Verifying the low-energy spectral response models of the CXO ACIS and the XMM-Newton EPIC CCDs

The flight calibration of the spectral response of CCD instruments below 1.5 keV is difficult in general because of the lack of strong lines in the on-board calibration sources typically used and the relatively poor spectral resolution at the lowest energies. We used 1E 0102.2-7219 (the brightest supernova remnant in the SMC) to evaluate the response models of the ACIS CCDs on the Chandra X-ray Observatory and the EPIC CCDs on the XMM-Newton Observatory. E0102 has strong lines of O, Ne, and Mg below 1.5 keV and very little or no Fe emission to complicate the spectrum. The spectrum of E0102 has been well-characterized using the gratings on the CXO and XMM-Newton. We have used the high-resolution spectral data from both gratings instruments to develop a spectral model for the CCD spectra. Fits with this model are sensitive to any problems with the gain calibration and the spectral redistribution model. We have also used the measured intensities of the lines to investigate the consistency of the detection efficiency models for the different instruments. We find that the gain of the three instruments is accurate to within 1.1% at ~ 570 eV and 0.9% at ~ 910 eV. We find that the measured flux of the Ne X Ly α line agrees at the 90% confidence limit for all detectors (within 6%). We find significant differences in the measured flux of the O VIII Ly α line, the largest discrepancy between one pair of datasets is 18%. Further analysis is required to investigate this discrepancy.