Context: Several new Ultra-High-Energy (UHE) gamma-ray sources have recently been discovered by the LHAASO collaboration, which represent a step forward in the search for the so-called galactic PeVatrons, the enigmatic sources of the Galactic cosmic rays up to PeV energies. However, it has been shown that multi-TeV gamma-ray emission does not necessarily prove the existence of a hadronic accelerator in the source, in fact it could also be explained as inverse Compton scattering from electrons in a radiation dominated environment. A clear distinction between the two major emission mechanisms would only be possible taking into account multi-wavelength data and detailed morphology of the source.Aims: We aim for understanding the nature of the unidentified source LHAASO J2108+5157, which is one of the few known UHE sources with no Very-High-Energy (VHE) counterpart.Methods: We observed LHAASO J2108+5157 in the X-ray band with XMM-Newton in 2021 for a total of 3.8 hours and at TeV energies with the Large-Sized Telescope prototype (LST-1), yielding 49 hours of good quality data. In addition, we analyzed 12 years of Fermi-LAT data, to better constrain emission of its High-Energy (HE) counterpart 4FGL J2108.0+5155. We used naima and jetset software packages to examine leptonic and hadronic scenario of the multi-wavelength emission of the source.Results: We found an excess (3.7σ) in the LST-1 data at energies E > 3 TeV. Further analysis in the whole LST-1 energy range assuming a point-like source, resulted in a hint (2.2σ) of hard emission which can be described with a single power law with photon index Γ = 1.6±0.2 between 0.3−100 TeV. We did not find any significant extended emission which could be related to a Supernova Remnant (SNR) or Pulsar Wind Nebula (PWN) in the XMM-Newton data, which puts strong constraints on possible synchrotron emission of relativistic electrons. We revealed a new potential hard source in Fermi-LAT data with a significance of 4σ and photon index Γ = 1.9 ± 0.2, which is not spatially correlated with LHAASO J2108+5157, but including it in the source model we were able to improve spectral representation of the HE counterpart 4FGL J2108.0+5155.Conclusions: The LST-1 and LHAASO observations can be explained as inverse Compton dominated leptonic emission of relativistic electrons with cutoff energy of 100 +70 −30 TeV. The low magnetic field in the source imposed by the X-ray upper limits on synchrotron emission is compatible with a hypothesis of a TeV halo. Furthermore, the spectral properties of the HE counterpart are consistent with a hypothesis of Geminga-like pulsar, which would be able to power the VHE-UHE emission. LST-1 and Fermi-LAT upper limits impose strong constraints on hadronic scenario of π 0 decay dominated emission from accelerated protons interacting with one of the two known molecular clouds in the direction of the UHE gamma-ray source, requiring hard spectral index, which is incompatible with the standard diffusive acceleration scenario. However, stochastic acceleration of protons...
A deep survey of the Large Magellanic Cloud at ∼0.1–100 TeV photon energies with the Cherenkov Telescope Array is planned. We assess the detection prospects based on a model for the emission of the galaxy, comprising the four known TeV emitters, mock populations of sources, and interstellar emission on galactic scales. We also assess the detectability of 30 Doradus and SN 1987A, and the constraints that can be derived on the nature of dark matter. The survey will allow for fine spectral studies of N 157B, N 132D, LMC P3, and 30 Doradus C, and half a dozen other sources should be revealed, mainly pulsar-powered objects. The remnant from SN 1987A could be detected if it produces cosmic-ray nuclei with a flat power-law spectrum at high energies, or with a steeper index 2.3–2.4 pending a flux increase by a factor >3–4 over ∼2015–2035. Large-scale interstellar emission remains mostly out of reach of the survey if its >10 GeV spectrum has a soft photon index ∼2.7, but degree-scale 0.1–10 TeV pion-decay emission could be detected if the cosmic-ray spectrum hardens above >100 GeV. The 30 Doradus star-forming region is detectable if acceleration efficiency is on the order of 1 − 10 per cent of the mechanical luminosity and diffusion is suppressed by two orders of magnitude within <100 pc. Finally, the survey could probe the canonical velocity-averaged cross section for self-annihilation of weakly interacting massive particles for cuspy Navarro-Frenk-White profiles.
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