Performance estimates and design characterization of space-based instruments for X-ray astrophysics

In X-ray astronomy we can collect informations from five different observables: intensity, source location, time variability, spectrum and polarization. In this work I will deal with the latest two, analyzing the performance of two different focal plane instruments: the X-ray Microcalorimeter Spectrometer (XMS) of the Athena mission, and the CdZnTe array of the NuSTAR mission. Both instruments are imaging cameras capable of non-dispersive spectroscopy in two different energy bands: XMS is capable of 2-3 eV resolution between 0.2-10 keV, while NuSTAR operates in the 3-80 keV energy band with resolution up to 0.5 keV. XMS is a microcalorimeters array capable of high resolution spectroscopy with imaging capability on a broad class of sources. It will actually provide the first Integral Field Spectrometer (IFS) in the X-ray domain. Some of the most important science cases that will be covered with XMS observations include extended sources like galaxy clusters and the Warm Hot Intergalactic Medium, to faint sources like high redshift Active Galactic Nuclei, and the instrument will be able to characterize these objects in terms of temperature, density, chemical composition, velocity and redshift. In this respect, having to deal with extremely low fluxes, it is mandatory to have an efficient background rejection system to increase the detector performances in terms of minimum detectable flux and signal to noise ratio. These detectors have never been placed in the L2 orbit, and consequently there are no experimental data on the background to be expected during the operative phase. In this thesis I will present analytical estimates and Montecarlo simulations, obtained with the Geant4 software, aimed to predict the background level to be expected and to find solutions to reduce it to a level actually much better than the scientific requirement of the mission. These estimates will be used to simulate the observations of some of the scientific targets of the mission, enlightening the background role and the effectiveness of the solutions found to reduce it. The NuSTAR mission, flown on 13 June 2012, does not foresee a detector dedicated to polarimetry. The only focal plane detector are two arrays of CZT, placed in the focus of two multilayer optics capable for the first time to focalize photons up to 80 keV. However the detector features allow to exploit the Compton effect to determine the polarization degree of the incident radiation, and the high effective area supplies the high photon fluxes necessary for these measurements. In this thesis the minimum detectable polarization for the instrument has been evaluated with a view to develop a dedicated polarimetric read-out mode, along with the effect of the background in Low Earth Orbit on this kind of measurements.