Energy Sources and Light Curves of Macronovae

TL;DR

This paper proposes an alternative energy source model for macronovae, suggesting they are powered by a central engine rather than radioactive decay, and develops analytical light curve formulas to compare with observations.

Contribution

It introduces an engine-powered model for macronovae, expanding the parameter space and providing better fits to observed light curves compared to the traditional r-process model.

Findings

Engine model fits observed light curves better.

Allows for smaller ejecta masses.

Future observations can distinguish energy sources.

Abstract

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.