2017A&A...597A..35P

Context. The flight calibration of the spectral response of charge-coupled device (CCD) instruments below 1.5keV is difficult in general because of the lack of strong lines in the on-board calibration sources typically available. This calibration is also a function of time due to the effects of radiation damage on the CCDs and/or the accumulation of a contamination layer on the filters or CCDs.
Aims. We desire a simple comparison of the absolute effective areas of the current generation of CCD instruments onboard the following observatories: Chandra ACIS-S3,XMM-Newton (EPIC-MOS and EPIC-pn), SuzakuXIS, and SwiftXRT and a straightforward comparison of the time-dependent response of these instruments across their respective mission lifetimes.
Methods. We have been using 1E 0102.2-7219, the brightest supernova remnant in the Small Magellanic Cloud, to evaluate and modify the response models of these instruments. 1E 0102.2-7219 has strong lines of O, Ne, and Mg below 1.5keV and little or no Fe emission to complicate the spectrum. The spectrum of 1E 0102.2-7219 has been well-characterized using the RGS gratings instrument onXMM-Newton and the HETG gratings instrument on Chandra. As part of the activities of the International Astronomical Consortium for High Energy Calibration (IACHEC), we have developed a standard spectral model for 1E 0102.2-7219 and fit this model to the spectra extracted from the CCD instruments. The model is empirical in that it includes Gaussians for the identified lines, an absorption component in the Galaxy, another absorption component in the SMC, and two thermal continuum components with different temperatures. In our fits, the model is highly constrained in that only the normalizations of the four brightest lines/line complexes (the OVII Heα triplet, OVIII Lyα line, the NeIX Heα triplet, and the NeX Lyα line) and an overall normalization are allowed to vary, while all other components are fixed. We adopted this approach to provide a straightforward comparison of the measured line fluxes at these four energies. We have examined these measured line fluxes as a function of time for each instrument after applying the most recent calibrations that account for the time-dependent response of each instrument.
Results. We performed our effective area comparison with representative, early mission data when the radiation damage and contamination layers were at a minimum, except for theXMM-Newton EPIC-pn instrument which is stable in time. We found that the measured fluxes of the OVII Heαr line, the OVIII Lyα line, the NeIX Heαr line, and the NeX Lyα line generally agree to within ±10% for all instruments, with 38 of our 48 fitted normalizations within ±10% of the IACHEC model value. We then fit all available observations of 1E 0102.2-7219 for the CCD instruments close to the on-axis position to characterize the time dependence in the 0.5-1.0keV band. We present the measured line normalizations as a function of time for each CCD instrument so that the users may estimate the uncertainty in their measured line fluxes for the epoch of their observations.