When less is more: the truncation of the optimal filter to reconstruct events in X-IFU/Athena-like TES detectors

Abstract

The X-ray Integral Field Unit (X-IFU) instrument to be on board the future ESA mission Athena X-ray Observatory is a cryogenic micro-calorimeter array of Transition Edge Sensor (TES) detectors aimed at providing spatially resolved high-resolution spectroscopy. As a part of the on-board Event Processor (EP), the reconstruction software will provide the energy, spatial location and arrival time of the incoming X-ray photons hitting the detector and inducing current pulses on it. Being the standard optimal filtering technique the chosen baseline reconstruction algorithm, a particular modification of this technique based on a truncation of the filter in the Time Domain (equivalent to 0-padding the pulse signal) was previously studied on simulated data, proving a better energy resolution results at a lower computational cost. However, the 0-padding technique also showed a larger sensitivity to instrumental conditions, making essential the analysis of its behaviour over real laboratory data. A comparative analysis of X-IFU-like TES laboratory data from NASA and NIST (at different instrumental conditions) with both the 0-padding filter and the standard optimal filters reconstruction shows that, once the corrections for the baseline drift and the jitter (phase introduced by the sampling rate) have been performed, the resolution values obtained using 0-padding are systematically lower than those of the standard filter of the same length and comparable or even lower to the values provided by the full-length filter. The shorter length of the 0-padding filter would be an additional benefit, reducing the computational cost of the reconstruction process.