The majority of the ordinary matter in the local Universe has been heated by strong structure formation shocks and resides in a largely unexplored hot, diffuse, X-ray emitting plasma that permeates the halos of galaxies, galaxy groups and clusters, and the cosmic web. We propose a next-generation "Cosmic Web Explorer" that will permit a complete and exhaustive understanding of these unseen baryons. This will be the first mission capable to reach the accretion shocks located several times farther than the virial radii of galaxy clusters, and reveal the out-of-equilibrium parts of the intra-cluster medium which are live witnesses to the physics of cosmic accretion. It will also enable a view of the thermodynamics, kinematics, and chemical composition of the circumgalactic medium in galaxies with masses similar to the Milky Way, at the same level of detail that $Athena$ will unravel for the virialized regions of massive galaxy clusters, delivering a transformative understanding of the evolution of those galaxies in which most of the stars and metals in the Universe were formed. Finally, the proposed X-ray satellite will connect the dots of the large-scale structure by mapping, at high spectral resolution, as much as 100% of the diffuse gas hotter than $10^6$ K that fills the filaments of the cosmic web at low redshifts, down to an over-density of 1, both in emission and in absorption against the ubiquitous cosmic X-ray background, surveying at least 1600 square degrees over 5 years in orbit. This requires a large effective area (~10 m$^2$ at 1 keV) over a large field of view ($\sim1$ deg$^2$), a megapixel cryogenic microcalorimeter array providing integral field spectroscopy with a resolving power $E/\Delta E$ = 2000 at 0.6 keV and a spatial resolution of 5 arcsec in the soft X-ray band, and a low and stable instrumental background ensuring high sensitivity to faint, extended emission.

Voyage through the Hidden Physics of the Cosmic Web.

X-ray Wolter focusing telescopes concentrate the light by means of reflection on smooth surfaces at small grazing
angle (below a couple of degrees). The traditional coatings for these kind of applications are heavy materials
that, due to their high density, present a high critical energy for total reflection. Recent works have shown how a
thin layer of a light material, like carbon, on top of a traditional reflecting coating, can enhance the reflectivity in
soft x-ray spectral region (below 5 keV), without degrading the performances for higher energies. We presented
at SPIE 2007 some experimental results about the reflectivity measurement at very low energies (200 eV) and
rather large angles (1-2 deg). In the present work we extend the former study, by the realization of a new set of
samples with coatings made of different materials (Pt, Au, W, Ir) and the measurement of their reflectivity for
the typical angles (< 1°) and energies (1-10 keV) employed in astronomical grazing incidence telescopes.

New developments in light material overcoating for soft x-ray reflectivity enhancement

ATHENA Silicon Pore Optics (SPO) Mirror Modules (MM) have to be tested and accepted prior to integration in the full ATHENA Mirror Assembly (MA). X-ray tests of the MMs are currently performed at the PTB laboratory of the BESSY synchrotron facility in pencil beam configuration, but they require a PSF reconstruction. Full illumination X-ray tests could be performed using a broad, low-divergent X-ray beam like the one in use at PANTER (MPE, Neuried, Germany), but the large volume to be evacuated makes it impossible to perform the functional tests at the MMs production rate (3 MM/day).
 
To overcome these limitations, we started in 2012 to design a facility aimed at generating a broad (170 x 60 mm2), uniform and low-divergent (1.5 arcsec HEW) X-ray beam within a small lab (∼ 9 x 18 m2), to characterize the ATHENA MM. BEaTriX (the Beam Expander Testing X-ray facility) makes use of an X-ray microfocus source, a paraboloidal mirror, a crystal monochromation system, and an asymmetrically-cut diffracting crystal for the beam expansion. These optical components, in addition to a modular low-vacuum level (10-3 mbar), enable to match the ATHENA SPO acceptance requirements.
 
The realization of this facility at INAF-OAB in Merate (Italy) is now on going. Once completed, BEaTriX can be used to test the Silicon Pore Optics modules of the ATHENA X-ray observatory, as well as other optics, like the ones of the Arcus mission. In this paper we report the advancement status of the facility.

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11180/1118026/BEaTriX-Beam-Expander-Testing-X-ray-facility-for-testing-ATHENAs/10.1117/12.2535997.pdf

BEaTriX (Beam Expander Testing X-ray facility) for testing ATHENA's SPO modules: advancement status

The construction of the BEaTriX (Beam Expander Testing X-ray) facility is ongoing at INAF/Osservatorio astronomico di Brera. The facility will generate a broad (170 x 60 mm 2 ), uniform and low-divergent (1.5 arcsec HEW) X-ray beam within a small lab (∼ 9 x 18 m 2 ), using an X-ray microfocus source, a paraboloidal mirror, a monochromation system based on a combination of symmetrically cut and asymmetrically-cut crystals in Bragg diffraction configuration. Once completed, BEaTriX can be used to test the Silicon Pore Optics modules of the ATHENA X-ray observatory, as well as other optics, like the ones of the Arcus mission. The facility is designed to operate at 1.49 keV and 4.51 keV, by using two fixed beam lines, equipped with the necessary optical elements. The first beam line to be completed will be at 4.51 keV and will prove the BEaTriX concept. Silicon crystals are used at this energy and four symmetric diffractions, with appropriate tilt of some crystals, will provide the spectral filtering at the required level to return the desired divergence. Owing to the quite short range necessary to obtain a parallel beam with this setup, a low vacuum level (10-3 mbar) can be used without a significant beam extinction. In addition to a modular vacuum approach, the low vacuum will allow us to reduce the time required to evacuate the tank, thus enabling to demonstrate a test rate that will match the ATHENA SPO production of 3 MM/day. In this paper, we report the design of the facility and the construction progress.

Progress in the realization of the beam expander testing x-ray facility (BEaTriX) for testing ATHENA's SPO modules

Low density overcoatings (mainly based on materials containing Carbon) onto usual high-density coatings (based i.e. on materials like e.g. like Ir, Au or Pt) have been proposed since many years ago in order to enhance the X- ray reflectivity at low energy (between 0.5 and 4 keV) of X-ray astronomical optics. The trick is to make use of the total reflection from the thin low-density material (which does not suffer much the photoelectric absorption) at low X-ray energies; the reflection of photons at higher energies (< 4 keV) occurs thanks to the much denser material under the overcoating. For several future projects, like e.g. ATHENA, LYNX and eXTP, it is foreseen the use of low-density overcoatings that will importantly increase the effective area at low X-ray energies. In this paper we will introduce the use of overcoatings based on materials different from the usual ones considered so far like C, B4C and SiC. In particular, we will discuss about a novel approach based on the use of thin layer of a Carbon-like materials deposited using a dip coating method. A possible combination with an intermediate thin layer of Chromium deposited e.g. via sputtering onto the usual high density material (Ir, Au or Pt) before the application of the Carbon-like material is also considered in the study, because it can further greatly enhance the soft X-ray effective area of future X-ray telescopes.

Soft x-ray reflectivity-enhancement in astronomical telescopes via overcoatings: alternative materials and deposition methods

In X-ray astronomical telescopes, the focalization of the radiation is achieved by means of grazing incidence Wolter I
(parabola + hyperbola) optics in total reflection regime. In general, high density materials (e.g. Au, Pt, Ir, W) are used as
reflecting coatings, in order to increase as much as possible the cut-off angles and energies for total reflection. However
these materials present an important reduction of the reflectivity between 0.2 and 5 keV, due to the photoabsorption, and
this phenomenon is particularly enhanced in correspondence of the M absorption edges (between 2 and 3.5 keV). In
general, this determines a strong decrease of the telescope effective area. To overcome the problem we suggested in
previous works the coating of the mirror surface by a low-density material such as carbon. Mirror samples with different
coatings made by high density materials: Au, Ir, Pt, and W with a carbon overcoating were manufactured and reflectivity
data in the soft X-ray band (100-2000 eV), performed both at the XACT facility in Palermo (Italy) and at BSRF
synchrotron in Beijing (China), are showed. In this paper we present some of the first results concerning the
measurements carried out at the photon energies of 200 eV (i.e. below the carbon K absorption edge) and 1280 eV (i.e.
the region just below the heavy material M absorption edge).

L. Zheng

Papers

Carbon overcoatings for soft x-ray reflectivity enhancement