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 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. DaiDan Zeng Dan-Zeng-Luo-BuD. 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. LiH. C. 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 SciencesUniversité de GenèveHebei Normal UniversityTibet UniversityTIANFU Cosmic Ray Research Center2022-08-042022-08-042021-10-01Astrophysical Journal Letters. Vol.919, No.2 (2021)20418213204182052-s2.0-85116440330https://repository.li.mahidol.ac.th/handle/123456789/76847We report the discovery of an ultrahigh-energy (UHE) gamma-ray source, LHAASO J2108+5157, by analyzing the LHAASO-KM2A data of 308.33 live days. A significant excess of gamma ray-induced showers is observed in both energy bands of 25-100 and >100 TeV with 9.5σ and 8.5σ, respectively. This source is not significantly favored as an extended source with an angular extension smaller than the point-spread function of KM2A. The measured energy spectrum from 20 to 200 TeV can be approximately described by a power-law function with an index of-2.83 ± 0.18stat. A harder spectrum is demanded at lower energies considering the flux upper limit set by Fermi-LAT observations. The position of the gamma-ray emission is correlated with a giant molecular cloud, which favors a hadronic origin. No obvious counterparts have been found, and deeper multiwavelength observations will help to cast new light on this intriguing UHE source.Mahidol UniversityEarth and Planetary SciencesPhysics and AstronomyArticleSCOPUS10.3847/2041-8213/ac2579