Athena will be a fully-fledged X-ray observatory, with a collecting area that dwarfs its predecessors XMM-Newton and Chandra, with a wide-range of astrophysical applications and a large user base. Its design is driven by two key astrophysical questions central to the Hot and Energetic Universe theme: how does baryonic matter assemble into the large-scale structures that we see today and how do black holes grow and influence that matter? The full science case for Athena is available as arXiv:1306.2307.
Athena’s science themes are thus closely aligned with current and future priorities in the UK’s science programme..
To achieve its science goals in the themes briefly outlined above, Athena requires spatially-resolved X-ray spectroscopy and deep wide-field X-ray spectral imaging with performance greatly exceeding that offered by current X-ray observatories like XMM-Newton and Chandra, or by missions recently launched such as SRG/eROSITA. This capability requires an X-ray telescope combining unprecedented collecting area (1.4m2 goal at 1 keV) with an excellent angular resolution (5”) and a wide field of view (40’x40’). New instrumentation providing spatially-resolved high resolution spectroscopy will yield the physical parameters of hot gas structures out to high redshift and map the intergalactic medium in the nearby Universe. A wide field instrument performing spectrally-resolved imaging over a broad energy band is required to determine the evolution of SMBH in the early Universe, and shed new light on black hole accretion and ejection processes, over a wide range of masses from Galactic compact objects to the largest supermassive black holes.
Figure 1: Schematic of the Athena mission highlighting the key elements.
All these capabilities combine in the Athena concept (see Fig.1). Athena consists of a single X-ray telescope with a fixed 12m focal length, based on ESA’s Silicon Pore Optics (SPO) technology. SPO provides an exceptionally high ratio of collecting area to mass, while still offering the necessary angular resolution. It also benefits from a high technology readiness level (TRL) and a modular design highly amenable to mass production, necessary to achieve the unprecedented collecting area. The X-ray optics are procured by ESA.
The telescope focuses X-ray photons onto one of two instruments. The first instrument, the X-ray Integral Field Unit (X-IFU), led by IRAP in France, provides spatially-resolved high resolution (~2.5 eV) spectroscopy. The instrument is based on cooled Transition Edge Sensors (TES). These can deliver the necessary energy resolution, while providing exceptional efficiency compared to the dispersive spectrometers flown on the current generation of X-ray observatories. The second instrument, the Wide Field Imager (WFI), led by MPE in Germany, is a silicon-based detector using DEPFET Active Pixel Sensor technology. As X-ray spectroscopic imaging devices, the DEPFETs provide almost Fano-noise-limited energy resolution (~140 eV at 7 keV) and minimal sensitivity to radiation damage. Because each pixel is addressed individually, readout modes can be highly flexible and extremely fast. The large field of view is achieved via a focal plane composed of four detector arrays, with an additional detector configured to enable fast readout to accommodate measurements of very bright targets. Fig.2 compares the Athena performance with current and future missions, amply demonstrating the large increase in capability afforded.
Figure 2: Scientific performance of Athena. The left panel illustrates the gain in survey capabilities afforded by Athena compared with XMM and Chandra. The right panel shows the gain in spectral line sensitivity when compared with the Japan-US XRISM mission due for launch in 2023.
As well as representing a major advance in its own right, Athena has strong synergies with all the current and future major astronomical facilities which have UK involvement: SKA, ALMA, JWST, LSST, E-ELT, CTA, LISA. These important science synergies are being explored in a number exercises initiated by the Athena project, see for example the ESO-Athena white paper (arXiv:1705.06064), the SKA-Athena white paper (arXiv:1807.09080) and the Piro et al. Multi-Messenger-Athenasynergy white paper (arXiv:2110.15677).