days
to hundreds of days in IR bands) allow most survey telescopes to have ample
time to detect an IKN. However, the peak brightness, peak time, and evolution
pattern of the light curve of an IKN are similar to a superluminous supernova
in a galactic nucleus and a tidal disruption event making it difficult to
distinguish between them. But it also suggests that IKNe might have been
present in recorded AGN transients.
