Synchrotron x-ray transmission measurements and modeling of filters investigated for Athena

Advanced Telescope for High-Energy Astrophysics is a large-class astrophysics space mission selected by the European Space Agency to study the theme "Hot and Energetic Universe."The mission essentially consists of a large effective area x-ray telescope and two detectors: the X-ray Integral Field Unit (X-IFU) and the Wide Field Imager (WFI). Both instruments require filters to shield from out-of-band radiation while providing high transparency to x-rays. The mission is presently in phase B; thus, to consolidate the preliminary design, investigated filter materials need to be properly characterized by experimental test campaigns. We report results from high-resolution x-ray transmission measurements performed using different synchrotron radiation beamlines to assess the filter calibration accuracy and mitigate the risk related to selecting a unique calibration facility. The main goals of these test campaigns are (i) to verify the compliance of the investigated filter design to the scientific requirements, (ii) to develop an accurate x-ray transmission model, and (iii) to start identifying suitable measurement facilities and achievable accuracy for the flight filters calibration program. In particular, the x-ray transmission model of the X-IFU and WFI filters has been refined within the edges of Al, C, N, and O by deriving the optical constants from two reference samples measured by synchrotron light. The achievable filter calibration accuracy has been estimated by evaluating the agreement between the best-fit according to the developed transmission model and the experimental data.