The Athena X-ray Integral Field Unit: a consolidated design for the system requirement review of the preliminary definition phase.

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The Athena X-ray Integral Unit (X-IFU) is the high resolution X-ray spectrometer studied since 2015 for flying in the mid-30s on the Athena space X-ray Observatory. Athena is a versatile observatory designed to address the Hot and Energetic Universe science theme, as selected in November 2013 by the Survey Science Committee. Based on a large format array of Transition Edge Sensors (TES), X-IFU aims to provide spatially resolved X-ray spectroscopy, with a spectral resolution of 2.5 eV (up to 7 keV) over an hexagonal field of view of 5 arc minutes (equivalent diameter). The X-IFU entered its System Requirement Review (SRR) in June 2022, at about the same time when ESA called for an overall X-IFU redesign (including the X-IFU cryostat and the cooling chain), due to an unanticipated cost overrun of Athena. In this paper, after illustrating the breakthrough capabilities of the X-IFU, we describe the instrument as presented at its SRR (i.e. in the course of its preliminary definition phase, so-called B1), browsing through all the subsystems and associated requirements. We then show the instrument budgets, with a particular emphasis on the anticipated budgets of some of its key performance parameters, such as the instrument efficiency, spectral resolution, energy scale knowledge, count rate capability, non X-ray background and target of opportunity efficiency. Finally, we briefly discuss the ongoing key technology demonstration activities, the calibration and the activities foreseen in the X-IFU Instrument Science Center, touch on communication and outreach activities, the consortium organisation and the life cycle assessment of X-IFU aiming at minimising the environmental footprint, associated with the development of the instrument. Thanks to the studies conducted so far on X-IFU, it is expected that along the design-to-cost exercise requested by ESA, the X-IFU will maintain flagship capabilities in spatially resolved high resolution X-ray spectroscopy, enabling most of the original X-IFU related scientific objectives of the Athena mission to be retained. The X-IFU will be provided by an international consortium led by France, The Netherlands and Italy, with ESA member state contributions from Belgium, Czech Republic, Finland, Germany, Poland, Spain, Switzerland, with additional contributions from the United States and Japan.
© The Author(s) 2023. Artículo firmado por 304 autores. This paper is based on the documentation assembled by the CNES project team and the Consortium partners for the X-IFU System Requirement Review, which has started at the end of June, albeit with reduced objectives (the review will be completed in September 2022). DB wishes to thank the CNES Project and Support teams for continuing the preparation of the SRR data pack, during the disruptive and unfortunate events that happened to Athena, which could have led to the termination of X-IFU. There is no doubt that their efforts will be rewarded in the upcoming design-to-cost exercise. DB would also like to express his gratitude to the CNES Management (Philippe Baptiste, President), Lionel Suchet (Chief Operating Officer), Caroline Laurent (Director of orbital systems and applications), Philippe Lier (Deputy-Director of orbital systems and applications), the French ESA SPC delegation (Olivier La Marle and Juliette Lambin) for their unfailing support to X-IFU. This support was instrumental in preserving an X-IFU on the new Athena mission and will remain precious in the upcoming phase of the reformulation of the mission. Special thanks to all the SPC delegations which supported the objective of keeping a flagship X-ray observatory in the ESA Science Program. DB is also thankful to Gilles Bergametti (President of the CNES Comit´e des Programmes Scientifiques, CPS), Athena Coustenis (President of the CNES Comit´e d’´evaluation de la recherche et de l’exploration spatiale, CERES) and Pierre-Olivier Petrucci (President of the Programme National des Hautes Energies) for their support and for providing the feedback from the French scientific community at large. The French contribution to X-IFU is funded by CNES, CNRS and CEA. This work has been also supported by ASI (Italian Space Agency) through the Contract 2019-27-HH.0, and by the ESA (European Space Agency) Core Technology Program (CTP) Contract No. 4000114932/15/NL/BW and the AREMBES - ESA CTP No.4000116655/16/NL/BW. This publication is part of grant RTI2018-096686-B-C21 funded by MCIN/AEI/ 10.13039/501100011033 and by “ERDF A way of making Europe”. This publication is part of grant RTI2018-096686-B-C21 and PID2020-115325GB-C31 funded by MCIN/AEI/10.13039/501100011033.
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