Constraining Anisotropic Diffusion between Geminga and Earth with the Cosmic-Ray Electron and Positron Spectrum
The gamma-ray halo around Geminga indicates significant suppression of cosmic-ray diffusion. One possible explanation for this phenomenon is the projection effect of slow diffusion perpendicular to the mean magnetic field (characterized by the diffusion coefficient D _⊥ ) within an anisotropic diffu...
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2025-01-01
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| Online Access: | https://doi.org/10.3847/1538-4357/ad9d39 |
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| author | Junji Xia Xiaojun Bi Kun Fang Siming Liu |
| author_facet | Junji Xia Xiaojun Bi Kun Fang Siming Liu |
| author_sort | Junji Xia |
| collection | DOAJ |
| description | The gamma-ray halo around Geminga indicates significant suppression of cosmic-ray diffusion. One possible explanation for this phenomenon is the projection effect of slow diffusion perpendicular to the mean magnetic field (characterized by the diffusion coefficient D _⊥ ) within an anisotropic diffusion framework. In this scenario, the diffusion coefficient parallel to the mean field ( D _∥ ) can still be large, enabling electrons and positrons ( e ^± ) produced by Geminga to efficiently travel to Earth along the magnetic field lines, possibly resulting in a detectable e ^± flux. In this work, we first determine the basic parameters of the anisotropic model using the morphology and spectral measurements of the Geminga halo and then predict the flux of e ^± produced by Geminga at the location of Earth. We find that the e ^− + e ^+ spectrum of DAMPE can give crucial constraint on the anisotropic diffusion model: to ensure that the predicted spectrum does not exceed the measurements, the Alfvén Mach number of the turbulent magnetic field ( M _A ) should not be less than 0.75, corresponding to D _∥ / D _⊥ ≲ 3 given that ${D}_{\perp }={D}_{\parallel }{M}_{A}^{4}$ . This implies that a significant suppression of D _∥ relative to the average value in the Galaxy may still be necessary. Furthermore, we find that under the anisotropic diffusion model, Geminga can produce a very sharp feature around 1 TeV in the e ^− + e ^+ spectrum, which could naturally explain the peculiar 1.4 TeV excess tentatively observed by DAMPE. |
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| language | English |
| publishDate | 2025-01-01 |
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| spelling | doaj-art-af3af98d7e2f4f2b9b7d7dadd194943c2025-08-20T02:45:00ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-01978216210.3847/1538-4357/ad9d39Constraining Anisotropic Diffusion between Geminga and Earth with the Cosmic-Ray Electron and Positron SpectrumJunji Xia0Xiaojun Bi1https://orcid.org/0000-0002-5334-9754Kun Fang2https://orcid.org/0000-0002-1209-349XSiming Liu3https://orcid.org/0000-0003-1039-9521The School of Physical Science and Technology, Southwest Jiaotong University , Chengdu 611756, People’s Republic of China ; liusm@swjtu.edu.cnKey Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences , Beijing 100049, People’s Republic of China ; bixj@ihep.ac.cn, fangkun@ihep.ac.cn; University of Chinese Academy of Sciences , Beijing 100049, People’s Republic of ChinaKey Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences , Beijing 100049, People’s Republic of China ; bixj@ihep.ac.cn, fangkun@ihep.ac.cnThe School of Physical Science and Technology, Southwest Jiaotong University , Chengdu 611756, People’s Republic of China ; liusm@swjtu.edu.cnThe gamma-ray halo around Geminga indicates significant suppression of cosmic-ray diffusion. One possible explanation for this phenomenon is the projection effect of slow diffusion perpendicular to the mean magnetic field (characterized by the diffusion coefficient D _⊥ ) within an anisotropic diffusion framework. In this scenario, the diffusion coefficient parallel to the mean field ( D _∥ ) can still be large, enabling electrons and positrons ( e ^± ) produced by Geminga to efficiently travel to Earth along the magnetic field lines, possibly resulting in a detectable e ^± flux. In this work, we first determine the basic parameters of the anisotropic model using the morphology and spectral measurements of the Geminga halo and then predict the flux of e ^± produced by Geminga at the location of Earth. We find that the e ^− + e ^+ spectrum of DAMPE can give crucial constraint on the anisotropic diffusion model: to ensure that the predicted spectrum does not exceed the measurements, the Alfvén Mach number of the turbulent magnetic field ( M _A ) should not be less than 0.75, corresponding to D _∥ / D _⊥ ≲ 3 given that ${D}_{\perp }={D}_{\parallel }{M}_{A}^{4}$ . This implies that a significant suppression of D _∥ relative to the average value in the Galaxy may still be necessary. Furthermore, we find that under the anisotropic diffusion model, Geminga can produce a very sharp feature around 1 TeV in the e ^− + e ^+ spectrum, which could naturally explain the peculiar 1.4 TeV excess tentatively observed by DAMPE.https://doi.org/10.3847/1538-4357/ad9d39Cosmic raysInterstellar mediumMagnetic fieldsPulsar wind nebulae |
| spellingShingle | Junji Xia Xiaojun Bi Kun Fang Siming Liu Constraining Anisotropic Diffusion between Geminga and Earth with the Cosmic-Ray Electron and Positron Spectrum The Astrophysical Journal Cosmic rays Interstellar medium Magnetic fields Pulsar wind nebulae |
| title | Constraining Anisotropic Diffusion between Geminga and Earth with the Cosmic-Ray Electron and Positron Spectrum |
| title_full | Constraining Anisotropic Diffusion between Geminga and Earth with the Cosmic-Ray Electron and Positron Spectrum |
| title_fullStr | Constraining Anisotropic Diffusion between Geminga and Earth with the Cosmic-Ray Electron and Positron Spectrum |
| title_full_unstemmed | Constraining Anisotropic Diffusion between Geminga and Earth with the Cosmic-Ray Electron and Positron Spectrum |
| title_short | Constraining Anisotropic Diffusion between Geminga and Earth with the Cosmic-Ray Electron and Positron Spectrum |
| title_sort | constraining anisotropic diffusion between geminga and earth with the cosmic ray electron and positron spectrum |
| topic | Cosmic rays Interstellar medium Magnetic fields Pulsar wind nebulae |
| url | https://doi.org/10.3847/1538-4357/ad9d39 |
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