2014A&A...569A..54L

Observations in the X-ray energy band are often limited by the background because of the low fluxes of typical sources. The background can easily be higher than the signal itself, and thus any mission with the scientific goal of observing faint and/or extended sources in the X-ray band must deal with the background problem. ESA has recently selected ``the hot and energetic universe'' as science theme for the second large-class mission in the Cosmic Vision science program, to be pursued with an advanced X-ray observatory to be launched in 2028, and at present ATHENA is the proposal that is most likely to be selected for this slot. The mission is aimed to place an X-ray telescope in the L2 orbit equipped with an X-ray Integral Field Unit (X-IFU) based on high spectral resolution transition-edge microcalorimeters, and has among its goals the detection and characterization of high-redshift active galactic nuclei (AGNs), cluster of galaxies and their outskirts, which is why great care must be taken to reduce the background impact on the detection/characterization of these sources. The background is composed of a diffuse component and an internal particle component for any satellite operating in the X-ray band. We take as reference the X-ray IFU instrument that will be placed onboard the ATHENA mission to analyze both these components and their variability for different orbits, observational conditions and/or design choices. We also show how different background levels affect the instrumental performance, and the scientific results obtainable with the instrument in the best configuration. The X-IFU is a cryogenic X-ray spectrometer, based on a large array of 3840 transition-edge sensors (TES) of 250µm side, with a spectral energy resolution of 2.5eV in the 0.2-10keV energy band, over a field of view of 5x5arcmin², high count rate capability and a 5-arcsec angular resolution. There are no experimental data about the background experienced by microcalorimeters in the L2 orbit, and thus the particle background levels were calculated by means of Monte Carlo simulations: we considered the original design configuration and an improved configuration aimed to reduce the unrejected background, and tested them in the L2 orbit and in the low Earth orbit, comparing the results with experimental data reported by other X-ray instruments. For the diffuse component, we used the background levels measured from a 1 sr region representative of typical high galactic latitude pointings and analyzed the variations expected with the different orbital conditions. To show the results obtainable with the improved configuration we simulated the observation of a faint, high-redshift, point source (F_0.5–10keV∼6.4x10^–16erg/cm2/s, z=3.7), and of a hot galaxy cluster at R₂₀₀ (S_b0.5–2keV=8.61x10^–16erg/cm2/s/arcmin2, T=6.6keV). First we confirm that implementing an active cryogenic anticoincidence reduces the particle background by an order of magnitude and brings it close to the required level. The implementation and test of several design solutions can reduce the particle background level by a further factor of 6 with respect to the original configuration. The residual background is dominated by secondary particles, and this component can be decreased by design solutions such as passive shielding with appropriate materials. The best background level achievable in the L2 orbit with the implementation of ad-hoc passive shielding for secondary particles is similar to that measured in the more favorable LEO environment without the passive shielding, allowing us to exploit the advantages of the L2 orbit. We define a reference model for the diffuse background and collect all the available information on its variation with epoch and pointing direction. With this background level the ATHENA mission with the X-IFU instrument is able to detect ∼4100 new obscured AGNs with F>6.4x10^–16erg/cm2/s during three years, to characterize cluster of galaxies with S_b(0.5-2keV)>9.4x10^–16erg/cm2/s/sr on timescales of 50ks (500ks) with errors ≲40% (≲12%) on metallicity, ≲16% (∼4.8%) on temperature, ∼2.6% (∼0.72%) on the gas density, and several single-element abundances. With respect to the original design the background level obtained significantly enhances the performance of the X-IFU. It allows us to improve the times needed for the detection and the characterization of high-redshift and/or faint Compton-thick AGNs and to reduce the limit flux of X-IFU, and thus to have a significant measurement of the fraction of these objects where current surveys are missing data. Moreover, the reduced background allows an improved characterization of diffuse and/or faint sources in terms of relative errors on the physical properties of the sources and observational times needed to obtain them.