Is the Macronova in GW170817 Powered by the Central Engine?

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<jats:title>Abstract</jats:title> <jats:p>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 <jats:italic>r</jats:italic>-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 (<jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaac4a8ieqn1.gif" xlink:type="simple"/> </jats:inline-formula>), than the <jats:italic>r</jats:italic>-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 <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaac4a8ieqn2.gif" xlink:type="simple"/> </jats:inline-formula>. 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 <jats:italic>r</jats:italic>-process nucleosynthesis.</jats:p>

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