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Institut für Astronomie und Astrophysik

Abteilung Astronomie

Sand 1, D-72076 Tübingen, Germany
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Dissertation Roberta Amato

Contribution of Pseudo-Focused Soft Protons to the Background of ATHENA

This PhD thesis explores the role of low energy protons, the so-called ‘soft protons’, as a component of the background in view of the future ESA’s X-ray mission Athena. As a matter of fact, a high level of soft proton flux at the focal plane of Athena can adversely affect the scientific goals of the mission. To prevent this, a correct estimate of the soft proton flux expected at the focal plane of the satellite is fundamental. Such an estimate can be achieved only if the reflectivity of soft protons from the optics is well understood, with efforts on both the experimental and the theoretical sides.
To this aim, I applied the model of reflectivity of particles at grazing incidence proposed by Remizovich et al. (1980), under the non-elastic approximation, to the experimental measurements of proton scattering at low incident angles from XMM-Newton and eROSITA mirror samples. The mismatch between the model and the experimental data led me to create a new analytical semi-empirical model, where the parameter s enclosing the micro-physics of the interaction between the protons and the mirror lattice is directly derived by fitting the data. This new model gives a more accurate estimate of the scattering efficiency and energy loss distributions, but depends on the specific materials eROSITA and XMM-Newton are made of. For the model to be applied to Athena, new experimental data on Athena’s optics, the Silicon Pore Optics (SPO), are necessary.
These new data were acquired during dedicated experimental campaigns carried out by the Institut für Astronomie und Astrophysik of the University of Tübingen. The experiment consisted in measuring the scattering of low energy protons at grazing incidence from an Athena SPO sample, at two different incident energies, ~ 470 keV and ~ 170 keV, and at four different incident angles, 0.6°, 0.8°, 1.0°, 1.2°. The new data are consistent, within the error bars, with the data from the eROSITA mirror sample, so that the same model can be used to estimate the scattering efficiency of SPO. A more accurate model can be built from a fit of the new data sets, provided that energy loss measurements are retrieved from the raw data.
The new semi-empirical model can be implemented in a ray-tracing code to build a specific response matrix for protons. The construction of a proton response matrix is a 2-years project that falls within the AHEAD2020 activities, in view of the launch of Athena. The project foresees the construction of a proton response matrix for XMM-Newton as a reliable tool for the deconvolution of observed soft protons spectra. If the validation of this response matrix is successful, then the same procedure can be used to produce an analogous proton response matrix for Athena. In this framework, I performed a Geant4 simulation of the interaction of soft protons with the focal plane of XMM-Newton, consisting in a detailed representation of the 7 CCDs of the MOS camera, the filters, and the proton shields surrounding the focal plane assembly. The coupling of the Geant4 simulation with the output of the aforementioned ray-tracing will bring to the proton response matrix for XMM-Newton.
To reach a round research profile, I also analysed observational X-ray data from two binary X-ray sources, which represent ideal cases to test to what extent soft protons can affect the quality of observational data. The low level of background required for Athena will improve the knowledge we have of these systems and will enhance advanced studies for a wider sample of X-ray binaries.
The first source is a very-faint millisecond pulsar in the globular cluster M22, for which I conducted a multi-wavelength search for counterparts. The lack of any optical counterpart returned an upper limit on the mass of the companion, allowing for a classification of the system as a so-called black widow binary, i.e., a low-mass X-ray binary with a companion star of mass M « 0.1M_sun. The analysis of the X-ray spectra favoured an intra-binary shock scenario as mechanism responsible for the X-ray emission.
The second source is the well-known high-mass X-ray binary Vela X-1, for which I performed a high-resolution spectroscopy study of a Chandra/HETGS archival data, taken when the line of sight is intersecting the photoionisation wake. Standard plasma diagnostic techniques and simulations with the photoionisation codes CLOUDY and PION (in SPEX) suggested the presence of a multi-component plasma, which is typical for high-mass X-ray binaries with clumpy winds.

Dissertation (8.34 MB PDF file including figures)

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