Is the Macronova in GW170817 Powered by the Central Engine?

The gravitational wave event GW170817 from a binary neutron star merger is accompanied by electromagnetic counterparts, and the optical and near-infrared emission is called a macronova (or kilonova). Although the radioactivity of synthesized r-process elements is widely discussed as an energy source, its decisive evidence is not clearly shown yet. We discuss a macronova powered by central engine activities such as jet activities and X-rays from the matter fallback and show that the engine model allows much broader parameter spaces, in particular, smaller ejecta mass (∼ {10}^-4-0.01 {M}_⊙ ), than the r-process model. The blue and red macronovae are naturally explained by various combinations of the ejecta, such as a cocoon and merger ejecta with the energy sources of jets and X-rays. The required energy injection is very similar to the X-ray excess observed in GRB 130603B, with a power-law slope of ∼ -1.3. The required lanthanoid fraction for the opacity can also be consistent with the Galactic one. Early or late multiwavelength observations are crucial for revealing the central engine of short gamma-ray bursts and the r-process nucleosynthesis.