Spatial Models of R-process Remnants and their γ-Ray Detectability

We investigate the detectability of gamma-ray emission from long-lived radioactive isotopes in r -process-enriched remnants, focusing on how assumptions about their spatial distribution introduce uncertainty into detection prospects. Using a suite of physically motivated models for the Galactic distribution of kilonova and supernova remnants, we simulate synthetic remnant populations and compute their time-evolving gamma-ray spectra. We then compare these flux predictions to the sensitivity limits of next-generation instruments such as a Compton Spectrometer and Imager (COSI) and High-Energy X-ray Probe (HEX-P). We find that even under optimistic assumptions, detection probabilities with COSI are extremely low (≪1%), and that marginal improvements are only possible with instruments like HEX-P if prior localization is available. The choice of spatial distribution model can lead to more than an order-of-magnitude variation in expected line fluxes at low instrument sensitivities, underscoring the role of spatial modeling as a dominant source of uncertainty. Nevertheless, instrumental capability remains the fundamental bottleneck, and a hybrid mission combining COSI-like sky coverage with HEX-P-level line sensitivity would be required to make detection more probable than not.