The X-ray Integral Field Unit (X-IFU) is the high resolution X-ray spectrometer of the ESA Athena X-ray observatory. Over a field of view of 5’ equivalent diameter, it will deliver X-ray spectra from 0.2 to 12 keV with a spectral resolution of 2.5 eV up to 7 keV on ∼ 5” pixels. The X-IFU is based on a large format array of super-conducting molybdenum-gold Transition Edge Sensors cooled at ∼ 90 mK, each coupled with an absorber made of gold and bismuth with a pitch of 249 μm. A cryogenic anti-coincidence detector located underneath the prime TES array enables the non X-ray background to be reduced. A bath temperature of ∼ 50 mK is obtained by a series of mechanical coolers combining 15K Pulse Tubes, 4K and 2K Joule-Thomson coolers which pre-cool a sub Kelvin cooler made of a 3He sorption cooler coupled with an Adiabatic Demagnetization Refrigerator. Frequency domain multiplexing enables to read out 40 pixels in one single channel. A photon interacting with an absorber leads to a current pulse, amplified by the readout electronics and whose shape is reconstructed on board to recover its energy with high accuracy. The defocusing capability offered by the Athena movable mirror assembly enables the X-IFU to observe the brightest X-ray sources of the sky (up to Crab-like intensities) by spreading the telescope point spread function over hundreds of pixels. Thus the X-IFU delivers low pile-up, high throughput (< 50%), and typically 10 eV spectral resolution at 1 Crab intensities, i.e. a factor of 10 or more better than Silicon based X-ray detectors. In this paper, the current X-IFU baseline is presented, together with an assessment of its anticipated performance in terms of spectral resolution, background, and count rate capability. The X-IFU baseline configuration will be subject to a preliminary requirement review that is scheduled at the end of 2018.
The X-ray Integral Field Unit (X-IFU) is a next generation microcalorimeter planned for launch onboard the Athena observatory. Operating a matrix of 3840 superconducting Transition Edge Sensors at 90 mK, it will provide unprecedented spectro-imaging capabilities (2.5 eV resolution, for a field of view of 5’) in the soft X-ray band (0.2 up to 12 keV), enabling breakthrough science. The definition of the instrument evolved along the phase A study and we present here an overview of its predicted performances and their modeling, illustrating how the design of the X-IFU meets its top-level scientific requirements. This article notably covers the energy resolution, count-rate capability, quantum efficiency and non X-ray background levels, highlighting their main drivers.
In the framework of the ESA Athena mission, the X-ray Integral Field Unit (X-IFU) micro-calorimeter will provide unprecedented spatially resolved high-resolution X-ray spectroscopy. For this purpose, the on-board Event Processor (EP) must initially trigger the current pulses induced by the X-ray photons hitting the detector to proceed with a reconstruction which provides the arrival time, spatial location and energy of each event. The current event triggering design is implemented in two stages: one initial trigger of the low-pass filtered derivative of the raw data to extract records containing pulses and a second stage performing a fine detection to look for all the pulses in the record. In order to establish the current baseline detection technique of the EP in the X-IFU instrument, an assessment of the capabilities of different triggering algorithms is required, both in terms of performance (detection efficiency) and computational load, as processing will take place on-board. We present a comparison of two detection algorithms, the Simplest Threshold Crossing (STC) and the model-dependent Adjusted Derivative (AD). The analysis also evaluates the (possible) negative effect of different instrumental scenarios as a reduced sampling rate. The evaluations point out that the simplest algorithm STC shows worse performance than AD for the smallest pulses separations and the lowest secondary energies. Nevertheless, checking the expected number of such pulses combinations in a typical bright source observation, we conclude that it does not have impact in the science. Moreover, the savings in the computational resources and calibration needs make STC a valuable option.