We report on our analysis of the 1 Ms Chandra observation of the supernova remnant Cas A in order to localize , characterize and quantify its non-thermal X-ray emission .
More specifically , we investigated whether the X-ray synchrotron emission from the inside of the remnant is from the outward shock , but projected toward the inner ring , or from the inner shell .
We tackle this problem by employing a Lucy-Richardson deconvolution technique and measuring spectral indices in the 4.2-6 keV band .

We show that most of the continuum emission is coming from an inner ring that coincides with the location of the reverse shock .
This inner ring includes filaments , whose X-ray emission has been found to be dominated by X-ray synchrotron emission .
The X-ray emission from these filaments , both at the forward shock and from the inner ring , have relatively hard spectra with spectral index > -3.1 .
The regions emitting hard X-ray continuum contribute about 54 % of the total X-ray emission in the 4.2-6 keV .
This is lower than suggested by extrapolating the hard X-ray spectrum as measured by BeppoSAX-PDS and INTEGRAL .
This can be reconciled by assuming a gradual steepening of the spectrum toward higher energies .
We argue that the X-ray synchrotron emission is mainly coming from the Western part of the reverse shock .
The reverse shock in the West is almost at rest in our observation frame , corresponding to a relatively high reverse shock velocity of \sim 6000 km s ^ { -1 } ~ { } in the frame of the freely expanding ejecta .