# Spitzer Mid-Infrared Detections of Neutron Star Merger GW170817 Suggests Synthesis of the Heaviest Elements

## TL;DR

This study reports Spitzer Space Telescope detections of GW170817, providing evidence that the merger synthesized the heaviest elements in the r-process, with late-time infrared emission indicating the presence of third peak isotopes.

## Contribution

It presents the first infrared observations of GW170817 at late times, constraining the composition of heavy elements synthesized and supporting the production of the heaviest r-process isotopes.

## Key findings

- Detection at 4.5um constrains heavy element synthesis
- Steep decline in infrared luminosity suggests third peak isotopes
- Evidence supports synthesis of the heaviest r-process elements

## Abstract

We report our Spitzer Space Telescope observations and detections of the binary neutron star merger GW170817. At 4.5um, GW170817 is detected at 21.9 mag AB at +43 days and 23.9 mag AB at +74 days after merger. At 3.6um, GW170817 is not detected to a limit of 23.2 mag AB at +43 days and 23.1 mag AB at +74 days. Our detections constitute the latest and reddest constraints on the kilonova/macronova emission and composition of heavy elements. The 4.5um luminosity at this late phase cannot be explained by elements exclusively from the first abundance peak of the r-process. Moreover, the steep decline in the Spitzer band, with a power-law index of 3.4 +/- 0.2, can be explained by a few of the heaviest isotopes in the third abundance peak with half-life around 14 days dominating the luminosity (e.g. 140Ba, 143Pr, 147Nd, 156Eu, 191Os, 223Ra, 225Ra, 233Pa, 234Th) or a model with lower deposition efficiency. This data offers evidence that the heaviest elements in the second and third r-process abundance peak were indeed synthesized. Our conclusion is verified by both analytics and network simulations and robust despite intricacies and uncertainties in the nuclear physics. Future observations with Spitzer and James Webb Space Telescope will further illuminate the relative abundance of the synthesized heavy elements.