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Insights from LHAASO and IceCube into the origin of the Galactic diffuse teraelectronvolt–petaelectronvolt emission

Nature Astronomy (2024)Cite this article

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Abstract

High-energy diffuse gamma-ray and neutrino emission are expected from the Galactic plane, generated by hadronuclear interactions between cosmic rays and the interstellar medium. Therefore, measurements of this diffuse emission will provide important clues to the origin and nature of Galactic cosmic rays. Comparing the latest observations of the Large High-Altitude Air Shower Observatory and IceCube on diffuse Galactic gamma-ray and neutrino emission, respectively, we suggest that the diffuse gamma-ray emission at multi-teraelectronvolt energies contains a considerable leptonic component. By modelling the gamma-ray halos powered by middle-aged pulsars in our Galaxy, taking into account the magnetic field configuration and the interstellar radiation field in the Galaxy, we demonstrate that the collective contribution of pulsar halos can account for the excess in the measured diffuse gamma-ray emission with respect to the predicted flux from cosmic-ray–interstellar medium interactions.

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Fig. 1: Comparison of the gamma-ray flux derived from neutrino measurement and the total flux of known Galactic gamma-ray sources excluding those with pulsar association.
Fig. 2: Comparison of the DGE fluxes derived from different analyses.
Fig. 3: Predicted gamma-ray intensity map in 25–100 TeV contributed by halos of middle-aged pulsars recorded in ATNF catalogue under the 2ID model.
Fig. 4: Predicted gamma-ray intensity map in 25–100 TeV contributed by halos of middle-aged pulsars recorded in ATNF catalogue under the anisotropic diffusion model.
Fig. 5: Expected contribution of DGE from pulsar halos and CR–ISM interactions.
Fig. 6: Galactic longitudinal gamma-ray profile contributed by pulsar halos.

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Data availability

Two templates for IceCube’s data analysis, the all-sky gamma-ray intensity maps predicted by CR–ISM interactions and two pulsar halo models are available at https://box.nju.edu.cn/d/a441e4a5b4cd4a71a2dd/. The data of the diffuse gamma-ray measured by LHAASO are available at ref. 13. Properties of the LHAASO sources and HESS sources can be found from refs. 12 and 50, respectively. The basic properties of the pulsars used in the calculation can be found at the ATNF pulsar catalogue via the link https://www.atnf.csiro.au/research/pulsar/psrcat/.

Code availability

GALPROP is available at https://galprop.stanford.edu. The codes generating figures in this study are available at https://box.nju.edu.cn/d/a441e4a5b4cd4a71a2dd/. The numerical code simulating the spectrum and 2D intensity profile of pulsar halos under two different models can be provided upon request.

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Acknowledgements

We are grateful to L. Lu for the helpful discussion on IceCube’s results and K. Fang for the valuable comment. This work is supported by the National Natural Science Foundation of China (grant nos. U2031105, 12393852, 12220101003, 12333006) and the Project for Young Scientists in Basic Research of Chinese Academy of Sciences (grant no. YSBR-061).

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Authors and Affiliations

  1. School of Astronomy and Space Science, Nanjing University, Nanjing, China

    Kai Yan, Ruo-Yu Liu, Chao-Ming Li & Xiang-Yu Wang

  2. Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing, China

    Kai Yan, Ruo-Yu Liu, Chao-Ming Li & Xiang-Yu Wang

  3. Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China

    Rui Zhang & Qiang Yuan

  4. School of Astronomy and Space Science, University of Science and Technology of China, Hefei, China

    Rui Zhang & Qiang Yuan

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Contributions

R.-Y.L. designed the study and led the writing of the paper. K.Y. modelled the emission of pulsar halos, and analysed the IceCube data and LHAASO data. R.Z. carried out the simulation of cosmic-ray propagation in the Galaxy and their radiation. C.-M.L. analysed the gamma-ray source contribution. Q.Y. and X.-Y.W. discussed the interpretation of the results. All the authors edited the paper.

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Correspondence to Ruo-Yu Liu.

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Extended data

Extended Data Fig. 1 Neutrino intensity map with the π0 template and KRA\({}_{\boldsymbol{\gamma }}^{\mathbf{5}}\) template.

(a) the all-sky intensity map with the π0 template. (b) the all-sky intensity map with the KRA\({}_{\gamma }^{5}\) template. (c) the intensity map in the same ROI of LHAASO’s DGE analysis, after removing the region out of the Galactic plane and masking the LHAASO source region, for the π0 template. (d) same as (c) but for the KRA\({}_{\gamma }^{5}\) template.

Extended Data Fig. 2 Distribution of the strength of the magnetic field in the Galactic plane.

The Galactic centre is located at (0,0). The positions of pulsars considered in this work are marked as blue dots. The boundary of the Galactic longitude of LHAASO’s ROI l = 15° and l = 235° are labeled for as dashed line for reference.

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Supplementary Information

Supplementary Figs. 1–5 and discussion.

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Yan, K., Liu, RY., Zhang, R. et al. Insights from LHAASO and IceCube into the origin of the Galactic diffuse teraelectronvolt–petaelectronvolt emission. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02221-y

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