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One of the most awaited features of the future of X-ray observatories is high resolution spectroscopy of the next generation of imaging devices based on calorimetric measurements of the photon energy in detectors cooled to few tens of mK. The two missions for which such detectors are envisioned and actually play a major role in the scope of these missions are Astro-H in Japan (with a possible launch in 2013 3) and the International X-ray Observatory (IXO) currently discussed by ESA, NASA, and JAXA as a mission for about 2018-2021 4. While Astro-H (Takahashi et al. 2009) is already being built, the IXO mission has not yet been fully approved and is still in the first phase of technical studies. Nevertheless, to give an outlook on the coming spectroscopic capabilities of these future X-ray missions, we will use the current prospective response files for the IXO mission with the planned so called narrow field detectors (NFI), which provide high resolution spectroscopy of about 2-3 eV energy resolution, to illustrate the prospects. In addition to providing high resolution spectroscopy IXO will also significantly increase the photon collecting power compared to XMM-Newton with a proposed collecting area around 1 keV of about 3 m2 to provide a better photon statistics to fully exploit the higher spectral resolution statistically. It will also increase the redshift range of cluster studies providing a detailed picture of galaxy cluster astrophysics up to redshifts of 2.

With the prospective energy resolution it will be possible to trace gas motions in the ICM with an accuracy of better than 100 km s-1 in the Fe lines in the X-ray spectra. This is made possible due to the fact that the thermal line broadening for lines from Fe ions is smaller by a factor of about 7 than the sound velocity which corresponds closely to the thermal velocity of the bulk of the baryons in the ICM, the protons. Fig. 40 shows the simulated spectrum as observed with the IXO NFI of a patch in a cluster with an aperture radius of 1 arcmin and a flux of 3 × 10-13 erg s-1 cm-2 (at 0.5-2 keV). With an exposure of about 100 ks (40 ks) a Gaussian shape turbulent velocity broadening can be measured with an accuracy of 40 (70) km s-1. Even for a cluster at redshift 1 with a flux of 10-14 erg s-1 cm-2 velocity broadening accuracy would still be 100 km s-1 in an exposure of 100 ks. This will provide many new insights into the ICM physics, answering questions such as for example: how much turbulent energy is injected into the ICM by the interaction of cool cores with the central AGN? How much of the cool core heating comes from the dissipation of this turbulent energy? How is the energy dissipated in merging clusters and how is this reflected in the turbulent and bulk motion structure of the ICM? How much can the mass estimate of clusters be improved if we can measure the energy content of the ICM in turbulent energy?

Figure 40

Figure 40. Simulated IXO NFI spectrum of cluster emission with a flux of 3 x 10-13 erg s-1 cm-2 in the NFI field-of-view at a redshift z ~ 0.2 with an exposure of 100 ksec. The inset shows the spectral structure of major Fe L-shell lines around 1 keV. Spectral line broadening for plasma with mean turbulent velocities of 100 km/s and 500 km/s are compared in this inset.

Finally Fig. 41 illustrates the potential of IXO to provide detailed information on very distant clusters. Common objects at redshift 2 having a sky density of about 1 per deg2 are poor clusters with masses of about 3 × 1013 Modot and probably temperatures around 2 keV. Fig. 41 shows the a spectrum of such an object observed with the IXO NFI instrument for 250 ks (see also Arnaud et al. 2009). It will allow the measurement of the bulk temperature with an accuracy better than 5% and provide abundance determinations with interesting precision: Fe (± 11%), Si (± 18%), O, Mg (± 30%). This will allow us to study the thermal and chemical enrichment history of galaxy clusters over most of the interesting cosmic epochs in which clusters existed. For the more nearby bright clusters IXO will very accurately measure the abundances of many chemical elements, including trace elements like Cr, Ti, Mn, or Co. The other elements, which we detect in clusters with XMM-Newton and Chandra (O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni) will be traced with an unprecedented accuracy, which will help us to put better constrains on supernova yields and thus on their explosion mechanisms. Furthermore, the wide field imager on IXO will allow us to resolve the 2-dimensional metal distribution down to the relevant mixing physical scales and study in detail the process of metal injection.

Figure 41

Figure 41. Simulated IXO NFI spectrum of a galaxy group with an ICM temperature of 2 keV at redshift 2. Also shown is the estimated background spectrum. ICM properties like the temperature and abundance measurements can well be determined from this simulated exposure of 250 ksec. See text for details.

The filamentary warm-hot intergalactic medium (WHIM) permeating the cosmic web might contain up to 50% of baryons at redshifts of z ltapprox 2. This intergalactic gas is heated to temperatures between 105 and 107 K as it gathers in regions of overdensity 10-100 (with respect to the mean baryon density of the Universe; at the current epoch the mean baryon density is < nb > = 2 ×10-7 cm-3). Because this gas has such a low density and because it is so highly ionized, it is very hard to detect. The current instrumentation only allows us to probe the highest overdensity regions of the WHIM, such as the filament between the clusters Abell 222 and 223 (Werner et al. 2008), or to look for absorption features toward bright continuum sources in sight lines with known large scale structure, such as H2356-309 behind the Sculptor Wall (Buote et al. 2009). The combination of the large collecting area of IXO and the high spectral resolution of its diffraction grating spectrometers will allow us to begin to probe the low-overdensity regions of the intergalactic medium. High-resolution X-ray absorption spectroscopy of sight lines towards bright continuum sources may allow us to probe overdensities of 10 or more (assuming a metallicity of 0.1 Solar), through the absorption features of H- and He-like ions of oxygen. Emission line imaging spectroscopy of the WHIM should be pursued with a large field-of-view (of order one degree), short focal length, large grasp (effective area times field-of-view) soft X-ray telescope, with a high resolution imaging X-ray spectrometer at the focus. A successful experiment will need a grasp of order several hundred cm2 deg2, and a spectral resolution of order 1-2 eV or better (Paerels et al. 2008).

Apart from these expectations, a new mission like IXO capable of opening a new parameter space for X-ray spectroscopy and simultaneously increasing the sensitivity substantially and thus the reach of the observations, has an enormous potential for new discoveries. The most important discoveries of such missions generally come as surprises. Since the concept of IXO is already realistic - apart from detailed technical problems still to be solved - and also not much more costly than previous missions, we can expect its realization. Therefore we can foresee a bright future for X-ray spectroscopic research also over the coming two decades, and clusters will surely be among the most interesting objects to study.

Acknowledgements. We like to thank A. Bykov, G. Pratt, Carles Badenes, and Aurora Simionescu for a critical reading of the manuscript and for discussions. NW acknowledges support provided by the National Aeronautics and Space Administration through Chandra Postdoctoral Fellowship Award Number PF8-90056 issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics and Space Administration under contract NAS8-03060. HB acknowledges support for the research group through The Cluster of Excellence "Origin and Structure of the Universe", funded by the Excellence Initiative of the Federal Government of Germany, EXC project number 153 and support from the DfG Transregio Programme TR33.

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