  A macronova (kilonova) was discovered with a short gamma-ray burst, GRB
130603B, which is widely believed to be powered by the radioactivity of
$r$-process elements synthesized in the ejecta of a neutron star binary merger.
As an alternative, we propose that macronovae are energized by the central
engine, i.e., a black hole or neutron star, and the injected energy is emitted
after the adiabatic expansion of ejecta. This engine model is motivated by
extended emission of short GRBs. In order to compare the theoretical models
with observations, we develop analytical formulae for the light curves of
macronovae. The engine model allows a wider parameter range, especially smaller
ejecta mass, and better fit to observations than the $r$-process model. Future
observations of electromagnetic counterparts of gravitational waves should
distinguish energy sources and constrain the activity of central engine and the
$r$-process nucleosynthesis.
