Is the macronova in GW170817 powered by the central engine?

TL;DR

This paper proposes that the macronova in GW170817 is powered by central engine activities like jets and X-rays, broadening the understanding of energy sources beyond radioactive decay, and explains observed features with various ejecta configurations.

Contribution

It introduces a central engine-powered model for the macronova, expanding the parameter space and providing explanations for observed emissions and features.

Findings

Engine model allows smaller ejecta mass than r-process model.

Blue and red macronovae explained by different ejecta combinations.

Energy injection similar to X-ray excess in GRB 130603B.

Abstract

The gravitational wave event GW170817 from a binary neutron star (NS) merger is accompanied by electromagnetic counterparts, and the optical and near-infared 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 the 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 ($\sim10^{-4}-0.01\,\Msun$) 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 the power-law slope of $\sim-1.3$. The required lanthanoid fraction for the opacity can be also consistent with the Galactic one. Early or late multi-wavelength observations are crucial for revealing the central engine of short gamma-ray bursts and the r-process nucleosynthesis.