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

We present the first results on the hard X-ray continuum image (up to 15 keV) of the supernova remnant Cas A measured with the EPIC cameras onboard XMM-Newton. The data indicate that the hard X-ray tail, observed previously, that extends to energies above 100 keV does not originate in localised regions, like the bright X-ray knots and filaments or the primary blast wave, but is spread over the whole remnant with a rather flat hardness ratio of the 8-10 and 10-15 keV energy bands. This result does not support an interpretation of the hard X-radiation as synchrotron emission produced in the primary shock, in which case a limb brightened shell of hard X-ray emission close to the primary shock front is expected. In fact a weak rim of emission near 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 width 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 from the reverse shock heated ejecta given the ejecta temperature and the age of the remnant. The uniquely high wavelength-dispersive RGS-spectrometer has allowed, for the first time, to extract monochromatic images in several highly ionised element species with high spectral resolution. We present here a preliminary result on the measurement of the OVIII Ly-$\alpha$ and Ly-$\beta$ brightness distribution 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 $(10\hbox{$^{\prime\prime}$ })$ variations in the $N_{\rm H}$ column over the remnant and the potential presence of resonance scattering of the OVIII Ly-$\alpha$ photons in the limb brightened shell.