1 Introduction

In this paper we present a detailed X-ray spectral analysis of the young supernova remnant Cassiopeia A with an angular resolution of the order 20 arcsec over a field of view covering the full remnant. The data were obtained from an 86 kilosec exposure of the XMM-Newton EPIC-MOS cameras to the source. The outstanding spectral grasp of XMM-Newton, i.e. the combination of sensitivity, X-ray bandwidth and spectral resolving power, coupled to this very long exposure time provides ample photon statistics for a full spectral modelling of each image pixel commensurate with the beam width of the XMM-Newton telescopes ($\sim$15 $^{\prime\prime}$ Half Power Width), even for source regions of low surface brightness. This is illustrated in Fig. 1 which shows a broad band high resolution Chandra image of Cas A (Hughes et al. 2000) on which the pixel grid used in this analysis has been superimposed. Also drawn on this image is a contour indicating the region with good statistics and where the flux is not dominated by scattering. In addition, two samples of raw spectral data are shown, indicating the typical statistical quality in regions of high and low surface brightness.

Figure 1: The pixel grid used in our analysis superimposed on the high angular resolution Chandra image of Cas A. The green contour indicates the region with good statistics and low scattering. Below are typical single pixel spectra from a high count and a low count region.

Figure 2: An example of a spectral fit within a single $20{\hbox {$^{\prime \prime }$ }} \times 20{\hbox {$^{\prime \prime }$ }}$ pixel - cool component in blue, hot component in green and full model in red.

The energy resolution, gain stability and gain uniformity of the MOS-cameras allows significant detection of emission line energy shifts of order 1 eV or greater for prominent lines like Si-K, S-K and Fe-K. Proper modelling of these line blends with the aid of broad band spectral fitting, taking into account the non-equilibrium ionisation balance (NEI), allows an assessment, with unprecedented accuracy, of Doppler shifts and abundance variations of the X-ray emitting material across the face of the remnant with an angular resolution adequate enough to discriminate the fine knot structure seen by Chandra. The implications for the dynamical model of the remnant and for the origin and shock heating of the X-ray emitting ejecta will be highlighted as the key result of this investigation.