Escape from Vela X

TL;DR

This paper presents a new model linking the structures of Vela X pulsar wind nebula, explaining its spectral features and predicting a high-energy cosmic ray electron signature, with implications for understanding positron excess.

Contribution

It introduces a novel model connecting the ERN and X-ray cocoon through particle injection and escape, explaining spectral observations and cosmic ray features.

Findings

Diffusive escape explains the steep Fermi-LAT spectrum.

Vela X should produce a detectable high-energy electron feature.

PWNe may account for the rising positron fraction in cosmic rays.

Abstract

Whilst the Vela pulsar and its associated nebula are often considered as the archetype of a system powered by a \sim10^4 year old isolated neutron star, many features of the spectral energy distribution of this pulsar wind nebula are both puzzling and unusual. Here we develop a model that for the first time relates the main structures in the system, the extended radio nebula (ERN) and the X-ray cocoon through continuous injection of particles with a fixed spectral shape. We argue that diffusive escape of particles from the ERN can explain the steep Fermi-LAT spectrum. In this scenario Vela X should produce a distinct feature in the locally-measured cosmic ray electron spectrum at very high energies. This prediction can be tested in the future using the Cherenkov Telescope Array (CTA). If particles are indeed released early in the evolution of PWNe and can avoid severe adiabatic losses, PWN provide a natural explanation for the rising positron fraction in the local CR spectrum.