eROSITA calibration

The on-ground calibration measurements of the eROSITA cameras were performed at the PUMA X-ray test facility at MPE. The energy calibration was derived for a global threshold of 46 adu (around 40 eV) at multiple emission lines. The charge transfer inefficiency (CTI) was found to be extremely low yielding excellent spectral resolution in a CCD, with a typical 1σ uncertainty of only 1 eV in the whole energy range (0.3-10 keV). There is a small temperature dependence of the gain and CTI, and no X-ray leaks generated by the ⁵⁵Fe calibration source were found. Further details on the energy resolution of the cameras are shown in Table 3 of Predehl et al. 2021. A detailed report on the eROSITA calibration can be found in Dennerl et al. 2020.

The calibration measurements of the eROSITA mirrors were performed at the PANTER 130 m long beam X-ray test facility of MPE. These calibration measurements helped to calibrate the on-axis and off-axis eROSITA PSF, to determine the eROSITA effective area and the eROSITA vignetting (off-axis effective area). The following figures depict more in detail the behaviour of the eROSITA on-axis and off-axis PSF and vignetting, as well as the effective area and grasp.

Fig. 1 Point spread function (PSF) mapping of telescope module 1. The measurements were taken at various energies and the RGB PSF images created by combining the following datasets: C-K and Al-K (red), Ag-L and Ti-K (green), Fe-K and Cu-K (blue). The overall brightness is proportional to the relative photon flux per pixel. © Predehl et al. 2021.

Fig. 2 Example of the vignetting curves at seven different energies for telescope module 1. The curves for the other telescope modules are similar. © Predehl et al. 2021.

Fig. 3 Comparison of the on-axis effective areas as a function of energy for eROSITA and other X-ray telescopes. © Predehl et al. 2021.

Fig. 4 Comparison of the grasp, defined as the product of field of view times (averaged) effective area, as a function of energy for eROSITA and other X-ray telescopes. © Predehl et al. 2021.

Fig. 5 Energy resolution (FWHM in eV) of all telescope modules (TMs) as a function of the photon energy, derived from single pixel events in ground calibration measurements. The dashed curves indicated the theoretical limit which can be achieved with X-ray CCDs.