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Evidence of cosmic-ray acceleration up to sub-PeV energies in the supernova remnant IC 443

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

TL;DR

The study addresses whether supernova remnants can accelerate cosmic rays to PeV energies, using LHAASO observations of IC 443 and a 3D likelihood analysis that separates a point-like component (C0) and an extended component (C1), with cross-checks against Fermi-LAT data to establish the GeV–TeV connection and π⁰-decay signatures. The results show C0 aligns with the Fermi-LAT π⁰-decay source and yields a 95% lower limit on the proton cutoff of p_cut ≈ 300 TeV, while C1 is extended and compatible with hadronic or leptonic emission, together implying sub-PeV proton acceleration in the SNR–MC system IC 443. These findings support the role of SNRs interacting with molecular clouds as Galactic PeVatrons and provide constraints on particle escape and diffusion around SNRs. The analysis robustly accounts for systematic uncertainties and leverages cross-band data to strengthen the hadronic interpretation of the observed γ-ray emission.

Abstract

Supernova remnants (SNRs) have been considered as the primary contributors to cosmic rays (CRs) in our Galaxy. However, the maximum energy of particles that can be accelerated by shocks of SNRs is uncertain observationally and theoretically, and the role of contribution to CRs around PeV energies by SNRs is unclear. In this study, we present observations of high-energy $γ$-ray emission from the SNR IC 443 using the Large High Altitude Air Shower Observatory (LHAASO). The morphological analysis reveals a pointlike source whose location and spectrum are consistent with those of the Fermi-LAT-detected compact source with $π^0$-decay signature, and a more extended source which is consistent with a newly discovered source, previously unrecognized by Fermi-LAT. The spectrum of the point source can be described by a power-law function with an index of $\sim3.0$, extending beyond $\sim 30$ TeV without apparent cutoff. Assuming a hadronic origin of the $γ$-ray emission, the $95\%$ lower limit of accelerated protons reaches about 300 TeV. The extended source might be coincident with IC 443, SNR G189.6+3.3 or the putative pulsar wind nebula CXOU J061705.3+222127, and can be explained by either a hadronic or leptonic model. The LHAASO results provide compelling evidence that CR protons up to sub-PeV energies can be accelerated by the SNR.

Evidence of cosmic-ray acceleration up to sub-PeV energies in the supernova remnant IC 443

TL;DR

The study addresses whether supernova remnants can accelerate cosmic rays to PeV energies, using LHAASO observations of IC 443 and a 3D likelihood analysis that separates a point-like component (C0) and an extended component (C1), with cross-checks against Fermi-LAT data to establish the GeV–TeV connection and π⁰-decay signatures. The results show C0 aligns with the Fermi-LAT π⁰-decay source and yields a 95% lower limit on the proton cutoff of p_cut ≈ 300 TeV, while C1 is extended and compatible with hadronic or leptonic emission, together implying sub-PeV proton acceleration in the SNR–MC system IC 443. These findings support the role of SNRs interacting with molecular clouds as Galactic PeVatrons and provide constraints on particle escape and diffusion around SNRs. The analysis robustly accounts for systematic uncertainties and leverages cross-band data to strengthen the hadronic interpretation of the observed γ-ray emission.

Abstract

Supernova remnants (SNRs) have been considered as the primary contributors to cosmic rays (CRs) in our Galaxy. However, the maximum energy of particles that can be accelerated by shocks of SNRs is uncertain observationally and theoretically, and the role of contribution to CRs around PeV energies by SNRs is unclear. In this study, we present observations of high-energy -ray emission from the SNR IC 443 using the Large High Altitude Air Shower Observatory (LHAASO). The morphological analysis reveals a pointlike source whose location and spectrum are consistent with those of the Fermi-LAT-detected compact source with -decay signature, and a more extended source which is consistent with a newly discovered source, previously unrecognized by Fermi-LAT. The spectrum of the point source can be described by a power-law function with an index of , extending beyond TeV without apparent cutoff. Assuming a hadronic origin of the -ray emission, the lower limit of accelerated protons reaches about 300 TeV. The extended source might be coincident with IC 443, SNR G189.6+3.3 or the putative pulsar wind nebula CXOU J061705.3+222127, and can be explained by either a hadronic or leptonic model. The LHAASO results provide compelling evidence that CR protons up to sub-PeV energies can be accelerated by the SNR.

Paper Structure

This paper contains 4 sections, 1 equation, 6 figures, 4 tables.

Table of Contents

  1. ROI of the analysis
  2. Systematic uncertainties
  3. LHAASO fluxes of sources C0 and C1
  4. Fermi-LAT analysis

Figures (6)

  • Figure 1: Panel (a) shows the LHAASO observed significance map of a $3\times3$ deg$^2$ region surrounding IC 443 for $E>0.5$ TeV, overlaid with the 1.4 GHz radio continuum contours in dark-green 2008AJ....135..796L. The dashed line in the bottom right corner shows the diameter of the PSF, weighted by the TS value of each energy bin. Red square labels the centroid of C0, and blue square and circle show the centroid and $68\%$ containment size (intrinsic) of C1. Panel (b) shows the one-dimensional distribution of the integrated $\gamma$-ray fluxes from the rectangle box shown in panel (a), together with the PSF convolved profiles of C0 and C1. The zero point is chosen as the midpoint between C0 and C1. The dotted line shows the PSF profile centered at C0. Panels (c) and (d) show the significance maps of C0 and C1 components, respectively. The centroids and $68\%$ extensions (if any) observed by other experiments are also shown for comparison. The cyan contours depict the shocked molecular gas distribution measured by the MWISP project with velocities ranging from $-10$ km/s to $10$ km/s. In panel (d), the centroid and extension of SNR G189.6+3.3 are shown in white.
  • Figure 2: The SEDs of the two sources C0 and C1, with statistical errors (red) and total errors including statistical and systematic ones (black). Arrows show the 95% confidence level upper limits, and solid lines show the best-fitting spectra of the two sources. Results measured by MAGIC 2007ApJ...664L..87A, VERITAS 2009ApJ...698L.133A, and HAWC 2025arXiv250112613A are also shown.
  • Figure 3: Gamma-ray spectra of C0 (top panel) and C1 (bottom panel) as measured by Fermi-LAT and LHAASO. Solid lines in the plots show the hadronic model predictions of the spectra, assuming no spectral cutoff of protons ($p_{\rm cut}=\infty$). In the top panel, the dashed line shows the hadronic model flux for $p_{\rm cut}=300$ TeV (for C0), and in the bottom panel, the dashed line shows the prediction of a leptonic model (for C1).
  • Figure 4: Cumulative probability distribution of parameter $1/p_{\rm cut}$ for source C0. The $95\%$ lower limit on $1/p_{\rm cut}$ is 0.0034 TeV$^{-1}$, as indicated by the vertical dashed line, corresponding to $p_{\rm cut} \approx 300$ TeV.
  • Figure S1: Left: the fan-shaped region of interest (ROI) of this analysis, with the white circle denoting a $4^{\circ}$ radius centered on IC 443. Right: the one-dimensional radial distribution of the integrated $\gamma$-ray fluxes above 1 TeV for different components, centered at the midpoint between C0 and C1.
  • ...and 1 more figures