J-GEM observations of an electromagnetic counterpart to the neutron star merger GW170817

TL;DR

This paper reports J-GEM follow-up observations of the electromagnetic counterpart SSS17a to GW170817, revealing rapid brightness and color evolution consistent with a kilonova powered by r-process nucleosynthesis, with indications of more ejecta mass or additional energy sources.

Contribution

First detailed optical follow-up of SSS17a showing rapid evolution, supporting kilonova models with r-process element synthesis and suggesting higher ejecta mass or extra energy sources.

Findings

SSS17a's brightness declined rapidly, unlike supernovae.

Color evolution indicates r-process nucleosynthesis.

Observed brightness suggests more ejecta mass or additional energy source.

Abstract

The first detected gravitational wave from a neutron star merger was GW170817. In this study, we present J-GEM follow-up observations of SSS17a, an electromagnetic counterpart of GW170817. SSS17a shows a 2.5-mag decline in the $z$-band from 1.7 days to 7.7 days after the merger. Such a rapid decline is not comparable with supernovae light curves at any epoch. The color of SSS17a also evolves rapidly and becomes redder for later epochs; the $z-H$ color changed by approximately 2.5 mag in the period of 0.7 days to 7.7 days. The rapid evolution of both the optical brightness and the color are consistent with the expected properties of a kilonova that is powered by the radioactive decay of newly synthesized $r$-process nuclei. Kilonova models with Lanthanide elements can reproduce the aforementioned observed properties well, which suggests that $r$-process nucleosynthesis beyond the second peak takes place in SSS17a. However, the absolute magnitude of SSS17a is brighter than the expected brightness of the kilonova models with the ejecta mass of 0.01 $\Msun$, which suggests a more intense mass ejection ($\sim 0.03 \Msun$) or possibly an additional energy source.