For illustrative purposes, Fig. 117 shows a comparison of an XMM-Newton EPIC and a Chandra ACIS-I observation of a cluster with a 6 keV thermal plasma spectrum, 0.3 solar metallicity, a redshift of 0.3 and an X-ray flux in the 0.1-10 keV band of erg cm s (thus a fairly luminous system). Such a comparison gives a good feel for the capabilities of both instruments for performing studies of faint objects. We simulated the response of the Chandra ACIS front-illuminated CCD imaging instrument, using the response matrices supplied for the guest observer proposal submission. With a 30 ksec observation, ACIS is able to measure the temperature of the cluster to about 10% accuracy (assuming only poissonian noise and neglecting systematic effects). Following the same procedure, i.e., up-to-date detector response matrices, input spectrum and observing time, the combined response of the 3 EPIC focal plane cameras on XMM-Newton allows a 2.5% accuracy on the estimated cluster temperature (neglecting systematic effects).
The bottom panel of Fig. 117 also illustrates the different line spectral resolution that XMM-Newton EPIC and RGS and the Chandra ACIS instruments can achieve.
The presented count spectra comparisons are based on year 2008 responses. For data of different mission epochs the actual appearance of the count spectra comparisons, especially at the low energies, varies due to the different level of contamination of various instruments at different epochs. The losses of effective areas due to contamination are corrected by the correspondent instrument calibrations.