Gamma-Ray Spectra of $R$-Process Nuclei

The radioactive decay of unstable nuclei created in the rapid neutron capture process releases a large amount of $\gamma$-rays. When the ejecta are optically thick, these $\gamma$-rays may contribute to an associated kilonova. Once transparent, prominent spectral features will be directly observable in current and future $\gamma$-ray detectors. In this work, we study and compare the $\gamma$-ray spectra of different representative $r$-process trajectories across a broad range of timescales, from hours to 50,000 years, identifying the nuclei which significantly contribute. We discuss these findings in the context of observability, noting that there are several practical challenges to connecting observed signatures to specific nuclei. However, if these challenges can be overcome, direct observation of $\gamma$-rays from $r$-process sites can provide insight into the fundamental physics underpinning the $r$-process.