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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| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
IOP Publishing
2025-01-01
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| Series: | The Astrophysical Journal |
| Subjects: | |
| Online Access: | https://doi.org/10.3847/1538-4357/ad9d39 |
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| Summary: | 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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| ISSN: | 1538-4357 |