X-Ray Measurements of the Particle Acceleration Properties at Inward Shocks in Cassiopeia A

Toshiki Sato, Satoru Katsuda, Mikio Morii, Aya Bamba, John P. Hughes, Yoshitomo Maeda, Manabu Ishida, Federico Fraschetti

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19 Scopus citations


We present new evidence that the bright nonthermal X-ray emission features in the interior of the Cassiopeia A supernova remnant are caused by inward-moving shocks, based on Chandra and NuSTAR observations. Several bright inward-moving filaments were identified using monitoring data taken by Chandra in 2000-2014. These inward-moving shock locations are nearly coincident with hard X-ray (15-40 keV) hot spots seen by NuSTAR. From proper-motion measurements, the transverse velocities were estimated to be in the range of ∼2100-3800 km s-1 for a distance of 3.4 kpc. The shock velocities in the frame of the expanding ejecta reach values of ∼5100-8700 km s-1, which is slightly higher than the typical speed of the forward shock. Additionally, we find flux variations (both increasing and decreasing) on timescales of a few years in some of the inward-moving shock filaments. The rapid variability timescales are consistent with an amplified magnetic field of B ∼ 0.5-1 mG. The high speed and low photon cut-off energy of the inward-moving shocks are shown to imply a particle diffusion coefficient that departs from the Bohm regime (k 0 = D 0/D 0,Bohm ∼ 3-8) for the few simple physical configurations we consider in this study. The maximum electron energy at these shocks is estimated to be ∼8-11 TeV, which is smaller than the values of ∼15-34 TeV that were inferred for the forward shock. Cassiopeia A is dynamically too young for its reverse shock to appear to be moving inward in the observer frame. We propose instead that the inward-moving shocks are a consequence of the forward shock encountering a density jump of 5-8 in the surrounding material.

Original languageEnglish (US)
Article number46
JournalAstrophysical Journal
Issue number1
StatePublished - Jan 20 2018

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • ISM: supernova remnants
  • X-rays: ISM
  • acceleration of particles
  • supernovae: individual (Cassiopeia A)


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