Hard X-ray emission from Cassiopeia A SNR

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 (similar to 5%) to the data with a power-law index, Gamma = 2.94+/-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 tbe X-ray thermal bremmstrahlung from accelerated MeV electrons at shock fronts [1] which would have Gamma similar or equal to 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+/-0.09. We discuss the implication of the hardest nonthermal X-rays detected from Cas A to the synchrotron radiation model.