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Natural SciencesAstronomyPhysicsAstrophysicsCosmologyRadiation PhysicsContinuum EmissionHigh-energy Astrophysical SourcesHigh-energy AstrophysicsObservational CosmologySpectral RadiationCosmic-ray SpectraGalactic Cosmic RaysHigh-frequency Radio ContinuumRadio Continuum EmissionUpper Limit Estimation