Escape of cosmic-ray electrons from supernova remnants

TL;DR

This paper studies how cosmic-ray electrons escape from supernova remnants, their energy losses, and how their radiation can be observed, providing insights into cosmic-ray origins and magnetic field amplification.

Contribution

It presents a detailed model of CR electron escape, energy spectrum, and radiation around SNRs, highlighting differences from CR nuclei and implications for gamma-ray observations.

Findings

CR electrons escape from SNRs starting at 10^3 - 7*10^3 years.

Maximum energy of runaway CR electrons is below 50 TeV due to synchrotron losses.

Runaway CR electrons produce synchrotron and inverse Compton radiation detectable by upcoming telescopes.

Abstract

We investigate escape of cosmic ray (CR) electrons from a supernova remnant (SNR) to interstellar space. We show that CR electrons escape in order from high energies to low energies like CR nuclei, while the escape starts later than the beginning of the Sedov phase at an SNR age of 10^3 - 7*10^3 yrs and the maximum energy of runaway CR electrons is below the knee about 0.3 - 50 TeV because unlike CR nuclei, CR electrons lose their energy due to synchrotron radiation. Highest energy CR electrons will be directly probed by AMS-02, CALET, CTA and LHAASO experiments, or have been already detected by H.E.S.S. and MAGIC as a cutoff in the CR electron spectrum. Furthermore, we also calculate the spatial distribution of runaway CR electrons and their radiation spectra around SNRs. Contrary to common belief, maximum-energy photons of synchrotron radiation around 1 keV are emitted by runaway CR electrons which have been caught up by the shock. Inverse Compton scattering by runaway CR electrons can dominate the gamma-ray emission from runaway CR nuclei via pion decay, and both are detectable by CTA and LHAASO as clues to the CR origin and the amplification of magnetic fluctuations around the SNR. We also discuss middle-aged and/or old SNRs as unidentified very-high-energy gamma-ray sources.