Energy response of Athena WFI prototype detectors

The Wide Field Imager instrument of ESA's next X-ray observatory Athena will consist of specifically developed DEPFET detectors which enable a low noise and fast readout operation. In order to confirm the required spectroscopic performance within the energy range of 0.2 keV to 15 keV, various emission lines were probed with prototype detectors on a 64 x64 pixel scale. These detectors include the first sensors representative for flight with respect to the transistor layout and fabrication technology. Four different detectors were tested and exhibit a Fano noise limited spectral performance. The required energy resolution was achieved by all detectors even at readout times as fast as 2.5 ps/row. Additionally, one of the detectors achieved an outstanding performance of a FWHM below 45 eV at C Ka (277 eV) and 6.1 ps/row. Complementary to the measurements, the charge cloud size was determined as a function of the photon energy based on Monte Carlo simulations. These simulations enable a quantification of the effective charge loss at different energies due to the thresholds applied at the event recombination.