This page describes the Athena observatory proposed to ESA as the second L (large) mission of its Cosmic Vision 2015-2025 programme to address the Hot and Energetic Universe science theme.
From mid 2015 to April 2016 (Phase A1) two industrial consortia under ESA contract, were asked to study two mission concepts: the As proposed mission (described below) and a concept introduced by ESA during the Phase 0 study dubbed CDF mission, which has a reduced mirror size (effective area at 1 keV of 1.4 m2, instead of the As proposed 2 m2, corresponding to a mirror structure of 15 rows - “15-row mirror”, hereafter). The main scope of the Phase A1 study was to decide the mirror size that would fit technically and programmatically within the L-class mission boundaries.
A Mission Concept Review (MCR) took place in April/May 2016, followed by a ΔMCR during autumn 2016 with the goal to define the mission baseline configuration for the rest of Phase A. The ΔMCR achieved an important consolidation of the mass budget.
The Status Review 1 (SR1), presented in January 2018, comprehensively reviewed the Phase A work performed so far at the system level, including technical, cost and schedule aspects.
The SR1 confirmed that the "15-row mirror" is mass compliant with a high level of system margin (30%). The industrial primes will perform a design of the "15-row mirror" concept during the forthcoming Phase A extension.
Phase A will run until late 2018, ending with the Instrument Preliminary Requirement Reviews (IPRRs). Phase B1 will then follow until Q3/2019, ending with the Mission Formulation Review (MFR). Mission adoption by ESA’s Science Programme Committee (SPC) is expected in second half of 2021, leading to launch in the early 2030s.
The Athena proposal describes a mission concept according to which Athena will be a large X-ray observatory offering 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 like Hitomi, XARM, and SRG/eROSITA.
A conceptual design for the Athena spacecraft derived from the ESA CDF study, designed to be accommodated in an Ariane 5 launcher.
Athena will be launched by an Ariane 6 vehicle, with equivalent or larger lift capability and fairing size to that of the Ariane 5 ECA. It will operate at the second Sun-Earth Lagrangian point (L2) in a large halo orbit, although the possibility of an L1 halo orbit is also being assessed. The operational orbit will be reached with a direct transfer trajectory towards L2, with limited delta-V demands, and it offers a very stable thermal environment as well as good instantaneous sky visibility and high observing efficiency.
Athena has a baseline mission lifetime of 4 years, although it is expected to be designed and have consumables for a longer time. Operations will be performed as in standard ESA science missions, with the Mission Operations Centre (MOC) at ESOC and the Science Operations Centre (SOC) at ESAC. The Instrument and Science Centre (ISC) associated to each of the two instruments will be in support of the SOC with regard to science ground segment activities.
Athena will be operated as an observatory, in a similar fashion to prior missions such as XMM-Newton and Herschel. Users will access the observatory via open proposal calls.
A detailed analysis of the scientific questions underlying the Hot and Energetic Universe theme sets the key performance parameters for the mission. Mapping the dynamics and chemical composition of hot gas in diffuse sources requires high spectral resolution (2.5 eV) imaging with large area and low background; the same capabilities also optimize the sensitivity to weak absorption and emission features needed to uncover the hot components of the intergalactic medium. High resolution X-ray spectroscopy of distant gamma-ray bursts (GRBs) will reveal the signature of the first generation of stars, provided that the observatory can be repointed within 4 hours of an external trigger. An angular resolution lower than 5” (Half Energy Width) is needed to disentangle contaminants (point-source and sub-clump) from extended thermal emission in clusters, groups and galaxies. The same angular resolution is needed to resolve the dominant core emission and smaller accreting structures in galaxy clusters and groups up to redshift z~2. This resolution, when combined with the mirror effective area, also provides the necessary flux sensitivity (~10-17 erg cm-2 s-1 in the 0.5-2 keV band) to uncover typical accreting SMBH at z>6. The area coverage needed to detect significant samples of these objects within a reasonable survey time demands a large field of view instrument, combined with excellent off-axis response for the X-ray optics. The spectral resolution of that instrument will reveal the most obscured black holes at the peak of the Universe’s activity at z=1-4. High timing resolution and high-count rate capability will shed new light on nearby accreting black hole systems.
The Athena observatory consists of a single X-ray telescope with a fixed 12 m focal length (Willingale et al. 2013), 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 telescope collecting area (see more details on the development of SPO at ESA). The telescope focuses X-ray photons onto one of two instruments, which can be put in and out of the focal plane using a movable mirror assembly.
One instrument, the X-ray Integral Field Unit (X-IFU), provides spatially-resolved high resolution spectroscopy. The instrument is a cryogenic X-ray spectrometer, based on a large array of Transition Edge Sensors (TES), offering 2.5 eV spectral resolution, with 5’’ pixels, over a field of view of 5 arc minutes in equivalent diameter. Background in the X-IFU is mitigated using an active anti-coincidence layer, which is important to achieve the science goals for spectroscopy of faint extended sources. For more information, visit the X-IFU web portal.
The other instrument, the Wide Field Imager (WFI), is a Silicon-based detector using DEPFET Active Pixel Sensor (APS) technology. As X-ray spectroscopic imaging devices, the DEPFETs provide almost Fano-noise-limited energy resolution and minimal sensitivity to radiation damage. Because each pixel is addressed individually, readout modes can be highly flexible and extremely fast. With the development of appropriate readout ASICs, a time resolution of around 10μs is achievable as well as a count rate capability sufficient to deal with the brightest X-ray sources in the sky. The large field of view is achieved via a focal plane composed of several chips, where one of them will be enable fast readout to accommodate measurements of very bright targets. For more information, visit the WFI web portal.
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