Photon shot-noise analysis for the NewAthena X-IFU reformulation

The X-ray Integral Field Unit (X-IFU) onboard the ESA NewAthena mission is designed to provide spatially resolved high-resolution X-ray spectroscopy. This instrument relies on a large array of Transition Edge Sensor (TES) microcalorimeters operating at cryogenic temperatures of approximately 50 mK. While the primary scientific targets are X-rays, the detector’s extreme sensitivity renders it susceptible to out-of-band electromagnetic radiation, ranging from the far infrared to the extreme ultraviolet. This ”optical loading” generates Photon Shot-Noise (PSN), which can severely degrade the instrument’s energy resolution and compromise scientific performance. Unwanted radiation stems from two primary sources: external optical loads from astrophysical sources entering the telescope, and internal thermal radiation generated by the instrument’s warmer structure. To mitigate this, the detector assembly employs a multi-stage cryogenic system and a specialized stack of thermal-optical filters designed to shield the sensor. However, verifying that the PSN remains within acceptable limits requires rigorous modeling of the filter properties, surface emissivity, temperatures, and system geometry. Specifically, the calculation must account for complex cavity effects, such as multiple reflections between internal surfaces, and handle the very low transmissivities of the blocking filters. A custom raytracing software, ray3cer, was developed to perform comprehensive PSN analysis. Its modeling capability proved essential during the recent transition from the original Athena concept to the NewAthena design. In this paper, we present the PSN analyses that shaped the reformulated filter configuration, evaluating the nominal baseline performance, conducting temperature and surface reflectivity sensitivity studies, assessing failure tolerance to localized punctures, and deriving quantitative straylight constraints.