Discovery of the Ultrahigh-energy Gamma-Ray Source LHAASO J2108+5157

• Published in: Astrophysical journal. Letters 2021-10, Vol.919 (2), p.L22
• Main Authors: Bai, L. X., Bi, X. J., Cai, J. T., Chang, J., Chang, J. F., Chen, E. S., Chen, J., Chen, S. Z., Chen, T. L., Chen, X. L., Cheng, N., Cui, Y. D., Dai, Z. G., Dan-Zeng-Luo-Bu, Dan-Zeng, Volpe, D. della, Fan, J. H., Fang, K., Feng, Y. L., Gao, B., Gao, C. D., Guo, Y. Y., Han, Y. A., He, S. L., He, X. B., He, Y., Heller, M., Hor, Y. K., Hou, C., Huang, X. T., Ji, X. L., Levochkin, K., Li, F., Li, H. B., Li, H. C., Li, H. Y., Li, X. R., Li, Xin, Li, Y., Liang, Y. F., Liu, B., Liu, H., Liu, J., Liu, J. L., Liu, Z. X., Lv, H. K., Ma, B. Q., Ma, L. L., Ma, X. H., Mitthumsiri, W., Pang, B. Y., Pei, Z. Y., Qiao, B. Q., Ruffolo, D., Rulev, V., Sáiz, A., Sheng, X. D., Sun, Q. N., Sun, Z. B., Tam, P. H. T., Tang, Z. B., Wang, B. D., Wang, C., Wang, H., Wang, L. P., Wang, W., Wang, X. Y., Wang, Y. J., Wang, Z. H., Wang, Z. X., Wang, Zhen, Wei, J. J., Wei, Y. J., Wen, T., Wu, C. Y., Wu, H. R., Wu, X. F., Xiao, D. X., Xiao, G., Xin, G. G., Xin, Y. L., Yan, J. Z., Yang, J. Y., Yang, S. B., Ye, Y. M., Yin, N., Yu, Y. H., Zha, M., Zhai, X. X., Zhang, B. B., Zhang, P. F., Zhang, P. P., Zhang, R., Zhang, Y. L., Zhang, Yi, Zhang, Yong, Zhou, J. N., Zhou, X. X., Zhu, F. R., Zhu, H., Zuo, X.
• Format: Article
• Language: English
• Subjects: Energy bands; Energy spectra; Gamma emission; Gamma ray sources; Gamma rays; Gamma-ray astronomy; Gamma-ray observatories; Molecular clouds; Point spread functions
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ac2579

We 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 9.5 σ and 8.5 σ , 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 ± 0.18 stat . 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.