X-IFU defocused

The X-ray Integral Field Unit (X-IFU) is the high resolution X-ray spectrometer that is planned for launch on board the European Space Agency Athena X-ray observatory. The instrument will deliver X-ray spectra in the 0.2-12 keV range with a spectral resolution of 2.5 eV up to 7 keV. The overall field of view of 5' equivalent diameter will be subdivided into several thousands of ~5" pixels.  The main scientific goals of the instrument are  to study the dynamical, physical, and chemical properties of hot plasmas, notably those found in galaxy clusters, and to study the extreme environment around Galactic stellar-mass and supermassive black holes, accretion discs, jets, outflows, and winds. In addition, the unprecedented capabilities of the X-IFU will enable the study of many Athena observatory science targets, such as planets, stars, supernovæ, compact objects, and the interstellar medium. 

The defocused observations of X-IFU

The capabilities of X-IFU will be improved by using the defocusing option offered by the Movable Mirror Assembly (MMA). A defocusing of 35 mm (with respect to the nominal focal length of 12 m) will enable the observations of extremely bright Galactic sources with fluxes up to ~1 Crab, with only limited spectral resolution degradation. In this case the point spread function (PSF) of the telescope will be spread over a large number of pixels so that each pixel receives a small fraction of the overall flux.

Due to the energy dependence of the PSF shape, this mode will generate energy-dependent artefacts increasing with count rate if not analysed properly. I developed a new technique that enables the analysis of defocused observations in which the PSF varies as a function of energy. I applied this method to the Athena/X-IFU observations of bright sources, operated out of focus. I  tested the method for different flux levels, exposure times, and spectral shapes. 

Read more here: Kammoun et al. (2022)