Inefficient cosmic ray diffusion around Vela X : constraints from H.E.S.S. observations of very high-energy electrons

TL;DR

This paper investigates the diffusion of high-energy electrons around Vela X, finding that diffusion is highly inefficient compared to standard models, which helps reconcile observed cosmic-ray fluxes with theoretical predictions.

Contribution

It introduces a two-zone diffusion model around Vela X showing that diffusion is significantly suppressed near the PWN, a novel insight into cosmic-ray propagation.

Findings

Diffusion coefficient near Vela X is less than 10^28 cm^2/s for 10 TeV electrons.

Inefficient diffusion around Vela X is similar to other PWNe like Geminga and Monogem.

Standard diffusion models overpredict cosmic-ray fluxes from Vela X.

Abstract

Vela X is a nearby pulsar wind nebula (PWN) powered by a $\sim 10^4$ 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 SNR 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 HESS at $\sim 10$ TeV if the standard diffusion coefficient for the interstellar medium 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 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 $<10^{28}{\rm cm^2 s^{-1}}$ for $\sim10$ TeV CR electrons, which is about two orders of magnitude lower than the standard value for 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 spanning a wide range of ages.