Systematic Study of Acceleration Efficiency in Young Supernova Remnants with Nonthermal X-ray Observations

TL;DR

This study systematically analyzes young supernova remnants to understand particle acceleration efficiency by examining the cutoff energy in their nonthermal X-ray spectra, revealing a relationship with shock speed and remnant age.

Contribution

It provides the first systematic analysis of cutoff energies in multiple young SNRs, linking acceleration efficiency to shock speed and remnant age, and suggests magnetic turbulence increases over time.

Findings

Cutoff energy scales with shock speed squared and turbulence factor.

A correlation between turbulence and remnant age/maturity.

Maximum particle energies may be higher considering time-dependent turbulence.

Abstract

Cutoff energy in a synchrotron radiation spectrum of a supernova remnant (SNR) contains a key parameter of ongoing particle acceleration. We systematically analyze 11 young SNRs, including all historical SNRs, to measure the cutoff energy, thus shedding light on the nature of particle acceleration at the early stage of SNR evolution. The nonthermal (synchrotron) dominated spectra in filament-like outer rims are selectively extracted and used for spectral fitting because our model assumes that accelerated electrons are concentrated in the vicinity of the shock front due to synchrotron cooling. The cutoff energy parameter ($\varepsilon_0$) and shock speed ($v_{\rm sh}$) are related as $ \varepsilon_0 \propto v_{\rm sh}^2 \eta^{-1}$ with a Bohm factor of $\eta$. Five SNRs provide us with spatially resolved $\varepsilon_0$-$v_{\rm sh}$ plots across the remnants, indicating a variety of particle acceleration. With all SNRs considered together, the systematic tendency of $\eta$ clarifies a correlation between $\eta$ and an age of $t$ (or an expansion parameter of $m$) as $\eta \propto t^{-0.4}$ ($\eta \propto m^{4}$). This might be interpreted as the magnetic field becomes more turbulent and self-generated, as particles are accelerated at a greater rate with time. The maximum energy achieved in SNRs can be higher if we consider the newly observed time dependence on $\eta$.