We investigate the non-thermal X-ray emission from Cas A, using BeppoSAX, CGRO-OSSE, and Chandra data. For the hard X-ray continuum we test the model proposed by Laming, which invokes nonthermal bremsstrahlung from electrons accelerated by lower hybrid plasma waves. The justification for this model comes from our determination of a lower limit to the average magnetic field of B > 0.5 mG. For such high magnetic fields the synchrotron losses are severe enough that most of the electron populations responsible for the radio emission have maximum electron energies well below the limit for which X-ray synchrotron emission is important. However, we do suggest that the rim surrounding Cas A, seen in Chandra continuum images, is X-ray synchrotron emission. The width of this rim of 1.5 ′′ to 4 ′′ , can be used to infer the magnetic field near the shock front, for which we estimate B = 0.08 − 0.16 mG, and electron energies of ∼ 57 − 40 TeV. This magnetic field strength is lower than the average magnetic field, but higher than what may be expected from shocked interstellar medium, suggesting either a high magnetic field in the wind of the progenitor, or rapid, post shock, field amplification by non-linear growth of plasma waves. Combining the two magnetic field measurements we have constructed a simple two zone model. Most of the radio emission comes from inside Cas A where the magnetic field is strong. In contrast, the inverse Compton emission is dominated by emission from near the shock front. Only for our lower limit on the magnetic field strength near the shock front is it possible to explain the recent detection of TeV emission by HEGRA with inverse Compton, for which, in addition, we have to assume a rather high far infrared photon density that should be twice as high as our best estimate of ∼ 70 cm −3 . Pion decay is therefore likely to be the dominant emission from Cas A at TeV energies.

We report the detection of the 44 Sc nuclear decay lines at 67.9 keV and 78.4 keV associated with the nuclear decay of 44 Ti in Cassiopeia A. The line emission was observed by the PDS instrument on board BeppoSAX, which recently observed the supernova remnant for over 500 ks. The detection of the line emission with a flux of (2.1 ± 0.7) 10 −5 ph/cm 2 /s in each line (90% confidence) is at the 5σ significance level, if we can assume that the 12-300 keV continuum is adequately represented by a single power law. However, as the nature of the continuum is not clear we investigate various other possibilities. A more conservative estimate of the line flux is made by assuming that a power law continuum is at least a good approximation to the continuum emission for a narrower 30-100 keV energy range. With this limitation the measured line flux is (1.9 ± 0.9) 10 −5 ph/cm 2 /s, with the detection still at the 3.4σ significance level. We suggest that together with the CGRO-COMPTEL measurement of the 44 Ca line at 1157 keV of (3.3 ± 0.6) 10 −5 ph/cm 2 /s a flux for all three lines of (2.5 ± 1.0) 10 −5 ph/cm 2 /s for Cas A can be adopted. This implies an initial 44 Ti mass of (0.8 − 2.5) 10 −4 M ⊙ . 5 The observational identification numbers for the additional archival data used for our analysis are 30011001, 30011002 and 30795005. They were chosen on the availability of spectra made with the variable rise time rejection method.

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We present new Chandra ACIS-S3 observations of Cassiopeia A which, when combined with earlier ACIS-S3 observations, show evidence for a steady ∼ 1.5-2% yr −1 decline in the 4.2-6.0 keV X-ray emission between the years 2000 and 2010. The computed flux from exposure corrected images over the entire remnant showed a 17% decline over the entire remnant and a slightly larger (21%) decline from regions along the remnant's western limb. Spectral fits of the 4.2-6.0 keV emission across the entire remnant, forward shock filaments, and interior filaments indicate the remnant's nonthermal spectral powerlaw index has steepened by about 10%, with interior filaments having steeper powerlaw indices. Since TeV electrons, which give rise to the observed X-ray synchrotron emission, are associated with the exponential cutoff portion of the electron distribution function, we have related our results to a change in the cutoff energy and conclude that the observed decline and steepening of the nonthermal X-ray emission is consistent with a deceleration of the remnant's ≃5000 km s −1 forward shock of ≈ 10-40 km s −1 yr −1 .