Measuring the Local Diffusion Coefficient with H.E.S.S. Observations of Very High-Energy Electrons

TL;DR

This paper uses H.E.S.S. observations of high-energy electrons to determine that cosmic-ray diffusion in the local interstellar medium is efficient, contrasting with the inhibited diffusion near certain pulsars, and supporting pulsar contributions to positron excess.

Contribution

It provides the first direct measurement indicating that the local diffusion coefficient is not as low as near some pulsars, challenging previous assumptions about cosmic-ray propagation.

Findings

Local diffusion coefficient is higher than near Geminga and Monogem

Cosmic rays diffuse efficiently through the local interstellar medium

Supports pulsars as significant sources of positron excess

Abstract

The HAWC Collaboration has recently reported the detection of bright and spatially extended multi-TeV gamma-ray emission from Geminga, Monogem, and a handful of other nearby, middle-aged pulsars. The angular profile of the emission observed from these pulsars is surprising, in that it implies that cosmic-ray diffusion is significantly inhibited within ~25 pc of these objects, compared to the expectations of standard Galactic diffusion models. This raises the important question of whether the diffusion coefficient in the local interstellar medium is also low, or whether it is instead better fit by the mean Galactic value. Here, we utilize recent observations of the cosmic-ray electron spectrum (extending up to ~20 TeV) by the H.E.S.S. Collaboration to show that the local diffusion coefficient cannot be as low as it is in the regions surrounding Geminga and Monogem. Instead, we conclude that cosmic rays efficiently diffuse through the bulk of the local interstellar medium. Among other implications, this further supports the conclusion that pulsars significantly contribute to the observed positron excess.