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The Athena X-ray Integral Field Unit (X-IFU)

Didier Barret, Thien Lam Trong, Jan-Willem den Herder, Luigi Piro, Xavier Barcons, Juhani Huovelin, Richard Kelley, J. Miguel Mas-Hesse, Kazuhisa Mitsuda, Stéphane Paltani, Gregor Rauw, Agata Rożanska, Joern Wilms, Marco Barbera, Enrico Bozzo, Maria Teresa Ceballos, Ivan Charles, Anne Decourchelle, Roland den Hartog, Jean-Marc Duval, Fabrizio Fiore, Flavio Gatti, Andrea Goldwurm, Brian Jackson, Peter Jonker, Caroline Kilbourne, Claudio Macculi, Mariano Mendez, Silvano Molendi, Piotr Orleanski, François Pajot, Etienne Pointecouteau, Frederick Porter, Gabriel W. Pratt, Damien Prêle, Laurent Ravera, Etienne Renotte, Joop Schaye, Keisuke Shinozaki, Luca Valenziano, Jacco Vink, Natalie Webb, Noriko Yamasaki, Françoise Delcelier-Douchin, Michel Le Du, Jean-Michel Mesnager, Alice Pradines, Graziella Branduardi-Raymont, Mauro Dadina, Alexis Finoguenov, Yasushi Fukazawa, Agnieszka Janiuk, Jon Miller, Yaël Nazé, Fabrizio Nicastro, Salvatore Sciortino, Jose Miguel Torrejon, Hervé Geoffray, Isabelle Hernandez, Laure Luno, Philippe Peille, Jérôme André, Christophe Daniel, Christophe Etcheverry, Emilie Gloaguen, Jérémie Hassin, Gilles Hervet, Irwin Maussang, Jérôme Moueza, Alexis Paillet, Bruno Vella, Gonzalo Campos Garrido, Jean-Charles Damery, Chantal Panem, Johan Panh, Simon Bandler, Jean-Marc Biffi, Kevin Boyce, Antoine Clénet, Michael DiPirro, Pierre Jamotton, Simone Lotti, Denis Schwander, Stephen Smith, Bert-Joost van Leeuwen, Henk van Weers, Thorsten Brand, Beatriz Cobo, Thomas Dauser, Jelle de Plaa, Edoardo Cucchetti

arXiv:1608.08105v1astro-ph.IMastro-ph.COastro-ph.GAastro-ph.HE

TL;DR

Athena’s X-IFU is intended to provide spatially resolved, high-resolution X-ray spectroscopy for studying the Hot and Energetic Universe and broader observatory science. The paper connects those objectives to the instrument’s requirements, describes its consolidated design and predicted performance, and evaluates improvements constrained by mass, complexity, and resources. Its simulated and projected capabilities include detailed cluster spectroscopy and WHIM-filament detection, while the design remains subject to mass and accommodation constraints.

  • Problem

    Athena needs X-ray observations to investigate hot baryons, cosmic feedback, and related phenomena across the Hot and Energetic Universe.

  • Method

    The paper reviews the science objectives driving X-IFU requirements, describes the baseline instrument design and predicted performance, and examines optimization options.

  • Results

    The X-IFU combines large collecting area and spectral resolution, with simulations showing detection of four WHIM filaments at redshifts 0.108, 0.350, 0.444, and 0.753.

  • Takeaways & Limitations

    The X-IFU is expected to support 3D mapping of hot plasmas, weak-line detection, and physical characterization of hot and energetic astrophysical sources.

  • Takeaways & Limitations

    The X-IFU design must fit spacecraft mass and focal-plane accommodation constraints, which are expected to drive further design evolution.

Abstract

from arXiv · show

The X-ray Integral Field Unit (X-IFU) on board the Advanced Telescope for High-ENergy Astrophysics (Athena) will provide spatially resolved high-resolution X-ray spectroscopy from 0.2 to 12 keV, with 5 arc second pixels over a field of view of 5 arc minute equivalent diameter and a spectral resolution of 2.5 eV up to 7 keV. In this paper, we first review the core scientific objectives of Athena, driving the main performance parameters of the X-IFU, namely the spectral resolution, the field of view, the effective area, the count rate capabilities, the instrumental background. We also illustrate the breakthrough potential of the X-IFU for some observatory science goals. Then we briefly describe the X-IFU design as defined at the time of the mission consolidation review concluded in May 2016, and report on its predicted performance. Finally, we discuss some options to improve the instrument performance while not increasing its complexity and resource demands (e.g. count rate capability, spectral resolution). The X-IFU will be provided by an international consortium led by France, The Netherlands and Italy, with further ESA member state contributions from Belgium, Finland, Germany, Poland, Spain, Switzerland and two international partners from the United States and Japan.

1. INTRODUCTION

Athena’s X-IFU is designed to address the Hot and Energetic Universe through spatially resolved, high-resolution X-ray spectroscopy. The paper links its scientific objectives to instrument requirements and illustrates its potential with simulated Perseus-cluster observations.

  • The X-IFU is a cryogenic imaging spectrometer providing spatially resolved, high-spectral-resolution X-ray spectroscopy across a 5 arc minute equivalent diameter field of view.
  • 3D integral-field mapping of hot cosmic plasmas targets bulk motions, turbulence, abundances, and spatially distributed physical properties.These goals drive requirements on field of view, spatial resolution, particle background, spectral resolution, and calibration accuracy.
  • Weak-line spectroscopy targets unresolved absorption and emission features from Warm and Hot Intergalactic Medium filaments and unusual ion species or states.This science drives spectral resolution, calibration, and throughput requirements.
  • The instrument also supports plasma diagnostics, AGN reverberation and black-hole spin measurements, and studies of winds and outflows.These objectives drive spectral resolution, calibration, and high-count-rate capability.
  • A simulated 100 kilo-second Perseus-cluster spectrum demonstrates the information X-IFU could measure on sub-arc minute scales.The spectrum is based on Hitomi SXS observations and includes iron L and K complex regions for studying hot-cluster-gas properties.
  • The paper reviews Athena’s objectives, describes the baseline X-IFU configuration and predicted performance, and discusses performance-optimization options.

2. SPECTROSCOPY OF THE HOT AND ENERGETIC UNIVERSE

This section introduces the core X-IFU-related scientific objectives of Athena and identifies observatory-science goals relevant to the instrument.

  • The section reviews Athena’s core scientific objectives that are relevant to the X-IFU.
  • It also describes observatory-science goals relevant for X-IFU observations.
  • The discussion covers both Athena’s core objectives and selected observatory-science applications.

2.1 The Hot Universe

Athena’s X-IFU is designed to characterize hot cosmic baryons through spatially resolved, high-resolution X-ray spectroscopy. Its measurements target gas dynamics, chemical enrichment, AGN feedback, and the missing warm-hot baryons across cosmic time.

  • Cluster bulk motions and turbulence: Hitomi measured modest Perseus-core turbulence below 164 ± 10 km s−1, while X-IFU is expected to probe 12-times smaller spatial scales with 25-times greater spectroscopic throughput.The comparison illustrates the anticipated improvement over previous spatially resolved high-resolution spectroscopy.
  • Cluster bulk motions and turbulence: X-IFU will map hot-gas bulk motions and turbulence with 5 arcsecond spatial and 2.5 eV spectral resolution, reaching 10–20 km s−1 precision for 100–1000 km s−1 velocities.This enables direct velocity-field measurements in galaxy groups and clusters.
  • Chemical enrichment: X-IFU’s spectral resolution will resolve line complexes and measure abundance ratios beyond cluster cores, determining heavy-element abundances to high redshift (z > 1) for the first time.O/Fe and Si/Fe evolution can distinguish enrichment by SNIa ejection from stripping of pre-enriched galaxies.
  • AGN feedback on cluster scales: Measurements of line profiles and centroids will map AGN-driven turbulence to about 20 km s−1 and constrain how jet power is distributed through the intracluster medium.The instrument can also detect shocked gas around expanding radio lobes and resolve their shock expansion speeds.
  • AGN feedback on cluster scales: Dynamics near cool filaments will measure material cooling from the hot phase and relate it to fuel available for AGN, helping connect heating, cooling, and black-hole growth.The resulting jet-power measurements can be compared with accretion rates of hot and cold material.
  • The missing baryons and the Warm-Hot Intergalactic Medium: X-IFU observations of bright AGN and GRB afterglows are predicted to characterize the warm-hot intergalactic medium and its baryon budget, including multiple detected filaments in simulations.A simulated GRB afterglow spectrum contains four WHIM filaments at redshifts 0.108, 0.350, 0.444, and 0.753.

2.2 The Energetic Universe

X-IFU observations target energetic processes across cosmic history, from WHIM and early metal enrichment to AGN winds, black-hole spins, and accretion physics. Its sensitivity, spectral resolution, imaging, and timing enable measurements spanning faint absorption systems, feedback, and rapidly varying disk winds.

  • AGN and star-formation driven winds and outflows: X-IFU observations will map gas, energy, and metals expelled by AGN and starbursts nearby, while characterizing high-redshift environments and obscured AGN redshifts.The program extends from the local Universe to z ∼3 and uses Fe K emission to unveil redshifts of obscured distant AGN.
  • High-z Gamma-ray Bursts: early metal enrichment of the Universe: Four WHIM filaments are detected in simulated spectra of the same z > 0.8 sightline, enabling direct measurements of gas temperature and turbulence.The simulations use either a bright GRB afterglow or 3C454.3 as the background source.
  • High-z Gamma-ray Bursts: early metal enrichment of the Universe: X-IFU will measure metal-abundance patterns in at least 10 medium-bright X-ray afterglows per year, reaching gas metallicities as low as 1% of solar.These measurements can test whether the primordial stellar IMF was top heavy by constraining typical early-star masses.
  • AGN and star-formation driven winds and outflows: A sample of at least ∼50−80 nearby bright Seyfert galaxies can distinguish radiation-driven, momentum-driven, and magnetically-driven accretion-disc wind models.The measurements target UFO properties including column density, ionization state, velocity, location, geometry, and covering factor.
  • SMBH spins: A 100 ks observation can recover the spin of the maximally rotating black hole in PB5062 at z = 1.77 with 20% precision.The method combines the relativistically broadened Fe Kα line with the soft X-ray reflection continuum and separates broad from narrow features.
  • Accretion physics: Hundreds-of-seconds X-IFU observations will constrain magnetic fields through winds and track outflow mass rates, kinetic power, and disk fields on dynamical time scales.The instrument is designed to obtain sensitive line spectra during rapidly changing accretion states.

2.3 Observatory science

Beyond the Hot and Energetic Universe, X-IFU’s combination of spectral resolution, throughput, spatial mapping, and fast timing supports investigations of planets, stars, stellar accretion, flares, supernova remnants, and transients.

  • Solar-system science: X-IFU will identify the ions producing Jupiter’s soft X-ray aurora and measure line broadening velocities to test ion-acceleration theories.The ions’ origins can be distinguished between the solar wind and Io’s volcanoes.
  • Massive stars: Time-resolved high-resolution spectroscopy will probe wind inhomogeneity and larger-scale structures in massive stars through short-term variations, line profiles, and Doppler mapping.This enables studies across a large sample beyond the reach of current X-ray spectrometers.
  • Young stars: For CTTSs, high-resolution spectroscopy down to 3 kilo-seconds will measure accretion variability, 100−400 km s−1 bulk velocities, and density stratification.The observations target variability on predicted hour time scales and modulation from accretion-stream shadowing.
  • Young stars: X-IFU will investigate intense flares on young active stars, including mass motions and their influence on circumstellar disks and early planetary evolution.The cited flare simulations reach peak luminosities of LX ∼1032 erg s−1 and peak temperatures of ∼2 × 108 K.
  • Supernova remnants: Spatially resolved X-IFU spectra of supernova remnants will map abundances for elements with Z=6-28 and Doppler shifts and broadening in young ejecta.These measurements address explosion mechanisms, hot non-equilibrium plasmas, and their impact on the interstellar medium.
  • Transient science: Athena’s fast Target of Opportunity capability will enable X-IFU observations of transient phenomena within hours of their triggers.Potential targets include high-redshift gamma-ray bursts and future gravitational-wave sources.

3.1 Top level X-IFU performance specifications

The X-IFU performance requirements are implemented with a large cryogenic TES absorber array, routine calibration, and active cryogenic anti-coincidence shielding.

  • Detector architecture: 3,840 TES detectors with 249 µm absorber pitch operate at approximately 90 mK in the X-IFU focal-plane array.The array uses actively cooled X-ray absorbers thermally coupled to Transition Edge Sensors.
  • Detector architecture: An active cryogenic anti-coincidence system shields the detector array while the instrument is routinely calibrated.These design elements support achieving the derived X-IFU performance requirements.

3.2 Functional block diagram

The X-IFU combines cryogenic TES detection, multiplexed readout, filtering, calibration, anti-coincidence protection, and mechanical cooling to meet its top-level performance requirements.

  • Filtering and calibration: The filter wheel supports detector protection, optical-load reduction, bright-target throughput optimization, background measurements, and calibration functions.It may provide up to eight filter positions.
  • Filtering and calibration: On-board modulated X-ray sources correct energy-scale changes by illuminating the TES array with known-energy X-rays.Example source energies are 5.4 keV and 8.0 keV.
  • Detector and focal plane: 3840 TES pixels with 249 µm absorber pitch operate near 90 mK and are protected by an active cryogenic anti-coincidence system.The TES absorbers are thermally coupled to transition-edge sensors in a large-format array.
  • Readout electronics: The readout combines frequency-domain multiplexing, SQUID amplification, warm electronics, digital processing, and event processing across the detection chain.Each SQUID chain nominally operates 40 pixels, while the WFEE provides 96 channels.
  • Cryogenic chain: The reference cryogenic chain uses redundant mechanical coolers, a 4 K stage, Joule–Thomson cooling, and a hybrid sorption He3-ADR to reach approximately 50 mK.The chain is divided into shield-cooling and detector-cooling parts.
  • Performance requirements: Meeting 2.5 eV resolution at 7 keV, operation to 12 keV, and 1 mCrab count rates requires an approximately 161.5 dB DAC dynamic range, making optimization challenging.At 20 MHz sampling, this corresponds to an effective 15 bits.

3.3 The X-IFU mechanical design

The Mission Consolidation Review design illustrates the X-IFU’s substantial system size and complexity, while remaining subject to mass-budget and accommodation changes.

  • Current design: 860 kg including margins and approximately 2.3 kW in observation and regeneration modes were estimated for the X-IFU at the Mission Consolidation Review.The mass allocation was exceeded, making consolidation a priority.
  • Design evolution: The Dewar assembly and digital electronics were identified as the largest contributors to the mass budget and likely targets for redesign.Potential changes include shield number, cryostat compactness, and cooler count.
  • Design evolution: Accommodation optimization around Athena’s two instruments may distribute electronics boxes more widely and produce a less compact integrated design.The X-IFU is the driving component of the focal plane module accommodation.
  • Current design: The illustrated mechanical accommodation does not include potential optimization of electronic-box placement on the focal plane module.The figure therefore represents the May 2016 configuration rather than a finalized arrangement.

4. X-IFU CURRENT PERFORMANCE

The current X-IFU design approaches its spectral-resolution target and supports demanding velocity measurements, but effective area, background, gain stability, and bright-source throughput remain important constraints.

  • Effective area: The effective-area prediction uses the proposed mirror configuration, a 0.9 contingency factor, and 245 µm absorbers on a 249 µm pitch for comparison with Hitomi/SXS.A multilayer mirror coating was being studied to increase effective area around 4–7 keV.
  • Spectral resolution: 2.5 eV spectral resolution was being approached with frequency-domain multiplexed readout, while single-pixel measurements had already achieved the required resolution.The reported demonstrations are associated with SRON readout development.
  • Velocity measurements: At 7 keV, a 0.4 eV gain-calibration error corresponds to about 17 km/s systematic uncertainty in bulk velocity, while S/N 5 permits approximately 75 km/s turbulent velocities with 15 km/s error.These expectations assume 2.5 eV spectral resolution.
  • Calibration: Maintaining gain error below 0.4 eV at 7 keV requires bath-temperature deviations no greater than 1.5 mK from the 55 mK reference.Gain depends on bath temperature and the AC bias point.
  • Count-rate capability: The baseline grading scheme meets the 1 mCrab count-rate requirement but does not simultaneously meet the 1 Crab requirement for 30% throughput.The 1 Crab requirement remained under consolidation because it was considered very challenging.
  • Background: Instrumental-background compliance remains challenging because estimates depend on CryoAC and focal-plane design, material modeling, particle screening, and optical focalization efficiency.The current estimates retain substantial uncertainty.
  • Optical loading: Using thick optical-blocking filters enables observations of stars as bright as mV = 2 but reduces low-energy effective area.A thinner filter supports stars fainter than approximately mV 7.5 without significant energy-resolution degradation.

5. X-IFU PERFORMANCE OPTIMIZATION

Performance-optimization studies focus on improving count-rate capability within the existing resource envelope through hybrid pixel arrays and mirror defocusing, with potential secondary benefits for spectral resolution and instrument demands.

  • Small Pixel Array: A Small Pixel Array can improve count-rate capability by spreading the point-spread function across more pixels, while remaining within the allocated resource envelope.The proposed hybrid combines a high-count-rate central field with a lower-count-rate bulk array.
  • Small Pixel Array: SPA pixels are about twice as fast as LPA 1 pixels while preserving the same spectral resolution, whereas LPA 2 pixels are slower.The critically damped time constants are approximately 79 µs for SPA, 160 µs for LPA 1, and 286 µs for LPA 2.
  • Small Pixel Array: The SPA provides about an order-of-magnitude count-rate improvement over the baseline LPA 1 configuration.It can relax count-rate requirements on the bulk array and potentially reduce readout-chain or DAC demands.
  • Defocusing: A 35 mm mirror defocus improves count-rate capability by more than a factor of 10, including with the slower LPA 2 pixels.The option could enable one pixel design, but meaningful improvement requires at least 30 mm and the MMA currently assumes a 15 mm maximum.
  • Combined optimization: Combining an SPA with short defocusing could improve count-rate capability while avoiding dependence on large, externally controlled MMA defocusing distances.This combination is being studied as an alternative to adopting defocusing alone.

5.2 Spectral resolution

The X-IFU meets its spectral-resolution requirement through TES optimization, while proposed improvements must balance sensitivity gains against calibration and system-complexity costs.

  • 5.2 Spectral resolution: 2.5 eV below 7 keV is achieved through TES physical-parameter tuning, with intrinsic TES resolution of 1.8–1.9 eV before later degradations.Further degradations arise from finite record length, readout electronics, and cooling-system perturbations.
  • 5.2 Spectral resolution: Increasing spectral resolution could improve weak-line sensitivity and reduce systematics, but may require mixed TES settings, separate heat sinks, and dedicated processing chains.The paper specifically links this trade-off to WHIM-filament detection and resolving AGN-wind velocity and ionisation components.
  • 5.2 Spectral resolution: Different pixel sizes would complicate in-flight calibration because small and large pixels receive different photon rates during MXS pulses.MXS characteristics would need adjustment so calibration photons remain temporally separated, high-resolution, and free of cross-talk.
  • 5.2 Spectral resolution: Removing one of five filters could improve low-energy response but would exceed the spectral-resolution photon-shot-noise allocation.Interface specifications must be consolidated before filter removal is considered.
  • 5.2 Spectral resolution: A 50 nm Au filter could reduce secondary-electron background by about 20% but would dramatically reduce effective area below 1 keV.The benefit of lower background must therefore be weighed against low-energy response loss.

5.4 Pulse reconstruction

The paper evaluates pulse-reconstruction strategies and bright-source operating options to improve count-rate handling while preserving useful spectral performance and telemetry feasibility.

  • 5.4 Pulse reconstruction: Pulse-reconstruction optimization can improve both spectral resolution and count-rate capability beyond the baseline optimal-filtering method.The comparison considers optimal filtering, resistance-space analysis, and covariance-based analysis.
  • 5.4 Pulse reconstruction: 0.04 eV at 7 keV is the improvement from covariance-based analysis, but its demanding calibration requirements may prevent X-IFU adoption.Resistance-space analysis offers a smaller gain with lower implementation cost.
  • 5.4 Pulse reconstruction: 0.02 eV at 7 keV is the improvement from resistance-space analysis, which has limited computational cost, no added calibration needs, and robust high-count-rate performance.These properties make it the preferred alternative to optimal filtering among the compared methods.
  • 5.5 Pushing bright X-ray source observations: ∼10,000 cps/s with better than 3 eV resolution is projected for a defocussed LPA 2 when a 1 Crab source is reduced fivefold.Combining high- and medium-resolution events yields throughput above 50%.
  • 5.5 Pushing bright X-ray source observations: A gray filter could reduce bright-source flux by factors of 100–1000, helping keep observations within on-board processing and telemetry limits.Its filter-wheel implementation remains subject to mechanical and mass constraints.
  • 5.5 Pushing bright X-ray source observations: Pile-up and deadtime must be evaluated for bright sources because pile-up hardens spectra while paralyzable deadtime affects flux recovery and time-series power spectra.Simulations indicate spectral deviations begin when pile-up exceeds a few percent.

5.6 Field of view

Field-of-view enlargement could substantially improve imaging and WHIM studies, but compressed observing modes impose spacecraft-agility and operational-availability costs.

  • 5.6 Field of view: ∼5.8 arc minutes is the projected equivalent-diameter field of view from 300 µm pixels, the largest pitch judged compatible with 2.5 eV resolution.The pitch corresponds to 5.1 arc seconds on the sky.
  • 5.6 Field of view: 34% more imaging efficiency and 34% observing-time savings for mosaicing follow from the larger field of view.The larger field also offers 80% greater effectiveness for WHIM autocorrelation studies.
  • 5.6 Field of view: 80% greater effectiveness for WHIM autocorrelation studies is expected because the autocorrelation signal scales with the fourth power of field-of-view diameter.The larger field could also permit a 5 arc minute on-sky field while reserving 15% blind pixels for background monitoring.
  • 5.7 ToO efficiency: At a slew rate of 1 degree/minute, the compressed 18–12 hour X-IFU/WFI cycle would reduce operational availability by 16%, violating the 90% requirement.Meeting the requirement may require at least 3 degrees/minute and possibly thrusters.
  • 5.7 ToO efficiency: Segmenting observations increases slews, reaction-wheel off-loadings, and instrument switching cycles, thereby burdening spacecraft operational availability.The operational implications of the compressed mode require further assessment.
  • 5.7 ToO efficiency: ToO capability is a top-level Athena requirement for WHIM-filament detection, high-redshift GRB studies, and discovery science such as gravitational-wave follow-up.Its consolidation must be considered globally alongside effective area, mass, and cost constraints.

6. SHORT TERM PLANS AND CONCLUSIONS

The short-term plan focuses on consolidating the X-IFU design within spacecraft resource constraints while pursuing performance improvements and technology maturation.

  • 6. SHORT TERM PLANS AND CONCLUSIONS: Mass-budget consolidation and optimized focal-plane accommodation are the next priorities for preserving scientific performance within spacecraft constraints.The approach begins with critical reviews of thermal, mechanical, and electrical budgets.
  • 6. SHORT TERM PLANS AND CONCLUSIONS: Performance options including count-rate capability, spectral resolution, and low-energy response are being studied without increasing resource demands or complexity.Some options, such as the hybrid array, have system-level impacts requiring longer analysis.
  • 6. SHORT TERM PLANS AND CONCLUSIONS: Technology development is advancing the readiness of the TES array, focal-plane assembly, electronics, filters, aperture assembly, and cryo-coolers.Some activities converge on development of the Detector Cooling System.
  • 6. SHORT TERM PLANS AND CONCLUSIONS: Hitomi SXS results demonstrated the transformational potential of high-resolution X-ray spectroscopy while highlighting the challenge of building the X-IFU.The X-IFU consortium includes key members of the SXS team.
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