Description:
<jats:title>Abstract</jats:title>
<jats:p>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 <jats:italic>Chandra</jats:italic> and <jats:italic>NuSTAR</jats:italic> observations. Several bright inward-moving filaments were identified using monitoring data taken by <jats:italic>Chandra</jats:italic> in 2000–2014. These inward-moving shock locations are nearly coincident with hard X-ray (15–40 keV) hot spots seen by <jats:italic>NuSTAR</jats:italic>. From proper-motion measurements, the transverse velocities were estimated to be in the range of ∼2100–3800 km s<jats:sup>−1</jats:sup> for a distance of 3.4 kpc. The shock velocities in the frame of the expanding ejecta reach values of ∼5100–8700 km s<jats:sup>−1</jats:sup>, 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 <jats:italic>B</jats:italic> ∼ 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 (<jats:italic>k</jats:italic>
<jats:sub>0</jats:sub> = <jats:italic>D</jats:italic>
<jats:sub>0</jats:sub>/<jats:italic>D</jats:italic>
<jats:sub>0,Bohm</jats:sub> ∼ 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.</jats:p>