The COSPIX mission: focusing on the energetic and obscured Universe

Tracing the formation and evolution of all supermassive black holes, including the obscured ones, understanding how black holes influence their surroundings and how matter behaves under extreme conditions, are recognized as key science objectives to be addressed by the next generation of instruments. These are the main goals of the COSPIX proposal, made to ESA in December 2010 in the context of its call for selection of the M3 mission. In addition, COSPIX, will also provide key measurements on the non thermal Universe, particularly in relation to the question of the acceleration of particles, as well as on many other fundamental questions as for example the energetic particle content of clusters of galaxies. COSPIX is proposed as an observatory operating from 0.3 to more than 100 keV. The payload features a single long focal length focusing telescope offering an effective area close to ten times larger than any scheduled focusing mission at 30 keV, an angular resolution better than 20 arcseconds in hard X-rays, and polarimetric capabilities within the same focal plane instrumentation. In this paper, we describe the science objectives of the mission, its baseline design, and its performances, as proposed to ESA.

💡 Research Summary

The paper presents the COSPIX mission concept, a proposed ESA M‑class observatory designed to address fundamental questions about supermassive black holes (SMBHs), the energetic and obscured Universe, and non‑thermal processes in astrophysics. The authors argue that current hard X‑ray facilities (e.g., INTEGRAL/IBIS) lack the sensitivity and angular resolution needed to resolve the Cosmic X‑ray Background (CXB) peak at ~30 keV, while upcoming missions such as NuSTAR and Astro‑H, although employing focusing optics, will still be limited to modest effective areas (~300 cm² at 30 keV) and angular resolutions of 45″–100″. COSPIX is conceived to overcome these limitations by delivering an order‑of‑magnitude larger collecting area (≈2000 cm² at 30 keV), sub‑20″ half‑energy width (HEW) imaging, and polarimetric capability within the same focal plane.

Science objectives are clearly enumerated: (1) resolve >70 % of the CXB at its peak, thereby uncovering the population of heavily obscured AGN that dominate the background; (2) obtain high‑quality broadband spectra (0.3–100 keV) for all classes of AGN and Galactic black holes, enabling time‑resolved spectroscopy on dynamical timescales and reverberation mapping; (3) study the present and historic activity of Sgr A* through high‑resolution imaging of flares and reflected emission from nearby molecular clouds; (4) measure black‑hole spin distributions for a large sample of SMBH and stellar‑mass black holes via Fe‑K line profiling; (5) investigate particle acceleration and non‑thermal emission in jets, clusters, supernova remnants, and colliding‑wind binaries, with the added dimension of hard‑X‑ray polarimetry to discriminate synchrotron versus inverse‑Compton processes.

To meet these goals, the mission adopts a single, long‑focal‑length (33 m) Wolter‑I telescope combining Silicon Pore Optics (SPO) for the outer shells (50–112 cm diameter) and Slumped‑Glass Optics (SGO) for the inner shells (down to 10 cm). Multilayer coatings extend reflectivity up to ~75 keV. The resulting effective area exceeds 3500 cm² at 10 keV, ~1800 cm² at 30 keV, and ~600 cm² at 75 keV, while maintaining a mass of ~394 kg. The focal plane consists of two superimposed detectors: the Low‑Energy Spectro‑Imager (LESI) covering 0.3–40 keV, based on a monolithic DEPFET array (32 k pixels, 520 µm pitch), and the High‑Energy Spectro‑Imager (HESI) covering 8–250 keV, built from an 8 × 8 array of Caliste CdTe modules (2 mm thick, 780 µm pitch). The LESI front‑side pixelated mask enables Compton polarimetry when photons scatter in LESI and are absorbed in HESI. Both detectors operate at modest cooling (−50 °C and −40 °C) and achieve the required spectral resolutions (≤150 eV at 6 keV, ≤1 keV at 68 keV). An active anti‑coincidence shield, a 3.1 m collimator, and a 3.5 m sky shield suppress particle and stray‑light backgrounds, allowing a minimum detectable polarization of <0.7 % (20–40 keV, 100 mCrab, 100 ks).

The mission architecture relies on formation‑flight of two spacecraft: one carrying the mirror module, the other the detector assembly, separated by the 33 m focal length. Metrology and micro‑propulsion systems, inherited from the Simbol‑X study, provide the required alignment and stability. An L2 halo orbit is selected for thermal stability and continuous sky access, avoiding the radiation belts encountered in high‑elliptical orbits. The total launch mass (≈2.06 t) fits within a Soyuz launch vehicle capability. Data downlink uses a 15 m X‑band antenna at 1.6 Mbps for ~4 h per day, sufficient for a 1 Crab source without compression.

Performance simulations show that COSPIX’s point‑source continuum sensitivity surpasses all existing and planned missions, reaching ≈2 × 10⁻¹⁵ erg cm⁻² s⁻¹ (10–40 keV, 1 Ms, 3σ). The polarimetric sensitivity enables detection of polarization fractions down to 0.7 % for a 100 mCrab source in 100 ks, opening a new observational window on high‑energy emission mechanisms. The angular resolution (<20″) ensures source confusion is negligible even at the deepest flux levels, crucial for resolving the CXB.

In summary, COSPIX offers a transformative step in hard X‑ray astronomy: a large‑area, high‑resolution, polarimetric focusing telescope that will uncover the hidden SMBH population, map the physics of accretion and ejection across the mass scale, and probe particle acceleration in a variety of astrophysical environments. Its design leverages mature technologies (SPO, SGO, DEPFET, Caliste) and a proven formation‑flight concept, making it a realistic and compelling candidate for ESA’s next medium‑class mission.