The Emergence of a Lanthanide-Rich Kilonova Following the Merger of Two Neutron Stars

TL;DR

This paper reports the discovery of a lanthanide-rich kilonova following a neutron star merger, confirming that such mergers are a major site of heavy element nucleosynthesis in the universe.

Contribution

It provides observational evidence of lanthanide-rich ejecta from a neutron star merger, supporting models of r-process nucleosynthesis and kilonova emission.

Findings

Detection of infrared features consistent with lanthanide-rich ejecta

Slower near-infrared evolution compared to optical wavelengths

Confirmation that neutron star mergers are key sites of heavy element formation

Abstract

We report the discovery and monitoring of the near-infrared counterpart (AT2017gfo) of a binary neutron-star merger event detected as a gravitational wave source by Advanced LIGO/Virgo (GW170817) and as a short gamma-ray burst by Fermi/GBM and Integral/SPI-ACS (GRB170817A). The evolution of the transient light is consistent with predictions for the behaviour of a "kilonova/macronova", powered by the radioactive decay of massive neutron-rich nuclides created via r-process nucleosynthesis in the neutron-star ejecta. In particular, evidence for this scenario is found from broad features seen in Hubble Space Telescope infrared spectroscopy, similar to those predicted for lanthanide dominated ejecta, and the much slower evolution in the near-infrared Ks-band compared to the optical. This indicates that the late-time light is dominated by high-opacity lanthanide-rich ejecta, suggesting nucleosynthesis to the 3rd r-process peak (atomic masses A~195). This discovery confirms that neutron-star mergers produce kilo-/macronovae and that they are at least a major - if not the dominant - site of rapid neutron capture nucleosynthesis in the universe.