University of Applied Science of North-West Switzerland
X-ray imaging with space detectors based on the moiré effect
X-ray astronomy is a powerful source of information about astrophysical sources, as X-rays are emitted by accelerated electrons and excited ions. Hence X-rays give us indirect information about the morphology and power of the source. In addition, the continuum emission from accelerated electrons reveals the characteristics of important aspects like the magnetic field configuration in stellar and compact sources, the expanding shells in supernova remnants, the accretion disk of a black hole, the jet morphology in active galactic nuclei, etc. Focusing X-ray optics (e.g by grazing incidence mirrors) is only feasible with long (10-20 m) focal distances. This requires either very long satellites, or formation flights in which the focusing optics is installed on a separate satellite from the one hosting the photon detectors, with the need of keeping very accurate and stable alignment and distance. Hence, space instruments based on focusing X-ray optics are extremely expensive. A simpler alternative is to use pairs of absorbing gratings made of high-Z matter, projecting "shadows" forming characteristic moiré patterns. The image is then reconstructed mathematically, starting from the distribution of photons recorded by detectors with coarse (or no) pixelization. This technique allows to build compact instruments achieving a very good resolution (of few arcseconds), with the drawback of a reduced dynamic range (two nearby sources are both visible only if one is not fainter than 1-2 orders of magnitude). The operating space detector with the best resolution is RHESSI. A new detector (STIX) is in construction, which will be installed on the ESA Solar Orbiter mission (to be launched in 2017). STIX will achieve similar performance as RHESSI with a more compact design. In addition, a new technique promises to make it possible to build gratings with period at the micron level, allowing for X-ray imagers which can fit a small satellite. Work is in progress to develop an X-ray imager which can be flown on a nanosatellite.