Evidence for a gamma-ray molecular target in the enigmatic PeVatron candidate LHAASO J2108+5157

Context. Peta-eV (PeV) astronomy emerged in 2021 with the discovery of ultra-high-energy gamma-ray sources associated with powerful natural particle accelerators known as PeVatrons. In order to determine the nature of their emission, namely whether it has a hadronic or leptonic origin, it is essential to characterise the physical parameters of the environment where it originates. Aims. We unambiguously confirm the association of molecular gas with the PeVatron candidate LHAASO J2108+5157 using unprecedented high angular-resolution (17″) 12, 13 CO( J = 1 → 0) observations carried out with the Nobeyama 45m radio telescope. Methods. We characterised a molecular cloud in the vicinity of the PeVatron candidate LHAASO J2108+5157 by determining its physical parameters from our 12, 13 CO( J = 1 → 0) line observations. We used an updated estimation of the distance to the cloud, which provided a more reliable result. The molecular emission was compared with excess gamma-ray images obtained with Fermi -LAT at energies above 2 GeV to search for spatial correlations and test a possible hadronic ( π 0 decay) origin for the gamma-ray emission. Results. We find that the morphology of the spatial distribution of the CO emission is strikingly similar to that of the Fermi -LAT excess gamma ray. By combining our observations with archival 21 cm HI line data, the nucleons (HI + H 2 ) number density of the target molecular cloud is found to be 133.0 ± 45.0 cm −3 , for the measured angular size of 0.55 ± 0.02° at a distance of 1.6 ± 0.1 kpc. The resulting total mass of the cloud is M (HI + H 2 ) = 7.5±2.9×10 3 M ⊙ . Under a hadronic scenario, we obtain a total energy of protons of W p = 4.3 ± 1.5 × 10 46 erg with a cutoff of 700±300 TeV, which reproduces the sub-PeV gamma-ray emission. Conclusions. We identified a molecular cloud in the vicinity of LHAASO J2107+5157 as the main target where cosmic rays from an unknown PeVatron produce the observed gamma-ray emission via π 0 decay.