Evolved Pulsar Wind Nebulae as Sources of (Mostly Leptonic) Cosmic Rays

Abstract

Pulsars, and more precisely their wind nebulae, are perhaps the most likely astrophysical sources of high-energy cosmic-ray electrons and especially positrons, and in particular of the excess positronic component above ∼20 GeV measured by PAMELA, Fermi-LAT and AMS-02. While e± pairs are created in pulsar magnetospheres, their acceleration to relevant cosmic-ray energies likely occurs at the pulsar wind termination shock, and they are subsequently confined in the pulsar wind nebula (PWN) until late stages of its evolution. We discuss the implications of radiative and adiabatic energy losses for the likely contributions of different stages of PWN evolution to the observed cosmic-ray e± spectrum, and highlight the plausibly dominant contribution of late, subsonic expansion and especially bow-shock PWN phases.The most recently opened observational window on PWNe, with implications for accelerated e± in the relevant energy range, is the domain of TeV γ-rays. I review the properties of the population of PWNe revealed in this energy domain, in particular by the H.E.S.S. array of Cherenkov telescopes and its Galactic Plane Survey, and discuss the constraints which these place on the evolution of middle-aged PWNe. Finally, I briefly review the possibility of hadron acceleration in PWNe, and discuss theoretical and observational constraints which limit a plausible major contribution of these objects to the bulk of the (hadronic) Galactic cosmic-ray spectrum.