  We report the results of extracting the hard X-ray continuum spectrum of Cas
A SNR from RXTE/PCA Target of Opportunity (TOO) observations and CGRO/OSSE
observations. The data can rule out the single thermal bremsstrahlung model for
Cas A continuum between 2 and 150 keV. The single power law model gives a
mediocre fit (~5%) to the data with a power-law index, $\Gamma$ =
2.94$\pm$0.02. A model with two component (bremsstrahlung + bremsstrahlung or
bremsstrahlung + power law) gives a good fit. The power law index is quite
constrained suggesting that this continuum might not be the X-ray thermal
bremmstrahlung from accelerated MeV electrons at shock fronts (Asvarov et al.
1989) which would have $\Gamma\simeq$2.26. With several SNRs detected by ASCA
showing a hard power-law nonthermal X-ray continuum, we expect a similar
situation for Cas A SNR which has $\Gamma$=2.98$\pm$0.09. We discuss the
implication of the hardest nonthermal X-rays detected from Cas A to the
synchrotron radiation model.
