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 (π⁰ 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 + H₂) 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 + H₂) = 7.5±2.9×10³ M_⊙. Under a hadronic scenario, we obtain a total energy of protons of W_p = 4.3 ± 1.5 × 10⁴⁶ 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 π⁰ decay.

As part of the thesis to be submitted by Toledano-Juárez as a partial fulfillment for the requirements of Ph. D. Degree in Physics, Doctorado en Ciencias (Física), CUCEI, Universidad de Guadalajara.