DAMPE Proton Spectrum Indicates a Slow-diffusion Zone in the nearby ISM

Abstract

Abstract The hardening and softening features in the DAMPE proton spectrum are very likely to have originated from a nearby supernova remnant (SNR). The proton spectrum from the nearby SNR is required to be very hard below ≈10 TeV. To reproduce this feature, we illustrate that an anomalously slow-diffusion zone for cosmic rays (CRs) must exist in the local interstellar medium (ISM) after also taking the dipole anisotropy of CRs into account. Assuming that the diffusion coefficient is homogeneous in the nearby ISM, we show that the diffusion coefficient is constrained to the magnitude of 1026 cm2 s−1 when normalized to 1 GeV, which is about 100 times smaller than the average value in the Galaxy. We further discuss the spatial distribution of the slow diffusion and find two distinct possibilities. In one case, the SNR is several hundred parsecs away from the solar system, and both the SNR and the solar system must be included in a large slow-diffusion zone. In the other case, the SNR is very close with a distance of ∼50 pc and the slow-diffusion zone is only limited around the SNR, while the current multiwavelength observations do not indicate such a close source. This work provides a new way of studying the CR diffusion in the local ISM.