Observation of the ultra-high-energy $\gamma$-ray emission from theG25.0+0.0 region with LHAASO

We report the detection of very-high-energy (VHE, 100 GeV < E < 100 TeV) and ultra-high-energy (UHE, E > 100 TeV) ^ emissions from the direction of the star-forming region G25.0+0.0, observed by the Large High Altitude Air Shower Observatory (LHAASO). We find that the emission observed by the Water Cherenkov Detector Array (WCDA) is best described by two distinct components, including the extended source J1837EXT with a significance of 50σ and the point-like source J1837PS with a significance of 14σ. In contrast, the 1.3 km 2 array (KM2A) detects only a single component, J1837EXT, at a significance of 30σ. The emission above 100 TeV is resolved into only one point-like source with a significance of 8.3σ, and smooth connections with the spectral energy distribution (SED) of J1837EXT below 100 TeV imply a possible common origin. By combining multiwavelength data, we attribute the emission origin from J1837PS to two pulsar wind nebula (PWN) systems: G25.24 dominates the VHE emission, while AX J1837 contributes the UHE component. The angular extension of the J1837EXT component is about 0.5 degrees. The observed GeV-TeV SED for J1837EXT can be well explained by a hadronic model with an exponential cutoff power-law spectral (ECPL) index of ∼ 2.1 and a cutoff energy of 66 TeV. Our findings suggest that the emissions from J1837EXT most likely originate from the interaction of particles accelerated by the supernova remnant (SNR) embedded in the young massive star cluster RSGC1 with ambient gas. However, a scenario where cosmic rays (CRs) are accelerated by the stellar winds of the young star-forming region W42, and then transported to interact with clouds to produce in the observed region, cannot be completely ruled out.