Performance of the 0-padding Optimal Filter Method in Non-linear Gain Calibration

The focal-plane detector, the X-ray Integral Field Unit (X-IFU), on-board ESA's Athena space telescope is a transition edge sensor (TES) microcalorimeter array with 1.5k pixels, designed to provide spatially-resolved, high-resolution spectroscopy over the energy range 0.2-12 keV. The onboard event processor uses a digital optimal filter to determine the pulse-height of the measured current pulse from every X-ray photon striking the array. A modified optimal filter called the 0-padding filter has recently been proposed. This is a truncated version of the standard optimal filter and has been shown to provide comparable energy resolution but with the benefit of reduced computational expense. Whereas the standard optimal filter has zero integral and is not sensitive to variations in the DC level of the measured signal, the integral of the 0-padded version is non-zero and thus is more sensitive to fluctuations in DC signal over time. In this work, we explore the effect of 0-padding on the energy scale calibration using data from 250-pixels in a prototype Athena X-IFU array, measured over the range 1.3-12 keV.