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Potential PeVatron supernova remnant G106.3+2.7 seen in the highest-energy gamma rays

Abstract

Cosmic rays (protons and other atomic nuclei) are believed to gain energies of petaelectronvolts (PeV) and beyond at astrophysical particle accelerators called ‘PeVatrons’ inside our Galaxy. Although a characteristic feature of a PeVatron is expected to be a hard gamma-ray energy spectrum that extends beyond 100 teraelectronvolts (TeV) without a cut-off, none of the currently known sources exhibit such a spectrum owing to the low maximum energy of accelerated cosmic rays or owing to insufficient detector sensitivity around 100 TeV. Here, we report the observation of gamma-ray emission from the supernova remnant G106.3+2.7 (refs. 1,2) above 10 TeV. This work provides flux data points up to and above 100 TeV and indicates that the very-high-energy gamma-ray emission above 10 TeV is well correlated with a molecular cloud3 rather than with the pulsar PSR J2229+6114 (refs. 4,5,6,7,8). Regarding the gamma-ray emission mechanism of G106.3+2.7, this morphological feature appears to favour a hadronic origin via the π0 decay caused by accelerated relativistic protons9 over a leptonic origin via the inverse Compton scattering by relativistic electrons10,11. Furthermore, we point out that an X-ray flux upper limit on the synchrotron spectrum would provide important information to firmly establish the hadronic scenario as the mechanism of particle acceleration at the source.

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Fig. 1: Significance map around SNR G106.3+2.7 as observed by Tibet AS+MD above 10 TeV.
Fig. 2: Projected angular distribution of events observed above 10 TeV.
Fig. 3: Differential energy spectrum of gamma-ray emissions from SNR G106.3+2.7.

Data availability

The data that support the plots within this paper and other findings of this study are available from the website of the Tibet ASγ Collaboration (https://www.tibet-asg.org) or from the corresponding authors upon reasonable request.

Code availability

The codes used in this work are embedded within the analysis framework of the Tibet AS+MD array, and it is not practically possible to extract them. The codes, therefore, are not publicly available.

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Acknowledgements

The collaborative experiment of the Tibet Air Shower Arrays has been conducted under the auspices of the Ministry of Science and Technology of China and the Ministry of Foreign Affairs of Japan. This work was supported in part by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Culture, Sports, Science and Technology, and by Grants-in-Aid for Science Research from the Japan Society for the Promotion of Science in Japan. This work was supported by the National Key R&D Program of China (grant no. 2016YFE0125500), the National Natural Science Foundation of China (grant nos. 11533007, 11673041, 11873065, 11773019, 11773014, 11633007, 11803011 and 11851305) and the Key Laboratory of Particle Astrophysics at the Institute of High Energy Physics of the Chinese Academy of Sciences. The research presented in this paper made use of data supplied through the Canadian Galactic Plane Survey. This work was also supported by the joint research programme of the Institute for Cosmic Ray Research at the University of Tokyo.

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Contributions

The entire Tibet ASγ Collaboration contributed to the publication in terms of various aspects of the research ranging from hardware-related issues such as the design, construction, maintenance and calibration of the instrument to software-related issues such as data reduction, data analysis, Monte Carlo simulation and astrophysical explanation. D.C., J.H., M.O., T.K.S., M.T. and X.Z. analysed the data and prepared the manuscript. All the authors in the collaboration discussed the results of this work and commented on the manuscript.

Corresponding authors

Correspondence to D. Chen or J. Huang or M. Ohnishi or T. K. Sako or M. Takita or X. Zhang.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Yutaka Fujita and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Spectral gamma-ray energy distribution of G106.3+2.7.

a, The flux data points with 1σ statistical error bars include measurements by Tibet AS+MD (red dots; this work), Fermi30 (blue squares), VERITAS14 (purple pentagons) and the Dominion Radio Astrophysical Observatory’s Synthesis Telescope2 (turquoise blue dots). The two red downward arrows above 1014 eV show 99% C.L. upper limits obtained by this work. Note that all the VERITAS data points are raised by a factor of 1.62 to account for the spill-over of gamma-ray signals outside their window size of 0.32 radius. The best-fit gamma-ray energy spectrum in the leptonic model is shown by the black solid curve, with the flux by the electron synchrotron radiation (the orange solid curve), the IC scattering of CMB photons (the green dashed curve) and the IC scattering of IR photons (the light blue dash-dotted curve). The gray open diamond shows the flux of PSR J2229+6114 obtained in the 2 − 10 keV range6. b, The best-fit gamma-ray energy spectrum in the hadronic model is shown by the turquoise blue solid curve. The lower panels show the residual Δσof the fit.

Supplementary information

Supplementary Information

Supplementary Fig. 1, Tables 1–2 and discussion.

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The Tibet ASγ Collaboration., Amenomori, M., Bao, Y.W. et al. Potential PeVatron supernova remnant G106.3+2.7 seen in the highest-energy gamma rays. Nat Astron (2021). https://doi.org/10.1038/s41550-020-01294-9

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