XMM science simulator: EPIC mode performance issues

We describe simulations of the XMM EPIC instruments which suggests the correct operating mode must be chosen to ensure that spectral analysis of the data is not compromised by 'pile-up' effects. We contrast the performance with the AXAF CCD imaging spectrometer, and show that the XMM EPIC instruments will access a larger range of source fluxes due to a combination of higher effective area and better over- sampling of its mirror response function. Targets exceeding a flux of a few 10-12 ergs cm-2s-1 will be compromised for spectral analysis in AXAF. For XMM, the corresponding flux levels will be 10-11 ergs cm-11 ergs cm-2s-1. This feature warrants careful attention in calibration.