Spectrum and Location of Ongoing Extreme Particle Acceleration in Cassiopeia A

Abstract

Young supernova remnants (SNRs) are believed to be the origin of energetic cosmic rays (CRs) below the “knee” of their spectrum at ∼3 PeV (1015 eV). Nevertheless, the precise location, duration, and operation of CR acceleration in young SNRs are open questions. Here, we report on multiepoch X-ray observations of Cassiopeia A (Cas A), a 350 yr old SNR, in the 15–50 keV band that probes the most energetic CR electrons. The observed X-ray flux decrease (15% ± 1% over 10 yr), 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 (Ecut = 36 TeV), than the relic electron spectrum (q = 2.44 ± 0.03, Ecut = 4 ± 1 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 the injected electrons. 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.