Escape of cosmic rays from perpendicular shocks in the interstellar magnetic field

TL;DR

This study examines how cosmic rays escape from perpendicular shocks in supernova remnants, finding that without magnetic field amplification, the maximum energy is limited to several 10 TeV, influencing observed spectral features.

Contribution

The paper provides a theoretical and simulation-based analysis of cosmic ray escape limits at perpendicular shocks, highlighting the need for magnetic field amplification to reach PeV energies.

Findings

Maximum CR energy limited to ~10 TeV without magnetic amplification

Perpendicular shock acceleration covers about 20% of shock surface during free expansion

Spectral break around 10 TeV may originate from escape-limited acceleration

Abstract

We investigate the escape process from a perpendicular shock region of a spherical shock in the interstellar medium (ISM). The diffusive shock acceleration in the perpendicular shock of supernova remnants (SNRs) has been expected to accelerate cosmic rays (CRs) to the PeV scale without an upstream magnetic field amplification. We estimate the maximum energy of CRs limited by the escape from the perpendicular shock region. By performing test particle simulations, we confirm the theoretical estimation, showing that the escape-limited maximum energy in the perpendicular shock is several 10 TeV for the typical type Ia SNRs. Therefore, in order for SNRs in the ISM to accelerate CRs to the PeV scale, an upstream magnetic field amplification is needed. The characteristic energy scale of several 10 TeV could be the origin of the spectral break around 10 TeV, which was reported by recent direct CR observations. In addition, we show that, in the free expansion phase, the rapid perpendicular shock acceleration works on about 20% area of the whole shock surface, which is larger than the size of the superluminal shock region. We also discuss the escape of CR electrons from the perpendicular shock.