Cassiopeia A: On the origin of the hard X-ray continuum and the implication of the observed O-VIII Ly-alpha/Ly-beta distribution

We present the first results on the hard X-ray continuum image (up to 15 ke V) of the supernova remnant Cas A measured with the EPIC cameras onboard XM M-Newton. The data indicate that the hard X-ray tail, observed previously, that extends to energies above 100 keV does not originate in localised regi ons, like the bright X-ray knots and filaments or the primary blast wave, b ut is spread over the whole remnant with a rather Aat hardness ratio of the 8-10 and 10-15 keV energy bands. This result does not support an interpret ation of the hard X-radiation as synchrotron emission produced in the prima ry shock, in which case a limb brightened shell of hard X-ray emission clos e to the primary shock front is expected. In fact a weak rim of emission ne ar the primary shock front is discernable in the hardest X-ray image but it contains only a few percent of the hard X-ray emissivity. The equivalent w idth of the FE-K line blend varies by more than an order of magnitude over the remnant, it is hard to explain this as Fe-emission horn the reverse sho ck heated ejecta given the ejecta temperature and the age of the remnant. T he uniquely high wavelength-dispersive RGS-spectrometer has allowed, for th e first time, to extract monochromatic images in several highly ionised ele ment species with high spectral resolution. We present here a preliminary r esult on the measurement of the O VIII Ly-alpha and Ly-beta brightness dist ribution and brightness ratios. The large observed decrease of the Ly-alpha /Ly-beta ratio going from the N to the SE can be explained by small-scale (10") variations in the N-H Column over the remnant and the potential prese nce of resonance scattering of the O VIII Ly-alpha photons in the limb brig htened shell.