Multiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157

Abstract

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The Large High Altitude Air Shower Observatory (LHAASO) collaboration recently discovered several new ultrahigh-energy (UHE) γ-ray sources, representing progress in the search for Galactic PeVatrons, the sources of Galactic cosmic rays up to PeV energies. However, distinguishing between hadronic accelerators and inverse Compton scattering as the emission mechanism requires multi-wavelength data and detailed source morphology. This study focuses on the unidentified source LHAASO J2108+5157, lacking a very high-energy (VHE) counterpart. Observations were conducted in the X-ray and TeV energy bands with XMM-Newton and the Large-Sized Telescope prototype (LST-1), respectively. Fermi-LAT data from 12 years were also analyzed to investigate its high-energy (HE) counterpart, 4FGL J2108.0+5155. The results showed an excess in the LST-1 data at E > 3 TeV and a hint of hard emission in the entire energy range. No significant extended emission related to a supernova remnant or pulsar wind nebula was found in the X-ray data. Additionally, a new potential hard source was discovered in Fermi-LAT data. The observations were explained as inverse Compton-dominated leptonic emission of relativistic electrons with a cutoff energy of 100−30+70 TeV. The low magnetic field inferred from X-ray upper limits is compatible with a pulsar wind nebula or a TeV halo. The spectral properties of the HE counterpart are consistent with a Geminga-like pulsar, which could power the VHE-UHE emission. However, the lack of a pulsar near the UHE source poses a challenge to this scenario. Alternatively, the UHE γ rays could be explained by hadronic emission resulting from interactions of relativistic protons with molecular clouds near the source. The origin of the HE γ-ray emission remains an open question. In conclusion, the study presents evidence of possible leptonic and hadronic emission scenarios for the UHE γ-ray source, with further investigations needed to determine the exact nature of this enigmatic source.