  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 arcsecond pixels. The X-IFU is
based on a large format array of super-conducting molybdenum-gold Transition
Edge Sensors cooled at about 90 mK, each coupled with an absorber made of gold
and bismuth with a pitch of 249 microns. A cryogenic anti-coincidence detector
located underneath the prime TES array enables the non X-ray background to be
reduced. A bath temperature of about 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.
