![]() We also study the propagation of errors in the PSF to the measurement of galaxy shapes. The latter enables us to propose the first method capable of building a polychromatic PSF model, using no information other than undersampled star images, their position and spectra. These rely on sparsity and numerical optimal transport. In addition, because of the very wide band of its visible instrument, variations of the PSF with the wavelength of incoming light will also need to be accounted for.The main contribution of this thesis is the building of novel PSF modelling approaches. The PSF model will thus need to perform a super-resolution step. In the case of Euclid, star images will suffer from undersampling. Unresolved stars in the field provide a measurement of the PSF at given positions, from which a PSF model can be built. ![]() This, in turn, requires precise knowledge of the PSF itself.The PSF varies depending on the position of objects within the instrument’s focal plane. The images obtained by any optical instrument are altered by its Point Spread Function (PSF), caused by various effects: diffraction, imperfect optics, atmospheric turbulence (for ground-based telescopes)… Since the PSF also alters galaxy shapes, it is crucial to correct for it when performing weak lensing measurements. The European Space Agency’s upcoming Euclid mission is a spaceborne telescope with weak lensing as one of its primary science objectives.In practice, the weak lensing signal is recovered from the measurement of the shapes of galaxies. This makes it a powerful source of cosmological insight, and can in particular be used to study the distribution of dark matter and the nature of Dark Energy. Measuring this effect, called weak gravitational lensing, allows us to probe the large scale structure of the Universe. This causes a distortion of the images of distant galaxies. However, the mixing of Chandra imaging and spectroscopic data resulted on rare artifacts present on flux mosaics and loss on spectroscopic resolution preventing from doing strong asseverations regarding flaring periods on the seen variability.Īs light propagates through the Universe, its path is altered by the presence of massive objects. From the analyzed spectra, X-ray emission variability is again evident. Such behavior is coincident with the interpretation of the Fe Kα line emission as reprocessed, fluorescence, emission on gas clouds surrounding Sgr A⋆. Specifically, spectra corresponding to the years 20 showed an interesting characteristic of a delay on the variability of the background emission and the Fe Kα line emission. The spectra were extracted from the region in the immediate vicinity of Sgr A⋆. This line is an important probe of high-energy phenomena and it is a prevalent feature of different galactic center emissions. Additionally, spectral analysis was performed to model the Galactic Center emission and characterize it with the Fe Kα line variability. Such variability is better visualized if the three flux maps are put together on an RGB map which showed complex variations within molecular clouds. Particularly, the flux maps constructed on three different periods showed similar structures of molecular clouds found on previous works and the X-ray emission variability is evident as illumination patterns on the maps. Specifically, flux maps of the galactic central region were constructed to see the emission variability and its correlation to greater structures such as molecular clouds. Extensive observations towards the Galactic Center have revealed a rather faint emission (∼ 1039 erg/s) coming from Sgr A⋆ with a particular high variability on the X-ray band showing flaring activity. Sgr A⋆ and the Galactic Center are the best laboratories to study galactic nuclei high-energy phenomena as it is the closest (∼ 8 kpc) galactic nucleus allowing high-resolution observation runs. Sgr A⋆ is a compact radio source at the center of the Milky Way with strong evidence to be a supermassive black hole with a mass of about ∼ 4 × 106 M⊙. This work presents an X-ray study of the Sagittarius A⋆ (Sgr A⋆) emission variability on Chandra observations data during the years 2001 to 2017.
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