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The Fermi Large Area Telescope has observed a gamma-ray excess toward the center of the Galaxy at ~ GeV energies. The spectrum and intensity of the excess are consistent with the annihilation of dark matter with a mass of ~100 GeV and a velocity-averaged cross section of ~ 1e-26 cubic centimeter per second. In the meantime, a population of unresolved millisecond pulsars (MSPs) in the Galactic center remains a possible source of the excess. Furthermore, recent analyses have shown that the excess prefers the spatial morphology of the stellar bulge distribution in the Galactic center, supporting a MSP origin. The new discovery makes it imperative to further study the signals from MSPs. This dissertation studies the gamma-ray emission from Galactic millisecond pulsars to provide new insights into the origin of the Galactic center excess. Using the GALPROP code, we simulate the propagation of e± injected by the putative MSPs in the Galactic bulge and calculate the inverse Compton (IC) emission caused by the e± losing energy in the interstellar radiation field. We find recognizable features in the spatial maps of the IC. Above TeV energies, the IC morphology tends to follow the distribution of the injected e±. Then, we study the Cherenkov Telescope Array (CTA) sensitivity to the IC signal from MSPs. We find that the CTA has the potential to robustly discover the IC signature when the MSP e± injection efficiencies are in the range ≈ 2.9-74.1%. The CTA can also discriminate between an MSP and a dark matter origin for the radiating e± based on their different spatial maps. Next, we analyze the Fermi data from directions of Galactic globular clusters. The globular clusters are thought to be shining in gamma rays because of the MSP population they host. By analyzing their gamma-ray spectra, we reveal evidence for an IC component in the high-energy tail of Fermi data. Based on the IC component in the globular cluster spectra, the e± injection efficiency of millisecond pulsars is estimated to be slightly smaller than 10%. Finally, we study the spatial morphology of the 511 keV signal toward the Galactic center using data from INTEGRAL/SPI. We confirm that the 511 keV signal also traces the old stellar population in the Galactic bulge, which is similar to the Fermi GeV excess. Using a 3D smoothing kernel, we find that the signal is smeared out over a characteristic length scale of 150 ± 50 pc. We show that positron propagation prior to annihilation can explain the overall phenomenology of the 511 keV signal. / Doctor of Philosophy / Dark matter means matter that does not interact with light; therefore, they are invisible to traditional observations. We know that dark matter exists based on plenty of gravitational evidence: the motions of stars in galaxies, the large-scale structure of the Universe, the temperature fluctuations in the cosmic microwave background. However, we still know very little about the particle nature of dark matter. Detecting dark matter is one of the most extensive missions of modern physics. In indirect detection, the dark matter particles are expected to annihilate or decay in the cosmos, producing messenger particles that include gamma rays, cosmic rays, and neutrinos. Astronomical observations could detect those signals and confirm the nature of dark matter. However, understanding the astrophysical sources is essential for indirect detection of dark matter as they may emit similar signals. For a recent example, the Fermi Large Area Telescope launched by NASA is the most sensitive gamma-ray telescope in the energy range of ~ 100 MeV to ~ 100 GeV. It has detected an excess of gamma-ray signals toward the Galactic center consistent with what we expect from dark matter annihilation. However, millisecond pulsars, a type of fast rotating neutron stars, may also generate similar gamma-ray signals. Therefore, the origin of the signal remains unsettled. In this dissertation, we study different prospective of the gamma-ray emission from the millisecond pulsars in the Milky Way. We first study the inverse Compton signal from the millisecond pulsars in the Galactic bulge, caused by the relativistic e± injected by the millisecond pulsars. We find that the signal traces the original distribution of the e± above TeV energies. Next generation ground-based gamma-ray observatories like the Cherenkov Telescope Array (CTA) could be used to detect the signal. We study the CTA sensitivity to such an inverse Compton signal. We find that CTA can detect the inverse Compton signal from millisecond pulsars and discriminate it from a dark matter signal. We also study the gamma-ray emission from globular clusters in the Milky Way. They are dense collections of old stars orbiting our Galaxy, and they are known for hosting many millisecond pulsars. We reveal evidence for inverse Compton emission from the gamma-ray data of globular clusters. Our discovery helps us better understand the high-energy property of millisecond pulsars. Last, we study the morphology of the Galactic 511 keV signal caused by positron annihilation. Compact objects including millisecond pulsars are potential sources of the positrons. We find that the old stellar distribution with a smearing scale of ~ 150 pc best describes the 511 keV signal. Positron propagation from their sources prior to annihilation could explain the measured smearing scale.

No / The three seismic sources, S1, S2, and S3, detected from MDI Dopplergrams using the time-distance (TD) diagram technique are presented with the locations, areas, and vertical and horizontal velocities of the visible wave displacements. Within the data cube of 120 Mm, the horizontal velocities and the wave propagation times vary slightly from source to source. The momenta and start times measured from the TD diagrams in sources S1-S3 are compared with those delivered to the photosphere by different kinds of high-energy particles with the parameters deduced from hard X-ray and ¿-ray emission, as well as by the hydrodynamic shocks caused by these particles. The energetic protons (power laws combined with quasi-thermal ones, or jets) are shown to deliver momentum high enough and to form the hydrodynamic shocks deep enough in a flaring atmosphere to allow them to be delivered to the photosphere through much shorter distances and times. Then the seismic waves observed in the sources S2 and S3 can be explained by the momenta produced by hydrodynamic shocks, which are caused by mixed proton beams and jets occurring nearly simultaneously with the third burst of hard X-ray and ¿-ray emission in the loops with footpoints in the locations of these sources. The seismic wave in source S1, delayed by 4 and 2 minutes from the first and second hard X-ray bursts, respectively, is likely to be associated with a hydrodynamic shock occurring in this loop from precipitation of a very powerful and hard electron beam with higher energy cutoff mixed with quasi-thermal protons generated by either of these two bursts.

Mes travaux de recherche portent sur l'étude du rayonnement (surtouts rayons X durs) provenant des trous noirs accrétant (dans les noyaux actifs de galaxies et les binaires X). L'objectif est d'en extraire des informations sur les conditions physique régnant dans l'environnement immédiat de ces objets. Les principales question auxquelles je tente de répondre sont les suivantes: Quelle est la structure et la géométrie de la matière accrétée au voisinage du trou noir ? Comment celle -ci évolue-t-elle avec le taux d'accrétion de masse ? Quel est la relation entre les processus d'accrétion et la formation de jets souvent observés dans ces systèmes ? Mon approche est fondée sur une comparaison précise entre les observations et les prédictions des divers modèles. Je présente les efforts poursuivis depuis près de dix ans afin de développer des outils de simulation numérique pour modéliser le transfert de rayonnement dans les plasma chauds des sources compactes X. Je montre comment ces outils ont été utilisés pour modéliser le continuum haute énergie et la variabilité des trous noirs accrétants et pour contraindre la structure du flot d'accrétion. Je présente également des résultats reposant sur l'analyse et l'interprétation d'observations menées avec des télescopes spatiaux tels que XMM-Newton et INTEGRAL ainsi que sur des d'observations simultanées à plusieurs longueurs d'ondes allant de la radio aux rayons X durs.

<p align="justify">The success of Big Bang Nucleosynthesis (BBN) combined with the detailed analysis of the small imperfections of the Cosmic Microwave Background blackbody spectrum lead to the conclusion that most of the matter content of our universe is made of some non-baryonic material, the dark matter!</p><p><p><p align="justify">In this thesis, we review the compiling indications of dark matter and the so-called freeze-out mechanism which may settle the relic density of the species in the framework of the standard Big Bang model. We also examine principally two methods of detection of dark matter, direct and indirect detection searches.</p><p><p><p align="justify">Let us stress that the Standard Model on its own is unable to provide enough aspirants for the role of dark matter. As a consequence, one has to dig into the tremendous domain of physics "Beyond the Standard Model" in order to have a chance to elucidate the problem of the missing mass.</p><p><p><p align="justify">In this thesis in particular, we consider the Inert Doublet Model (IDM) which includes an additional Higgs doublet, enclosing two neutral scalars candidates for dark matter. We invoke the Standard freeze-out mechanism for the production of dark matter. We get then dark matter candidates in two rather separate mass ranges, one between 40 and 80 GeV, the other one between 400 GeV and 1 TeV. We also show that dark matter annihilation at the galactic center can be at the origin of a gamma-ray flux which can be probed by the future GLAST experiment.</p><p><p><p align="justify">We address a low reheating temperature scenario for the genesis of dark matter in a Left-Right symmetric extension of the Standard Model. The candidate for dark matter is a MeV right-handed neutrino and we show that a baryon-dark matter interaction at the galactic center can be the source of the low energy positrons responsible for the 511 keV gamma-ray excess observed by the INTEGRAL experiment in the galactic bulge region.</p><p><p><p align="justify">Finally, prompted by the possibility to explain the baryon and dark matter rather similar abundances by one single "Matter Genesis" mechanism, we study a non-thermal production mechanism for dark matter. The framework is also Left-Right symmetric and dark candidate is a ~3 GeV right handed neutrino.</p> <p><p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The recent discovery of a new population of Active Galactic Nuclei (AGN) – the Radio Loud Narrow Line Seyfert 1 (RL NLS1) galaxies – at g-ray energies by Fermi has prompted intense interest among researchers, as evidence mounts that these objects may in fact compose a new class of blazars. If RL NLS1s are indeed a new class of blazars, or at least analogous to them, then the detection of certain blazar-like properties would be expected. These properties include significant variability at all wavelengths across the electromagnetic spectrum and on timescales from minutes to years, significant & variable polarization in the radio and optical regimes, significant & variable high-energy emissions (especially in the g-ray regime), and a double-peaked structure of their spectral energy distributions. This dissertation seeks to characterize several of these properties for RL NLS1s as a class. These include the degree to which these objects are polarized and the variability of this property, the detection and characterization of these sources at g-ray energies with the Large Area Telescope aboard the Fermi spacecraft, and the degree to which these properties are interdependent. A photopolarimetric survey (the first of its kind for this class of objects) and g-ray monitoring program were conducted by the author in order to obtain the data necessary for this project. The measurements obtained via these observations are used to characterize this interesting class of objects with respect to a sample of blazars. In general, it was found that the polarizations of these objects fall between radio-quiet NLS1s and FSRQ-type blazars, and were most similar to HBL-type blazars. The 7 RL NLS1s in this sample that had been detected above a Test Statistic (TS) of 25 by Fermi/LAT were most similar to FSRQs, while 9 objects detected in the interval 9 < TS < 25 shared several properties with HBLs. Two RL NLS1s - J1443+4725 and J1644+2619 - are identified as high-confidence (TS > 25) g-ray sources for the first time, bringing the total number of members of this class firmly detected at g-ray energies to 8. The gamma-ray spectra of RL NLS1s are similar to FSRQs, though some have steeper spectra.

Multi-wavelength observations of mature supernova remnants (SNRs), especially with recent advances in gamma-ray astronomy, make it possible to constrain energy distribution of energetic particles within these remnants. In consideration of the SNR origin of Galactic cosmic rays and physics related to particle acceleration and radiative processes, we use a simple one-zone model to fit the nonthermal emission spectra of three shell-type SNRs located within 2 degrees on the sky: RX J1713.7-3946, CTB 37B, and CTB 37A. Although radio images of these three sources all show a shell (or half-shell) structure, their radio, X-ray, and gamma-ray spectra are quite different, offering an ideal case to explore evolution of energetic particle distribution in SNRs. Our spectral fitting shows that (1) the particle distribution becomes harder with aging of these SNRs, implying a continuous acceleration process, and the particle distributions of CTB 37A and CTB 37B in the GeV range are harder than the hardest distribution that can be produced at a shock via the linear diffusive shock particle acceleration process, so spatial transport may play a role; (2) the energy loss timescale of electrons at the high-energy cutoff due to synchrotron radiation appears to be always a bit (within a factor of a few) shorter than the age of the corresponding remnant, which also requires continuous particle acceleration; (3) double power-law distributions are needed to fit the spectra of CTB 37B and CTB 37A, which may be attributed to shock interaction with molecular clouds.

The extensive catalogue of gamma-ray selected flat-spectrum radio quasars (FSRQs) produced by Fermi during a four-year survey has generated considerable interest in determining their gamma-ray luminosity function (GLF) and its evolution with cosmic time. In this paper, we introduce the novel idea of using this extensive database to test the differential volume expansion rate predicted by two specific models, the concordance Lambda cold darkmatter (Lambda CDM) and R-h = ct cosmologies. For this purpose, we use two well-studied formulations of the GLF, one based on pure luminosity evolution (PLE) and the other on a luminosity-dependent density evolution (LDDE). Using a Kolmogorov-Smirnov test on one-parameter cumulative distributions (in luminosity, redshift, photon index and source count), we confirm the results of earlier works showing that these data somewhat favour LDDE over PLE; we show that this is the case for both Lambda CDM and R-h = ct. Regardless of which GLF one chooses, however, we also show that model selection tools very strongly favour R-h = ct over Lambda CDM. We suggest that such population studies, though featuring a strong evolution in redshift, may none the less be used as a valuable independent check of other model comparisons based solely on geometric considerations.

Blazars are variable emitters across all wavelengths over a wide range of timescales, from months down to minutes. It is therefore essential to observe blazars simultaneously at different wavelengths, especially in the X-ray and gamma-ray bands, where the broadband spectral energy distributions usually peak. In this work, we report on three " target-of-opportunity" observations of Mrk 421, one of the brightest TeV blazars, triggered by a strong flaring event at TeV energies in 2014. These observations feature long, continuous, and simultaneous exposures with XMM-Newton (covering the X-ray and optical/ultraviolet bands) and VERITAS (covering the TeV gamma-ray band), along with contemporaneous observations from other gamma-ray facilities (MAGIC and Fermi-Large Area Telescope) and a number of radio and optical facilities. Although neither rapid flares nor significant X-ray/TeV correlation are detected, these observations reveal subtle changes in the X-ray spectrum of the source over the course of a few days. We search the simultaneous X-ray and TeV data for spectral hysteresis patterns and time delays, which could provide insight into the emission mechanisms and the source properties (e. g., the radius of the emitting region, the strength of the magnetic field, and related timescales). The observed broadband spectra are consistent with a one-zone synchrotron self-Compton model. We find that the power spectral density distribution at greater than or similar to 4 x 10(-4) Hz from the X-ray data can be described by a power-law model with an index value between 1.2 and 1.8, and do not find evidence for a steepening of the power spectral index (often associated with a characteristic length scale) compared to the previously reported values at lower frequencies.