The Hot and Energetic Universe revealed by the X-IFU
ESA’s Athena X-ray observatory mission was selected in 2014 to address the Hot and Energetic Universe science theme (Ref. 1). The Hot Universe refers to the baryons in the Universe at temperatures above 105-6 K and that amount to about half of the total baryonic content. There are as many baryons at T >107 K trapped in dark-matter potential wells which trace the large-scale structures of the Universe as there are locked into stars. Gas at lower temperatures (down to T ~ 105 K) is expected to reside in filamentary structures pervading the intergalactic medium. The hot gas distributed on large-scales is strongly influenced by phenomena occurring in the immediate vicinity of black holes (the Energetic Universe) through a poorly understood process called Cosmic Feedback. As X-rays are copiously produced by hot gas and accretion around black holes, the best observational handle on the Hot and Energetic Universe is through X-ray observations. In addition X-rays can escape relatively unimpeded from significantly obscured environments and are only weakly contaminated by phenomena other than those mentioned. To address these and other science objectives Athena is conceived as a large observatory combining an unprecedented combination of sensitive X-ray imaging, timing and high-resolution spectroscopy.
The X-IFU is a cryogenic imaging spectrometer, offering spatially-resolved high-spectral resolution X-ray spectroscopy over a 5 arc minute equivalent diameter field of view. The breadth of the science affordable with the X-IFU encompasses key scientic issues of the Hot and Energetic Universe science theme and beyond. In a nutshell, the X-IFU will provide:
- 3D integral field spectroscopic mapping of hot cosmic plasmas, enabling measurements of gas bulk motions and turbulence, chemical abundances and the spatial distribution of these and other physical parameters. This drives the X-IFU field of view and spatial resolution, particle background, spectral resolution and calibration accuracy.
- Weak spectroscopic line detection, enabling the detection of unresolved absorption and emission lines from Warm and Hot Intergalactic Medium filaments and weak spectral features produced by unusual ion species or states. This drives the X-IFU spectral resolution, calibration and throughput.
- Physical characterization of the Hot and Energetic Universe, including plasma diagnostics using emission line multiplets, AGN reverberation and black hole spin measurements, winds in galactic sources in outburst, AGN winds and out ows, stellar out ows, solar wind charge exchange, etc. This drives the X-IFU spectral resolution, calibration and high-count rate capability.
The breakthrough capabilities of the X-IFU are illustrated in the figure below, which shows the expected X-IFU spectrum of the core of the Perseus cluster, based on very recent results obtained with the Hitomi Soft X-ray Spectrometer (Ref. 2).
The core of the Perseus cluster seen by X-IFU
The simulated X-IFU spectrum of the core of the Perseus cluster, based on Hitomi SXS observations (Ref. 2, Courtesy of A.C. Fabian and C. Pinto). The exposure time is 100 kilo-seconds. The grey area indicates the region not fully explored by SXS, due to the fact that the Hitomi SXS Perseus observation was performed with gate valve closed (blocking most X-rays below 2 keV). The insets show the region around the iron L and K complexes. The wealth of information provided by such a spectrum, that will be measured on sub-arc minute scales enables in depth studies of the physical properties of the hot cluster gas (e.g. temperature, density, turbulence, bulk motion, abundance, . . . ).
 Nandra, K., Barret, D., Barcons, X., Fabian, A., den Herder, J.-W., Piro, L., Watson, M., Adami, C., Aird, J., Afonso, J. M., and et al., The Hot and Energetic Universe: A White Paper presenting the science theme motivating the Athena+ mission, ArXiv e-prints (June 2013).
 Hitomi Collaboration, Aharonian, F., Akamatsu, H., Akimoto, F., Allen, S. W., Anabuki, N., Angelini, L., Arnaud, K., Audard, M., Awaki, H., Axelsson, M., et al., The quiescent intracluster medium in the core of the Perseus cluster, Nature 535, 117 121 (July 2016).
The X-IFU will achieve superior sensitivity compared to previous high-resolution spectrometers.
The science case presented here has been assembled by the X-IFU Science Advisory Team (see X-IFU Consortium), led by Massimo Cappi and Etienne Pointecouteau. We acknowledge support from the Athena Science Study Team, the Athena Working Group Chairs, the Athena Topical Panel Chairs and the Topical Panel members in strengthening the X-IFU science case and its top level performance requirements.
Latest reference: D. Barret and the Science Advisory Team, 2019, Science with the Athena X-ray Integral Field Unit, X-IFU Preliminary Requirements Review.