The role of ionization in the shock acceleration theory

TL;DR

This paper investigates how ionization processes influence the acceleration of heavy nuclei at supernova remnant shocks, revealing that photo-ionization limits maximum energies and produces secondary electrons that explain observed synchrotron radiation.

Contribution

It introduces the impact of ionization, especially photo-ionization, on diffusive shock acceleration, altering maximum energy predictions and explaining secondary electron production.

Findings

Maximum ion energies are lower than standard DSA predictions due to ionization.

Secondary electrons produced can account for observed synchrotron radiation.

Ionization affects the cosmic ray spectrum in the knee region.

Abstract

We study the acceleration of heavy nuclei at SNR shocks taking into account the process of ionization. In the interstellar medium atoms heavier then hydrogen which start the diffusive shock acceleration (DSA) are never fully ionized at the moment of injection. We will show that electrons in the atomic shells are stripped during the acceleration process, when the atoms already move relativistically. For typical environment around SNRs the dominant ionization process is the photo-ionization due to the background galactic radiation. The ionization has two interesting consequences. First, because the total photo-ionization time is comparable to the beginning of the Sedov-Taylor phase, the maximum energy which ions can achieve is smaller than the standard result of the DSA, which predict $E_{\max}\propto Z_N$. As a consequence the structure of the CR spectrum in the {\it knee} region can be affected. The second consequence is that electrons are stripped from atoms when they already move relativistically hence they can start the DSA without any pre-acceleration mechanism. We use the linear quasi-stationary approach to compute the spectrum of ions and electrons accelerated after being stripped. We show that the number of these secondary electrons is enough to account for the synchrotron radiation observed from young SNRs, if the amplification of the magnetic field occurs.