Inefficient Cosmic-Ray Diffusion around Vela X: Constraints from H.E.S.S. Observations of Very High-energy Electrons

Abstract

Abstract Vela X is a nearby pulsar wind nebula (PWN) powered by a ∼104 year old pulsar. Modeling of the spectral energy distribution of the Vela X PWN has shown that accelerated electrons have largely escaped from the confinement, which is likely due to the disruption of the initially confined PWN by the supernova remnant reverse shock. The escaped electrons propagate to the Earth and contribute to the measured local cosmic-ray (CR) electron spectrum. We find that the escaped CR electrons from Vela X would hugely exceed the measured flux by High Energy Stereoscopic System (HESS) at ∼10 TeV if the standard diffusion coefficient for the interstellar medium (ISM) is used. We propose that the diffusion may be highly inefficient around Vela X and find that a spatially dependent diffusion can lead to CR flux that is consistent with the HESS measurement. Using a two-zone model for the diffusion around Vela X, we find that the diffusion coefficient in the inner region of a few tens of parsecs should be ≲1028 cm2 s−1 for ∼10 TeV CR electrons, which is about two orders of magnitude lower than the standard value for the ISM. Such inefficient diffusion around PWN resembles the case of the Geminga and Monogem PWNe, suggesting that inefficient diffusion may be common in the vicinity of PWNe that span a wide range of ages.