Work

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

Year: 2026

Type: article

Abstract: 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.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202661188