The unidentified PeV source LHAASO J2108+5157: A microquasar remnant?

Context. LHAASO J2108+5157 is one of the most intriguing Galactic ultra-high-energy γ-ray sources. It is detected up to hundreds of TeV but lacks a clear counterpart at lower frequencies. Fermi-LAT observations have revealed a compact GeV source spatially coincident with the PeV emission, while VERITAS and LST-1 impose stringent upper limits in the intermediate TeV band, producing a pronounced spectral suppression between the GeV and TeV domains. Aims. We investigate a time-dependent hadronic scenario in which a past episode of microquasar jet activity injected relativistic protons that are now observed in a relic phase. Methods. Energy-dependent diffusion naturally segregates the particle population: Lower-energy protons remain confined near dense molecular gas and account for the GeV emission, whereas higher-energy particles reach more distant material, producing the hard TeV--PeV component. Results. For representative interstellar conditions, the model is simultaneously consistent with the broadband spectral energy distribution, the spatial compact--extended dichotomy, and the lack of detectable radio or X-ray counterparts. Conclusions. The observed bimodal γ-ray phenomenology thus emerges as a transport effect of a single injected proton population, supporting the interpretation of LHAASO J2108+5157 as a fossil microquasar remnant observed at an intermediate evolutionary stage and highlighting relic jet systems as a viable class of Galactic PeVatrons.