Advanced Telescope for High Energy Astrophysics
The Wide Field Imager for the Athena X-ray Observatory

The Hot and Energetic Universe

The Athena mission will be designed to address the two key astrophysical questions of the Hot and Energetic Universe Science Theme: 1) How do black holes grow and shape the Universe? and 2) How does ordinary matter assemble into the large-scale structures we see today?

The first question will be addressed through revealing supermassive black holes, even in obscured environments, out in to the early Universe, and by studying inflows and outflows of matter and energy as the black holes grow. The second question requires mapping the hot gas structures in the Universe, e.g., the gas in clusters and groups of galaxies, and the intergalactic medium, determining their physical properties and evolution through cosmic time.

Science with the Wide Field Imager

The WFI, with its large FoV and very good angular and spectral resolution, will provide the important capabilities to address the questions of the Hot and Energetic Universe science theme.

A new discovery space for supermassive black holes will be accessed with a suitably designed multi-tiered WFI survey. Such a survey is expected yield over 400 X-ray selected AGN and redshifts z=6-8 and around 30 at z>8 and thus place essential constraints on physical models for early growth and formation of supermassive black holes. For the first time, this high redshift Universe will be accessible, with a survey power a factor ~100 better then existing surveys (i.e., it would take approximately 100 years of dedicated survey observations with currently existing X-ray facilities). Crucially, these will include obscured objects, and those whose host galaxy light prevents identification of the AGN in other wavebands.

The same survey will also identify distant galaxy groups and clusters and thus allow to measure their gas entropy profiles on all mass scales out to z~2. This will be essentially to determine the non-gravitational energy input over the whole cluster volume from the centre to the outskirts back to the epoch when star formation and accretion activity were most active. Observations with the WFI will also provide temperature maps of clusters around radio-loud AGN out to intermediate redshifts and map shock structures and test jet evolution models and infer their impact at the epoch of group and cluster formation.

The physics of accretion under strong gravity conditions is not well known, and the geometry of the accretion flow is still to be determined. Black hole spins yield important information about the accretion-ejection process and the history of the black hole formation and evolution. In addition to its survey power the WFI will also allow to observe very bright X-ray sources with high throughput and low pileup. This will enable studies of the geometry of the hot corona-accretion disk system through reverberation mapping, to constrain the origin of the hot corona, and to measure spins in supermassive and Galactic black holes.

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