Thèse doctorat : Astrophysique : Strasbourg 1 : 2006. / Thèse soutenue sur un ensemble de travaux. Titre provenant de l'écran-titre. Bibliogr. 8 p.

Sgr A*, le trou noir supermassif le plus proche de nous, émet une luminosité quiescente très faible ainsi que des éruptions en infrarouge proche (NIR), rayons X et radio. Cette thèse a pour but d'étudier l'effet du passage de DSO/G2 près de Sgr A* sur les éruptions. J'ai utilisé et amélioré trois méthodes pour l'étude en rayons X : les blocs Bayésiens en deux passes pour détecter les éruptions avec une certaine probabilité, le lissage des courbes de lumières pour diminuer le bruit de Poisson et la méthode de Monte Carlo par chaînes de Markov pour l'ajustement des spectres des éruptions. J'ai contraint les paramètres physiques de la source pour une des 3 éruptions détectées en rayons X en 2011 et pour 3 éruptions détectées en rayons X et NIR durant la campagne multi-longueurs d'onde de février-avril 2014. L'activité en rayons X et NIR de février-avril 2014 correspond à celle observée avant le passage de DSO/G2 près de Sgr A*. J'ai calculé le taux d'éruption intrinsèque en rayons X de Sgr A* en 1999-2015 et détecté une plus faible activité à partir du 28 octobre 2013. L'énergie stockée pendant cette période peut expliquer la plus forte activité observée du 30 août au 9 septembre 2014. / Sgr A*, the closest supermassive black hole, is an extremely low luminosity black hole emitting flares in near-infrared (NIR), X-rays and radio. The goal of this Ph.D. is to study the impact of the pericenter passage of the Dusty S-cluster Object DSO/G2 close to Sgr A* on the flaring activity. I used and improved three methods for the study in X-rays: the two-steps Bayesian blocks method to detect flares with a given false detection probability, the light curve smoothing to reduce the Poisson noise and the Monte Carlo Markov chains method for the fitting of the flare spectra. I constrained the physical parameters of the flaring region for one of the three X-ray flares detected in 2011 and for three NIR/X-ray flares detected during the 2014 Feb.-Apr. multiwavelength campaign. The X-ray and NIR activity during the 2014 Feb.-Apr. is not different from those observed before the DSO/G2 pericenter passage. I computed the intrinsic flaring rate in X-rays from Sgr A* in 1999-2015 and I detected a smaller flaring activity beginning on 2013 Oct. 28. The energy saved during this time period could explain the largest activity observed from 2014 Aug. 30 to Sept. 9.

<p>This thesis investigates the X-ray properties of the entire Stanghellini et al. (1998) complete sample of GHz Peaked Spectrum galaxies with redshift lower than 1. In total 19 sources are included mainly from observations made by the European space telescope, XMM-Newton. Out of these the analysis of seven "new" observations made between 2006 and 2008 are throughout described. Data from the new observations shows consistency with already analysed data. As a new result a tentative discovery of a bi-modal structure in the X-ray to radio luminosity ratio within the sample is presented.</p>

This thesis investigates the X-ray properties of the entire Stanghellini et al. (1998) complete sample of GHz Peaked Spectrum galaxies with redshift lower than 1. In total 19 sources are included mainly from observations made by the European space telescope, XMM-Newton. Out of these the analysis of seven "new" observations made between 2006 and 2008 are throughout described. Data from the new observations shows consistency with already analysed data. As a new result a tentative discovery of a bi-modal structure in the X-ray to radio luminosity ratio within the sample is presented.

Pulsar wind nebulae (PWNe), nebulae harbouring a rotation-powered neutron star that was born in a supernova, provide opportunities to study highly relativistic pulsar winds and their interaction with the surrounding medium. Particularly interesting are PWNe that do not show any sign of the expected surrounding SNR shell and were thought to be born in subenergetic explosions or with unusual progenitors. The detection of a shell around one such PWN suggested that shells are indeed produced but may be faint due to unseen shocked ejecta, a low density environment, and/or a young age that has not yet allowed the shell to brighten and become visible. Here, by using observational X-ray data from modern telescopes with excellent spatial and energy resolution (Chandra and XMM-Newton), we target PWNe that do not have prominent SNR shells, and are known to be in varied environments, to further explore the characteristics of this growing, but poorly explored, class of PWNe. By combining imaging and spectroscopic results, we study the morphology of the PWNe, search for thermal emission from shock-heated material, investigate the energetics of the nebulae, and search for candidates for the neutron stars powering the nebulae. We find that while the faint shell surrounding G21.5–0.9 can be explained as a young PWN evolving in a low density medium, CTB 87 (G74.9+1.2) appears to be in an advanced stage of evolution, and G63.7+1.1 appears to be both in an advanced stage of evolution and in a dense environment. By performing spatially resolved spectroscopy, we have shown how the spectral characteristics vary across the PWNe, and note that more data will place better constraints on possible thermal emission in these remnants. The imaging portion of these studies has revealed intriguing large-scale morphologies for CTB 87 and G63.7+1.1, as well as a torus-jet structure in CTB 87 and neutron star candidates in both CTB 87 and G63.7+1.1. We conclude that both CTB 87 and G63.7+1.1 are likely interacting with the supernova remnant reverse shock, and CTB 87 may be additionally influenced by the motion of its neutron star.

Using the Chandra X-ray Observatory, Martini et al. (2006) found that the AGN fraction of galaxy clusters was five times higher than previous optical studies suggested. Using visual observations only, Dressler et al. (1985) estimated the AGN fraction of field galaxies to be 5%, while that of clusters was thought to be 1%. To understand the role that the environment plays in AGN fueling, the author studied a variety of environments, ranging from the field to groups to clusters. Will the AGN fraction of groups also be higher than that of the field? The author demonstrates how the AGN fraction of groups compares to that of clusters. In the following sections, the author describes the mechanics of X-ray astronomy, the group environment, and the characteristics of active galactic nuclei. The author briefly describes the possible mechanisms for AGN fueling.

We obtained four pointings of over 100 ks each of the well-studied Wolf-Rayet star WR 6 with the XMM-Newton satellite. With a first paper emphasizing the results of spectral analysis, this follow-up highlights the X-ray variability clearly detected in all four pointings. However, phased light curves fail to confirm obvious cyclic behavior on the well-established 3.766 d period widely found at longer wavelengths. The data are of such quality that we were able to conduct a search for "event clustering" in the arrival times of X-ray photons. However, we fail to detect any such clustering. One possibility is that X-rays are generated in a stationary shock structure. In this context we favor a co-rotating interaction region (CIR) and present a phenomenological model for X-rays from a CIR structure. We show that a CIR has the potential to account simultaneously for the X-ray variability and constraints provided by the spectral analysis. Ultimately, the viability of the CIR model will require both intermittent long-term X-ray monitoring of WR 6 and better physical models of CIR X-ray production at large radii in stellar winds.

Using the Chandra X-ray Observatory, Martini et al. (2006) found that the AGN fraction of galaxy clusters was five times higher than previous optical studies suggested. Using visual observations only, Dressler et al. (1985) estimated the AGN fraction of field galaxies to be 5%, while that of clusters was thought to be 1%. To understand the role that the environment plays in AGN fueling, the author studied a variety of environments, ranging from the field to groups to clusters. Will the AGN fraction of groups also be higher than that of the field? The author demonstrates how the AGN fraction of groups compares to that of clusters. In the following sections, the author describes the mechanics of X-ray astronomy, the group environment, and the characteristics of active galactic nuclei. The author briefly describes the possible mechanisms for AGN fueling.

Using the Chandra X-ray Observatory, Martini et al. (2006) found that the AGN fraction of galaxy clusters was five times higher than previous optical studies suggested. Using visual observations only, Dressler et al. (1985) estimated the AGN fraction of field galaxies to be 5%, while that of clusters was thought to be 1%. To understand the role that the environment plays in AGN fueling, the author studied a variety of environments, ranging from the field to groups to clusters. Will the AGN fraction of groups also be higher than that of the field? The author demonstrates how the AGN fraction of groups compares to that of clusters. In the following sections, the author describes the mechanics of X-ray astronomy, the group environment, and the characteristics of active galactic nuclei. The author briefly describes the possible mechanisms for AGN fueling.

Ultraluminous X-ray sources (ULXs) are extragalactic, non-nuclear, point-like X-ray sources whose luminosity supersedes that of the Eddington limit of an accreting stellar mass black hole (L&gt; 10 ^ 39 erg / s). Most of them are powered by black holes and neutron stars undergoing genuine super-Eddington accretion, with a small handful of candidates being consistent with sub-Eddington accretion on an intermediate mass black hole. In this thesis, we explore the populations of ULXs in the sky surveys of ESA's X-ray satellite, XMM-Newton, and the MPE's newly launched X-ray telescope, eROSITA. We do so by correlating them with the HECATE list of galaxiesto build two X-ray non-nuclear catalogs, and comparing the yields with very expensive surveys and previous works. To build a catalog, we useother reference lists of contaminant objects, such as the Gaia data releases, the SIMBAD database or the SDSS survey to look for contaminating objects of diverse nature, such as foreground stars or background quasars, in order to make sure that our resulting ULX samples are as clean as possiblewith catalog data only. Our results include the attestation that the XMM-Newton ninth data release provides an improvement in quantity and quality with respect to older data releases used in previous works, and that the eROSITA survey is currently in a very preliminary stage. The two new catalogs contain 12,952 and 3,720 non-nuclear X-ray sources, out of which 914 and 132 are ULX candidates with an expected ~ 25% fraction of undetected contaminants. This constitutes a very significant contribution to the already known 300 ULX candidates. Since the sky coverage and depth of the XMM-Newton and eROSITA surveys are vastly different, only 19 of the ULX candidates are shared between the catalogs. ULX candidates are preferentially found in star-forming galaxies, but a subset of very bright objects (L&gt; 5x10 ^ 40 erg / s) try to be more common in elliptical galaxies, in contradiction to what has been established in the literature. / <p>This thesis was written under the joint supervision of Erin O'Sullivan at Uppsala University and Axel Schwope at the Leibniz Institute for Astrophysics in Potsdam. The presentation was held online due to the COVID-19 circumstances.</p> / Master Thesis