The
Athena X-ray observatory consists of a single X-ray telescope with a 12 m
focal length, with an effective area of approx. 1.4 m2 (at 1 keV) and a spatial resolution of 5
arcseconds on-axis, degrading gracefully to less than 10 arcseconds at 30 arcminutes off-axis. The mirror is based on ESA's Silicon Pore Optics (SPO) technology. SPO provides an excellent ratio of collecting area to mass, while still offering a good angular resolution. It also benefits from a high
technology readiness level and a modular design highly amenable to mass production necessary to achieve the unprecedented telescope collecting area. A movable mirror assembly can focus X-rays onto either one of
Athena two instruments (WFI and X-IFU, see below) at any given time. Both the WFI and X-IFU successfully passed their Preliminary Requirements Reviews, on 31 October 2018 and 11 April 2019 respectively.
Wide Field Imager (WFI) The Wide Field Imager (WFI) is a large field of view spectral-imaging camera based on the unique Silicon DEPFET technology, developed in the semiconductor laboratory of the
Max Planck Society. The DEPFETs provide an excellent energy resolution (<170eV at 7keV), low noise, fast readout and high time resolution, with good radiation hardness. The instrument combines the Large Detector Array, which is optimized for a wide field of view observations over a 40' x 40' instantaneous sky area, with a separate Fast Detector tailored to observe the brightest point sources of the X-ray sky with high throughput and low pile-up. These capabilities, in combination with the unprecedented effective area and wide field of the
Athena telescope, will provide breakthrough capabilities in X-ray imaging spectroscopy. The WFI is developed by an international consortium composed of ESA member states. It is led by the
Max Planck Institute for Extraterrestrial Physics (DEU) with partners in Germany (ECAP, IAA Tübingen), Austria (
University of Vienna),
Denmark (DTU), France (
CEA Saclay,
Strasbourg), Italy (
INAF,
Bologna,
Palermo), Poland (SRC PAS, NCAC PAS), the United Kingdom (
University of Leicester,
Open University), the United States (
Pennsylvania State University (Penn State), SLAC,
Massachusetts Institute of Technology (MIT), SAO), Switzerland (
University of Geneva), Portugal (IA), and
Greece (Athens Observatory,
University of Crete). The principal investigator is Kirpal Nandra, Director of the High-Energy Group at MPE.
X-ray Integral Field Unit (X-IFU) The X-ray Integral Field Unit is the cryogenic X-ray spectrometer of
Athena X-IFU will deliver spatially resolved
X-ray spectroscopy, with a spectral resolution requirement of 2.5 eV up to 7 keV over a hexagonal field of view of 5 arc minutes (equivalent diameter). The prime detector of X-IFU is made of a large format array of Molybdenum Gold
transition-edge sensors coupled to absorbers made of
Au and
Bi to provide the required stopping power. The pixel size corresponds to slightly less than 5 arc seconds on the sky, thus matching the angular resolution of the X-ray optics. A large part of the X-IFU related
Athena science objectives relies on the observation of faint extended sources (e.g. hot gas in
cluster of galaxies to measure bulk motions and turbulence or its chemical composition), imposing the lowest possible instrumental background. This is achieved by the addition of a second cryogenic detector underneath the prime focal plane array. This way non-X-ray events such as particles can be vetoed using the temporal coincidence of detecting energy in both detectors simultaneously. The focal plane array, the sensors and the cold front end electronics are cooled at a stable temperature less than 100 mK by a multi-stage cryogenic chain, assembled by a series of mechanical coolers, with interface temperatures at 15 K, 4 K and 2 K and 300 mK, pre-cooling a sub Kelvin cooler made of a
3He adsorption cooler coupled with an Adiabatic Demagnetization Refrigerator. Calibration data are acquired along with each observation from modulated X-ray sources to enable the energy calibration required to reach the targeted spectral resolution. Although an integral field unit where each and every pixel delivers a high resolution X-ray spectrum, the defocussing capability of the
Athena mirror will enable the focal beam to be spread over hundreds of sensors. The X-IFU will thus be able to observe very bright X-ray sources. It will do so either with the nominal resolution, e.g. for detecting the baryons thought to reside in the
Warm Hot Intergalactic Medium, using bright gamma-ray burst afterglows, as background sources shining through the cosmic web, or with a spectral resolution of 3–10 eV, e.g. for measuring the spins and characterizing the winds and outflows of bright
X-ray binaries at energies where their spectral signatures are the strongest (above 5 keV). As of December 2018, when the X-IFU consortium was formally endorsed by ESA as being responsible for the procurement of the instrument to
Athena, the X-IFU consortium gathered 11 European countries (
Belgium,
Czech Republic,
Finland, France, Germany, Ireland, Italy, Netherlands, Poland, Spain, Switzerland), plus Japan and the United States. More than 50 research institutes are involved in the X-IFU consortium. The principal investigator of X-IFU is Didier Barret, Director of research at the research institute in astrophysics and planetology of
Toulouse (
IRAP-OMP,
CNRS UT3-Paul Sabatier/
CNES, France). Jan-Willem den Herder (
SRON, The Netherlands) and Luigi Piro (
INAF-IAPS, Italy) are co-principal investigators of the X-IFU.
CNES manages the project, and on behalf of the X-IFU consortium, is responsible for the delivery of the instrument to ESA. ==
Athena science goals ==