CORRAREA: cross-calibration of the XMM-Newton EPIC pn and MOS effective areas

Abstract

The remaining relative uncertainties on the calibration of the XMM-Newton telescope effective areas can still yield differences of the order of ten percent for the astrophysical parameters derived from the EPIC pn and MOS data. The aim of the CORRAREA project is to provide an empirical correction function determined by cross-calibration of the EPIC effective areas. The correction will be integrated as a default multiplicative energy-dependent correction factor to the ARFs in the SAS software package. It is determined by cross-calibrating the on-axis effective areas using a sample of more than 250 bright but non-piled-up observations of point-like sources. A fit-and-stack approach is used for cross-calibration with physically motivated models defined separately for each observation in the sample. Data-to-model ratios are calculated for each of the three detectors by dividing the detector data with the pn model convoluted with the detector response. A fit to the ratio of the MOS and pn data-to-model ratios then yields the correction function. The EPIC pn camera was chosen as the reference instrument as most of its calibration parameters have remained stable over the lifetime of the XMM-Newton mission. A highly automated pipeline was developed to allow for easy recalculation of the correction function to account for any future changes introduced in the XMM-Newton calibration files or SAS. The automated pipeline also allows to easily add future observations to the sample with only two manual tasks per observation: selection of a model and visual screening including the selection of source and background regions.