# Global Constraints on Diffusive Particle Acceleration by Strong Nonrelativistic Shocks

## TL;DR

This paper explores the relationship between cosmic-ray acceleration efficiency and radio spectral indices in supernova remnants, proposing methods to estimate efficiency and explaining spectral variations with SNR evolution.

## Contribution

It introduces a theoretical framework linking shock conditions to acceleration efficiency and spectral indices, providing new insights into particle acceleration in SNRs.

## Key findings

- Anticorrelation between acceleration efficiency and spectral index.
- Threshold injection rate for significant adiabatic index change.
- Older SNRs have lower injection rates and higher efficiencies.

## Abstract

Estimating the cosmic-ray acceleration efficiency $ \epsilon $ in supernova remnants (SNRs) through observations is a challenging task in general. Based on the Rankine-Hugoniot shock conditions, we find an anticorrelation between $ \epsilon $ and the power-law spectral index $ \alpha $ of relativistic particle distribution produced via diffusive particle acceleration by nonrelativistic shocks, implying more efficient acceleration in older SNRs with harder radio spectra. Then $ \epsilon $ may be estimated from some hard radio spectral index measurements. Assuming the particle distribution in downstream of strong shocks to be a nonrelativistic Maxwellian plus a relativistic power law with a high-energy cutoff, we also find that the injection rate for relativistic particles $ \eta $ needs to $ \gtrsim 10^{-6} $ for a prominent decrease of the adiabatic index in SNRs, which implies higher compression ratio and lower values of $ \alpha $. This threshold of $ \eta $ increases with the shock speed $ u_1 $, which may explain the relatively harder radio spectra of older SNRs with lower $ u_1 $. We show that $ \eta $ and/or the relativistic cutoff momentum $ p_\text{m} $ need to be low for old SNRs, and expect a gradual increase of $ \epsilon $ as SNR evolves with gradually decreasing $ \eta $ and $ p_\text{m} $.