Spectrum and location of ongoing extreme particle acceleration in Cassiopeia A

TL;DR

This study uses multi-epoch X-ray observations of Cassiopeia A to provide direct evidence of ongoing cosmic ray electron acceleration, revealing a harder electron spectrum and higher cutoff energy than previously observed, consistent with the modified nonlinear diffusive shock acceleration mechanism.

Contribution

It demonstrates for the first time a self-consistent link between multi-wavelength spectra and the mNLDSA acceleration mechanism in a young supernova remnant.

Findings

Observed a 15% X-ray flux decrease, indicating active CR electron acceleration.

Found a significantly harder electron spectrum with a higher cutoff energy than relic electrons.

Spectral modeling supports the mNLDSA mechanism as the driver of particle acceleration in Cas A.

Abstract

Young supernova remnants (SNRs) are believed to be the origin of energetic cosmic rays (CRs) below the "knee" of their spectrum at $\sim3$ petaelectronvolt (PeV, $10^{15}$ eV). Nevertheless, the precise location, duration, and operation of CR acceleration in young SNRs are open questions. Here, we report on multi-epoch X-ray observations of Cassiopeia A (Cas A), a 350-year-old SNR, in the 15-50 keV band that probes the most energetic CR electrons. The observed X-ray flux decrease $(15\pm1\%)$, contrary to the expected $>$90\% decrease based on previous radio, X-ray, and gamma-ray observations, provides unambiguous evidence for CR electron acceleration operating in Cas A. A temporal model for the radio and X-ray data accounting for electron cooling and continuous injection finds that the freshly injected electron spectrum is significantly harder (exponential cutoff power law index $q=2.15$), and its cutoff energy is much higher ($E_{cut}=36$ TeV) than the relic electron spectrum ($q=2.44\pm0.03$, $E_{cut}=4\pm1$ TeV). Both electron spectra are naturally explained by the recently developed modified nonlinear diffusive shock acceleration (mNLDSA) mechanism. The CR protons producing the observed gamma rays are likely accelerated at the same location by the same mechanism as those for the injected electron. The Cas A observations and spectral modeling represent the first time radio, X-ray, gamma ray and CR spectra have been self-consistently tied to a specific acceleration mechanism -- mNLDSA -- in a young SNR.