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LHAASO Detection of Ultra-High-Energy Gamma-Ray Emission toward the Giant Molecular Clouds

Zhen Cao, F. Aharonian, Y. X. Bai, Y. W. Bao, D. Bastieri, X. J. Bi, Y. J. Bi, W. Bian, A. V. Bukevich, C. M. Cai, W. Y. Cao, Zhe Cao, J. Chang, J. F. Chang, A. M. Chen, E. S. Chen, G. H. Chen, H. X. Chen, Liang Chen, Long Chen, M. J. Chen, M. L. Chen, Q. H. Chen, S. Chen, S. H. Chen, S. Z. Chen, T. L. Chen, X. B. Chen, X. J. Chen, Y. Chen, N. Cheng, Y. D. Cheng, M. C. Chu, M. Y. Cui, S. W. Cui, X. H. Cui, Y. D. Cui, B. Z. Dai, H. L. Dai, Z. G. Dai, Danzengluobu, Y. X. Diao, X. Q. Dong, K. K. Duan, J. H. Fan, Y. Z. Fan, J. Fang, J. H. Fang, K. Fang, C. F. Feng, H. Feng, L. Feng, S. H. Feng, X. T. Feng, Y. Feng, Y. L. Feng, S. Gabici, B. Gao, C. D. Gao, Q. Gao, W. Gao, W. K. Gao, M. M. Ge, T. T. Ge, L. S. Geng, G. Giacinti, G. H. Gong, Q. B. Gou, M. H. Gu, F. L. Guo, J. Guo, X. L. Guo, Y. Q. Guo, Y. Y. Guo, Y. A. Han, O. A. Hannuksela, M. Hasan, H. H. He, H. N. He, J. Y. He, X. Y. He, Y. He, S. Hernández-Cadena, B. W. Hou, C. Hou, X. Hou, H. B. Hu, S. C. Hu, C. Huang, D. H. Huang, J. J. Huang, T. Q. Huang, W. J. Huang, X. T. Huang, X. Y. Huang, Y. Huang, Y. Y. Huang, X. L. Ji, H. Y. Jia, K. Jia, H. B. Jiang, K. Jiang, X. W. Jiang, Z. J. Jiang, M. Jin, S. Kaci, M. M. Kang, I. Karpikov, D. Khangulyan, D. Kuleshov, K. Kurinov, B. B. Li, Cheng Li, Cong Li, D. Li, F. Li, H. B. Li, H. C. Li, Jian Li, Jie Li, K. Li, L. Li, R. L. Li, S. D. Li, T. Y. Li, W. L. Li, X. R. Li, Xin Li, Y. Li, Y. Z. Li, Zhe Li, Zhuo Li, E. W. Liang, Y. F. Liang, S. J. Lin, B. Liu, C. Liu, D. Liu, D. B. Liu, H. Liu, H. D. Liu, J. Liu, J. L. Liu, J. R. Liu, M. Y. Liu, R. Y. Liu, S. M. Liu, W. Liu, X. Liu, Y. Liu, Y. Liu, Y. N. Liu, Y. Q. Lou, Q. Luo, Y. Luo, H. K. Lv, B. Q. Ma, L. L. Ma, X. H. Ma, J. R. Mao, Z. Min, W. Mitthumsiri, G. B. Mou, H. J. Mu, A. Neronov, K. C. Y. Ng, M. Y. Ni, L. Nie, L. J. Ou, P. Pattarakijwanich, Z. Y. Pei, J. C. Qi, M. Y. Qi, J. J. Qin, A. Raza, C. Y. Ren, D. Ruffolo, A. Sáiz, D. Semikoz, L. Shao, O. Shchegolev, Y. Z. Shen, X. D. Sheng, Z. D. Shi, F. W. Shu, H. C. Song, Yu. V. Stenkin, V. Stepanov, Y. Su, D. X. Sun, H. Sun, Q. N. Sun, X. N. Sun, Z. B. Sun, N. H. Tabasam, J. Takata, P. H. T. Tam, H. B. Tan, Q. W. Tang, R. Tang, Z. B. Tang, W. W. Tian, C. N. Tong, L. H. Wan, C. Wang, G. W. Wang, H. G. Wang, J. C. Wang, K. Wang, Kai Wang, Kai Wang, L. P. Wang, L. Y. Wang, L. Y. Wang, R. Wang, W. Wang, X. G. Wang, X. J. Wang, X. Y. Wang, Y. Wang, Y. D. Wang, Z. H. Wang, Z. X. Wang, Zheng Wang, D. M. Wei, J. J. Wei, Y. J. Wei, T. Wen, S. S. Weng, C. Y. Wu, H. R. Wu, Q. W. Wu, S. Wu, X. F. Wu, Y. S. Wu, S. Q. Xi, J. Xia, J. J. Xia, G. M. Xiang, D. X. Xiao, G. Xiao, Y. L. Xin, Y. Xing, D. R. Xiong, Z. Xiong, D. L. Xu, R. F. Xu, R. X. Xu, W. L. Xu, L. Xue, D. H. Yan, T. Yan, C. W. Yang, C. Y. Yang, F. F. Yang, L. L. Yang, M. J. Yang, R. Z. Yang, W. X. Yang, Z. H. Yang, Z. G. Yao, X. A. Ye, L. Q. Yin, N. Yin, X. H. You, Z. Y. You, Q. Yuan, H. Yue, H. D. Zeng, T. X. Zeng, W. Zeng, X. T. Zeng, M. Zha, B. B. Zhang, B. T. Zhang, C. Zhang, F. Zhang, H. Zhang, H. M. Zhang, H. Y. Zhang, J. L. Zhang, Li Zhang, P. F. Zhang, P. P. Zhang, R. Zhang, S. R. Zhang, S. S. Zhang, W. Y. Zhang, X. Zhang, X. P. Zhang, Yi Zhang, Yong Zhang, Z. P. Zhang, J. Zhao, L. Zhao, L. Z. Zhao, S. P. Zhao, X. H. Zhao, Z. H. Zhao, F. Zheng, W. J. Zhong, B. Zhou, H. Zhou, J. N. Zhou, M. Zhou, P. Zhou, R. Zhou, X. X. Zhou, X. X. Zhou, B. Y. Zhu, C. G. Zhu, F. R. Zhu, H. Zhu, K. J. Zhu, Y. C. Zou, X. Zuo, Y. H. Yu

TL;DR

This paper reports the first detection of very-high-energy gamma-ray emission from five nearby giant molecular clouds via a stacking analysis of 4.5 years of LHAASO data, with significant signals coming from the combined clouds (TS up to 30.0 for WCDA and 40.1 for KM2A). The observed spectra are consistent with hadronic gamma-ray production from cosmic-ray interactions with the interstellar medium, in agreement with predictions based on the local CR spectrum. A search for a spectral knee by introducing a break in the gamma-ray spectrum finds no evidence for a knee in the current data, but favors a proton knee position above $0.9$~PeV, aligning with recent direct CR measurements. The work demonstrates GMCs as in-situ tracers of the Galactic CR density and suggests future observations (e.g., SWGO) could directly locate the proton knee and map CR distribution across the Galaxy.

Abstract

The $γ$-ray from Giant molecular clouds (GMCs) is regarded as the most ideal tool to perform in-situ measurement of cosmic ray (CR) density and spectra in our Galaxy. We report the first detection of $γ$-ray emissions in the very-high-energy (VHE) domain from the five nearby GMCs with a stacking analysis based on a 4.5-year $γ$-ray observation with the Large High Altitude Air Shower Observatory (LHAASO) experiment. The spectral energy distributions derived from the GMCs are consistent with the expected $γ$-ray flux produced via CR interacting with the ISM in the energy interval 1 - 100 $~\rm$ TeV. In addition, we investigate the presence of the CR spectral `knee' by introducing a spectral break in the $γ$-ray data. While no significant evidence for the CR knee is found, the current KM2A measurements from GMCs strongly favor a proton CR knee located above 0.9$~\rm$ PeV, which is consistent with the latest measurement of the CR spectrum by ground-based experiments.

LHAASO Detection of Ultra-High-Energy Gamma-Ray Emission toward the Giant Molecular Clouds

TL;DR

This paper reports the first detection of very-high-energy gamma-ray emission from five nearby giant molecular clouds via a stacking analysis of 4.5 years of LHAASO data, with significant signals coming from the combined clouds (TS up to 30.0 for WCDA and 40.1 for KM2A). The observed spectra are consistent with hadronic gamma-ray production from cosmic-ray interactions with the interstellar medium, in agreement with predictions based on the local CR spectrum. A search for a spectral knee by introducing a break in the gamma-ray spectrum finds no evidence for a knee in the current data, but favors a proton knee position above ~PeV, aligning with recent direct CR measurements. The work demonstrates GMCs as in-situ tracers of the Galactic CR density and suggests future observations (e.g., SWGO) could directly locate the proton knee and map CR distribution across the Galaxy.

Abstract

The -ray from Giant molecular clouds (GMCs) is regarded as the most ideal tool to perform in-situ measurement of cosmic ray (CR) density and spectra in our Galaxy. We report the first detection of -ray emissions in the very-high-energy (VHE) domain from the five nearby GMCs with a stacking analysis based on a 4.5-year -ray observation with the Large High Altitude Air Shower Observatory (LHAASO) experiment. The spectral energy distributions derived from the GMCs are consistent with the expected -ray flux produced via CR interacting with the ISM in the energy interval 1 - 100 TeV. In addition, we investigate the presence of the CR spectral `knee' by introducing a spectral break in the -ray data. While no significant evidence for the CR knee is found, the current KM2A measurements from GMCs strongly favor a proton CR knee located above 0.9 PeV, which is consistent with the latest measurement of the CR spectrum by ground-based experiments.

Paper Structure

This paper contains 9 sections, 3 equations, 7 figures, 2 tables.

Table of Contents

  1. The $\gamma$-ray flux of the giant molecular clouds
  2. Comparison with the predicted gamma-ray flux and constraining to the CR spectrum
  3. Conclusion
  4. The Giant Molecular Clouds
  5. LHAASO data analysis
  6. Likelihood analysis
  7. Testing Spectral Break Predictions at the CR 'Knee' with KM2A Data
  8. Stacking technique
  9. Systematic uncertainties

Figures (7)

  • Figure 1: Stacked TS profile for GMC sample containing 5 sources. The spectrum of GMCs is assumed to follow a power-law distribution. The TS value is color-coded for each flux and index combination. The 'x' sign shows the maximum value. The two solid contours represent 68$\%$ and 90$\%$ confidence level.
  • Figure 1: The total column density (in units of cm$^{-2}$) of GMCs derived from the Plack dust opacity.
  • Figure 2: The SED of the GMC as measured by LHAASO-KM2A. The differential flux has been normalized to a column density of $1\times10^{22}$ cm $^{-2}$ according to the column density of each GMC. The red solid (dashed ) line shows the best-fit power-law function of the KM2A (WCDA) data, and the gray shaded band is the $\pm1\sigma$ statistical uncertainty. The measurement by Fermi (gray points) is shown for comparison, which has been scaled based on the column density and distance of the cloud 2017AA...606A..22N. The blue shaded band represents the expected $\gamma$-ray flux produced via CR interacting with ISM, assuming a spectrum of CR which coincides with that used in the 2023KM2A_diffuse.
  • Figure 2: The significance map of the GMC regions at energy above 25 TeV. The contours represent the regions in gas column density larger than 7 $\times$10$^{21}$ cm$^{-2}$ (for Hercules is 4 $\times$10$^{21}$ cm$^{-2})$.
  • Figure 3: Stacked TS profile for GMCs obtained by fitting KM2A data with the predicted gamma-ray spectrum using the $AAFRAG$ package, assuming various values of $E_{knee}$ of cosmic rays and the post-knee index. The spectral shape was fixed during the fit, with the flux normalization left free. Significance contours are overlaid on the plot showing the 90$\%$ and 95$\%$ confidence levels.
  • ...and 2 more figures