## 6 Future Instruments for Astrophysics and Fundamental Physics Research
### Other Future X-ray Space Missions  
After the European mission Athena, other X-ray observatories are currently being proposed in China, Japan, and USA. The Hot Universe Baryon Surveyor (HUBS) [170], proposed in China, is designed to explore and characterize the circumgalactic medium with a large FoV. HUBS will be complementary to Athena. Thanks to the larger FoV (\(1^{\prime}\) angular resolution pixels over a \(1\,\mathrm{deg}^{2}\) FoV, while X-IFU has \(\sim\)5\({}^{\prime\prime}\) pixels over a FoV of 5\({}^{\prime}\) equivalent diameter), it is optimized to measure the spatial profile of extended warm-hot gas around galaxies and clusters of galaxies, and, potentially, the filamentary structures in the cosmic web, as well. The array of TES microcalorimeters is designed to have \(60\times 60\) pixels with an area of \(1\,\mathrm{mm}^{2}\) for each pixel. A \(12\times 12\) central array with smaller pixels is foreseen. The large Au/Bi absorbers required for this mission will be an interesting fabrication and optimization challenge [171]. The X-ray spectrometer will cover an energy range from 0.2-2 keV with expected energy resolution of \(2\,\mathrm{eV}\) and \(0.6\,\mathrm{eV}\) for the outer and inner array, respectively. The multiplexing read-out will be chosen between TDM, MHz-FDM and GHz-FDM depending on the overall system requirement. Orders of magnitude larger pixels arrays are envisaged for the future proposed large X-ray mission beyond Athena such as Lynx, Super-DIOS, and the Cosmic Web Explorer that will be briefly discussed here below. The Lynx X-ray Observatory [84], is one of four  
Figure 13: (a) X-IFU Focal Plane Assembly Demonstration Model. (b) Photograph of a X-IFU prototype array from NASA-GSFC with more than 3000 pixels on a pitch of \(260\,\mathrm{\SIUnitSymbolMicro m}\). Courtesy of the X-IFU FPA team at SRON and NASA-GSFC. large satellite mission concepts currently being studied by NASA. Lynx will provide a significant increases in sensitivity, FoV with subarcsecond imaging, and spectral resolution over Chandra and Athena. The three main science objectives of Lynx are: (i) the observation of the dawn of black holes, (ii) he understanding of the driving of the galaxy formation and evolutions, (iii) the study of the energetic side of the stellar evolution and the stellar ecosystem. One of the instruments on board of Lynx is an imaging spectrometer composed of a very large array of TES microcalorimeters [102]. To meet all the Lynx requirements, the focal plane array is expected to be divided in different sub-arrays, which will make it unique. Its main array will consist of 86,400, 50 \(\upmu\)m over an area of 1.5 \(\times\) 1.5 cm\({}^{2}\), with an energy resolution \(dE=3\) eV at 0.2-7 keV. At the innermost central 1 arc-min region, an array of 12,800, smaller, 25 \(\upmu\)m pixels will provide \(dE=2\) eV up to 7 keV. Finally, there will also be an ultra-high resolution subarray, optimized to achieve high energy resolution of 0.3 eV under 1 keV. To read-out such a large array of 100,000 pixels will set a challenge not only for the multiplexing, but for the layout of the wiring on the detector and bias circuit chips as well. NASA-GSFC has developed a process to combine planarized multi-stack buried wiring layers from Massachusetts Institute of Technology Lincoln Laboratory (MIT/LL) [98; 99] with their TES array design [115]. It will not remain a dream any more to realize a 100 k pixels instrument for a future space mission, by combining the heritage from X-IFU (Athena), the TES hydra concept discussed in Section 4.1, with the state-of-the-art of GHz-FDM [129], or a hybrid configuration including the traditional multiplexing read-out. In Japan, ISAS/JAXA is considering the Super Diffuse Intergalactic Oxygen Surveyor (DIOS) space mission [172] for a launch after 2030. The science case is similar to HUBS, but the satellite will perform wide field X-ray spectroscopy with a comparable FoV of about 0.5-1 deg, but with a better angular resolution of about 10-15 arcseconds.