X-ray Line Diagnostics of Ion Temperature at Cosmic-Ray Accelerating Collisionless Shocks

TL;DR

This paper models collisionless shock physics in supernova remnants, linking ion temperature and cosmic-ray acceleration, and proposes observational diagnostics to distinguish shock acceleration effects using upcoming X-ray missions.

Contribution

It introduces a new shock jump condition based on entropy production and connects ion temperature relaxation with cosmic-ray acceleration in SNRs.

Findings

Constrained cosmic-ray nuclei amounts consistent with Galactic CRs.

Ion temperatures are significantly lower when CR acceleration occurs.

Synthetic X-ray line observations can differentiate CR acceleration presence.

Abstract

A novel collisionless shock jump condition is suggested by modeling the entropy production at the shock transition region. We also calculate downstream developments of the atomic ionization balance and the ion temperature relaxation in supernova remnants (SNRs). The injection process and subsequent acceleration of cosmic-rays (CRs) in the SNR shocks are closely related to the formation process of the collisionless shocks. The formation of the shock is caused by wave-particle interactions. Since the wave-particle interactions result in energy exchanges between electromagnetic fields and charged particles, the randomization of particles associated with the shock transition may occur with the rate given by the scalar product of the electric field and current. We find that order-of-magnitude estimates of the randomization with reasonable strength of the electromagnetic fields in the SNR constrain the amount of the CR nuclei and ion temperatures. The constrained amount of the CR nuclei can be sufficient to explain the Galactic CRs. The ion temperature becomes significantly lower than in the case of no CRs. To distinguish the case without CRs, we perform synthetic observations of atomic line emissions from the downstream region of the SNR RCW~86. Future observations by {\it XRISM} and {\it Athena} can distinguish whether the SNR shock accelerates the CRs or not from the ion temperatures.