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The present work consists in the analyse of X-ray flares from the supermassive black hole Sgr A* at the centre of the Milky Way. To do that, the data from three space telescopes (XMM-Newton, Chandra, Swift) have been reduced to get the light curves of the observations of Sgr A* done during the time period 2016-2018. Once the curves obtained, it has been possible to detect the flares. Then, we gather the results with the ones corresponding to the period 1999-2015 to compute the intrinsic distribution of flares in terms of fluxes and durations. This allows us to correct the results from the instrumental bias. Then, we determined the variations of flaring rate considering first, the more and less luminous flares and then, the more and less energetic ones. These variations have been compared with mechanisms proposed to explain the formation of flares. Finally, a spectral analyse of the flares have been done to compare with the literature. Le travail présenté consiste en l’analyse des éruptions en rayons X issues du disque d’accrétion autour du trou noir supermassif Sgr A* situé au centre de la Voie Lactée. Pour cela, les données de trois télescopes spatiaux (XMM-Newton, Chandra, Swift) on été réduites pour obtenir les courbes de lumière des observations de Sgr A* durant la période 2016-2018. Une fois les courbes obtenues, nous avons pu détecter les éruptions. En regroupant les données avec celles de la période 1999-2015, il a été possible de remonter à la distribution intrinsèque des éruptions en termes de flux et de durée. Cela permet de corriger les résultats des biais liés aux instruments utilisés. Puis, nous avons déterminé les variations des taux d’éruptions en considérant les plus lumineuses et les moins lumineuses puis les plus énergétiques et moins énergétiques. Nous avons ensuite comparé les variations avec des mécanismes proposés pour expliquer la formation des éruptions. Enfin, une analyse spectrale des éruptions a été réalisée puis comparée à la littérature.

Galaxy: centre --- X-rays: individuals: Sgr A* --- X-rays: general --- methods: data analysis --- black hole --- accretion --- Galaxie : center --- rayons X : Sgr A* --- rayons X : général --- méthodes : analyse de données --- trou noir --- accrétion --- Physique, chimie, mathématiques & sciences de la terre > Aérospatiale, astronomie & astrophysique

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When lensed AGNs are discovered, the primary focus is usually on their lensing properties
rather than their physical properties. This Master thesis aims at finding new estimates of the
physical properties of a sample of ten gravitationally lensed quasars using a uniform method based on spectral modelling and scaling relations. The sample is comprised of doubly and quadruply lensed quasars at redshifts roughly between 1.5 and 2.6 showing various spectral properties. The physical properties computed here are the black hole mass, the Eddington ratio and the size of the BLR. The latter allows an estimation of the sensitivity of the source to the microlensing effect assuming a theoretical lens mass. 
The basis on which the determination of the physical properties rests is the optical/UV
spectra obtained from various instruments such as the MUSE (Multi-Unit Spectroscopic Explorer)
or the FORS (FOcal Reducer and low dispersion Spectrograph) instruments of the VLT. The
methodology can be summarized as follows. The first step is the modelling of the considered
spectrum and the identification of known broad and narrow emission lines. Secondly, the black
hole mass estimate and the size of the BLR are computed using known scaling relations based
on the virial theorem. These involve the calibrated intrinsic luminosity of the continuum either
at 1450 or 3000 Å depending on the chosen emission line, either C IV at 1549Å or Mg II at 2798
Å. Finally, the accretion efficiency is evaluated by the ratio between the Eddington luminosity,
which is the upper limit on the luminosity an AGN can theoretically have, and the bolometric
luminosity. The sensitivity to the microlensing effect is then evaluated by the ratio of the size of
the Broad Line Region to the projected Einstein radius of the microlens. The uncertainties of each of the physical quantity are also computed. Finally, a discussion about the implications of this method and its limitation is given.

Physique, chimie, mathématiques & sciences de la terre > Aérospatiale, astronomie & astrophysique