Time-division multiplexing switching transients and bandwidth effects on the X-ray Integral Field Unit readout performance

The X-IFU (X-ray Integral Field Unit), on the future European X-ray observatory NewAthena, will perform high-resolution imaging spectroscopy over an array of 1,504 transition-edge sensors operated at 55 mK. Its readout is performed via time-division multiplexing (TDM) at a few MHz, which relies on fast (but not instantaneous) switching between pixels of the same column. Differences in the currents of consecutive rows lead to signal-dependent transients in the readout. These transients, and how they settle before sampling of the final row signal, are in turn governed by the open-loop bandwidths in the system. Limitations of the bandwidths smear the transients and contaminate the settling of photon pulses, resulting in a frequency- and amplitude-dependent error on the output signal. Switching transients have thus ultimately a direct impact on the instrument's energy scale. Although the origin of these perturbations is well-understood, quantifying their end-to-end impact on performance remains challenging. We compare here experimental data with simulations to assess the accuracy of numerical predictions. We also present an end-to-end analysis of nonlinearity in the X-IFU energy scale due to switching transients, and compare it against current performance budget allocations. These simulations show that the X-IFU can meet its energy scale requirements.