Zhen CaoF. AharonianQ. AnAxikeguL. X. BaiY. X. BaiY. W. BaoD. BastieriX. J. BiY. J. BiH. CaiJ. T. CaiZhe CaoJ. ChangJ. F. ChangB. M. ChenE. S. ChenJ. ChenLiang ChenLiang ChenLong ChenM. J. ChenM. L. ChenQ. H. ChenS. H. ChenS. Z. ChenT. L. ChenX. L. ChenY. ChenN. ChengY. D. ChengS. W. CuiX. H. CuiY. D. CuiB. D'Ettorre PiazzoliB. Z. DaiH. L. DaiZ. G. DaiDanzengluobuD. Della VolpeX. J. DongK. K. DuanJ. H. FanY. Z. FanZ. X. FanJ. FangK. FangC. F. FengL. FengS. H. FengY. L. FengB. GaoC. D. GaoL. Q. GaoQ. GaoW. GaoM. M. GeL. S. GengG. H. GongQ. B. GouM. H. GuF. L. GuoJ. G. GuoX. L. GuoY. Q. GuoY. Y. GuoY. A. HanH. H. HeH. N. HeJ. C. HeS. L. HeX. B. HeY. HeM. HellerY. K. HorC. HouH. B. HuS. HuS. C. HuX. J. HuD. H. HuangQ. L. HuangW. H. HuangX. T. HuangX. Y. HuangZ. C. HuangF. JiX. L. JiH. Y. JiaK. JiangZ. J. JiangC. JinT. KeD. KuleshovK. LevochkinB. B. LiCheng LiCong LiF. LiH. B. LiState Key Laboratory of Particle Detection & ElectronicsNanjing UniversityShanghai Astronomical Observatory Chinese Academy of SciencesInstitute for Nuclear Research of the Russian Academy of SciencesShandong UniversityWuhan UniversityYunnan UniversityInstitute of High Energy Physics Chinese Academy of ScienceUniversity of Chinese Academy of SciencesGuangzhou UniversityTsinghua UniversitySun Yat-Sen UniversityUniversity of Science and Technology of ChinaZhengzhou UniversityDublin Institute for Advanced StudiesUniversità degli Studi di Napoli Federico IISichuan UniversityNational Astronomical Observatories Chinese Academy of SciencesMax-Planck-Institut für KernphysikSouthwest Jiaotong UniversityPurple Mountain Observatory Chinese Academy of SciencesHebei Normal UniversityTibet UniversityUniversit'e de Gen veTIANFU Cosmic Ray Research Center2022-08-042022-08-042021-08-10Astrophysical Journal Letters. Vol.917, No.1 (2021)20418213204182052-s2.0-85113363927https://repository.li.mahidol.ac.th/handle/20.500.14594/76856We report the discovery of a new unidentified extended γ-ray source in the Galactic plane named LHAASO J0341+5258 with a pretrial significance of 8.2 standard deviations above 25 TeV. The best-fit position is R.A. = 55. 34 0. 11 and decl. = 52. 97 0. 07. The angular size of LHAASO J0341+5258 is 0. 29 0. 06stat 0. 02sys. The flux above 25 TeV is about 20% of the flux of the Crab Nebula. Although a power-law fit of the spectrum from 10 to 200 TeV with the photon index α = 2.98 0.19stat 0.02sys is not excluded, the LHAASO data together with the flux upper limit at 10 GeV set by the Fermi-LAT observation, indicate a noticeable steepening of an initially hard power-law spectrum with a cutoff at ≈50 TeV. We briefly discuss the origin of ultra-high-energy gamma rays. The lack of an energetic pulsar and a young supernova remnant inside or in the vicinity of LHAASO J0341+5258 challenge, but do not exclude, both the leptonic and hadronic scenarios of gamma-ray production.Mahidol UniversityEarth and Planetary SciencesPhysics and AstronomyArticleSCOPUS10.3847/2041-8213/ac0fd5