L'astronomie de haute énergie se concentre sur l'étude des phénomènes les plus violents de l'univers à partir d'observations dans une gamme d'énergie allant des rayons X aux rayons gammas de très hautes énergies (1 keV - 100 TeV). Ces phénomènes incluent par exemple les explosions de supernovae et leurs vestiges, les pulsars et les nébuleuses de vent de pulsar ou encore la formation de jets ultrarelativistes au niveau des noyaux actifs de galaxie. Leur compréhension fait appel à des processus de physique des particules bien connus qui seront décrits dans cette thèse. Par l'intermédiaire de photons de haute énergie, l'étude de ces phénomènes de haute énergie ouvre donc une fenêtre originale pour la recherche de physique au delà du modèle standard. Les concepts relatifs à l'émission et la propagation de photons de haute énergie sont introduits dans cette thèse et appliqués à l'étude de l'émission de sources extragalactiques ainsi que du fond de lumière extragalactique, affectant la propagation des photons de haute énergie dans l'univers. Dans le cadre de cette thèse, ces sources extragalactiques de photons de haute énergie sont observées afin de rechercher de nouveaux bosons légers, tels que ceux appartenant à la famille des particules de type axion (PTA). Les bases théoriques décrivant cette famille de particules sont présentées, ainsi que la phénoménologie associée. Notamment, en raison de leur couplage à deux photons, ces particules ont la propriété d'osciller avec des photons en présence de champ magnétique externe. Une nouvelle signature de la présence de telles oscillations dans des champs magnétiques turbulents, sous la forme d'irrégularités stochastiques dans le spectre en énergie, est étudiée et discutée. Cette signature est appliquée à la recherche de PTA avec le réseau de télescopes HESS, permettant d'obtenir pour la première fois des contraintes sur ces modèles à partir d'observations en astronomie gamma. La recherche de la même signature dans des observations en rayons X permet d'améliorer les contraintes actuelles pour les PTA de très basse masse et l'extension de ces contraintes à des modèles de modification de la gravité comme explication de la nature de l'énergie noire est également évoquée. Enfin, la recherche de PTA avec l'instrument d'astronomie gamma du futur, CTA, est discutée; en particulier, une nouvelle observable est proposée qui tire partie du grand nombre de sources attendu avec cet instrument. / High-Energy astronomy studies the most violent phenomena in the universe with observations in a large spectrum of energies ranging from X rays to very high energy gamma rays (1 keV - 100 TeV). Such phenomena could be for instance supernovae explosions and their remnants, pulsars and pulsar wind nebulae or ultra relativistic jets formation by active galactic nuclei. Understanding these phenomena requires to use well-known particle physics processes. By means of high energy photons, studying such phenomena enables one to search for physics beyond the standard model. Concepts regarding the emission and propagation of high-energy photons are introduced and applied to study their emission by extragalactic sources and to constrain the extragalactic background light which affects their propagation. In this thesis, these high-energy extragalactic emitters are observed in order to search for new light bosons such as axion-like particles (ALPs). The theoretical framework of this family of hypothetical particles is reviewed as well as the associated phenomenology. In particular, because of their coupling to two photons, ALPs oscillate with photons in an external magnetic field. A new signature of such oscillations in turbulent magnetic fields, under the form of stochastic irregularities in the source energy spectrum, is introduced and discussed. A search for ALPs with the HESS telescopes with this new signature is presented, resulting in the first constraints on ALPs parameters coming from high-energy astronomy. Current constraints on ALPs at very low masses are improved by searching for the same signature in X-ray observations. An extension of these constraints to scalar field models for modified gravity in the framework of dark energy is then discussed. The potential of the search for ALPs with CTA, the prospected gamma-ray astronomy instrument, is eventually studied; in particular, a new observable is proposed that relies on the high number of sources that are expected to be discovered with this instrument.

The constant flux of cosmic rays that bombard Earth from within our own galaxy are understood to come from both shell-type supernova remnants and pulsar wind nebulae (PWNe). Multiwavelength study of these objects can help us to understand what types of particles are accelerated, and gamma-ray emission is key to understanding the highest energy cosmic rays. In this thesis, I analyze and interpret observations made with the Very Energetic Radiation Imaging Telescope Array System (VERITAS), a gamma-ray telescope located in Southern Arizona. LS 5039 and HESS J1825-137 occupy the same field of view on the sky and were observed for about 8 hours with VERITAS. LS 5039 is a gamma-ray binary, and the observations supports theories that the compact object hosts a PWN which is continuously interacting with the nearby star. HESS J1825-137 is a very extended PWN with an extent of diameter greater than 1 degree on the sky. Using the VERITAS observations, I am able to measure the radial profile and compare the gamma-ray luminosity to other PWN. DA 495, a "Crab-like" PWN with unusually strong magnetic fields, was observed for about 70 hours with VERITAS. In this study, results are combined with radio and X-ray spectral information to allow for detailed astrophysical modeling of the region. This broadband spectral modeling places constraints on the properties of the particle population in this PWN and allows for both leptonic and hadronic emission scenarios to be evaluated. Hadronic scenarios instil doubt on the pure PWN interpretation and favor a previously undetected shell-type remnant being present.

This thesis studies the possibility of using the Monte Carlo simulation program FLUKA to determine the neutron induced radioactivity of concrete walls at the The Svedberg Laboratory (TSL) in Uppsala. If a simulation of the activation would produce reliable results, it would be a useful complement to measurements for the decommissioning and clearance of the buildings of the facility. An experiment was performed in which a concrete core was taken from one of the non-activated walls in the facility. The core was cut into samples and irradiated with a neutron beam. The samples were then measured in a gamma-ray spectroscopy setup, by which the produced radioactive nuclides were identified and their activities determined. The same setup was then simulated in FLUKA. A comparison of the simulations and the measurements shows that the average activity for all nuclides obtained with FLUKA is similar to the measured one, however with large differences for some nuclides. The average ratio of the simulated and measured activities or all nuclides is 1.07 with a standard deviation of 0.55. The obtained results may be useful for future radiological clearance work at TSL.

Ultraluminous X-ray (ULX) sources are non-nuclear extragalactic accreting compact objects whose X-ray luminosities exceed the Eddington limit for stellar mass black hole binaries (BHB). Their high luminosities suggest they are either intermediate mass black holes, that their emission is beamed, or that they are emitting at super-Eddington rates. We observed the ULX IC 342 X-1 simultaneously in X-ray and radio with Chandra and the VLA to investigate previously reported unresolved radio emission coincident with the ULX. The Chandra spectrum appears to be consistent with an accretion disc-dominated thermal state and suggests a mass of the black hole using the modeled inner disc temperature to be 157Mʘ ≤ M √ (cosi) ≤ 200 Mʘ. No significant radio emission was observed, consistent with the source being in a thermal disc-dominated state. Reanalysis of previous X-ray observations of the source shows that high energy curvature often interpreted as evidence for supercritical accretion cannot confidently be identified using the 2-10 keV energy band. Black hole systems such as BHBs, ULXs, and AGN represent the greatest test labs in the universe for the study of extreme gravity. Emission from the accretion disk and scattering from the surrounding corona allow study of the ultra-strong gravity and magnetic fields very near the central BH engine. However, many of these effects are imprinted as polarization of the emission and are invisible to spectral and timing studies alone. The outflows from AGN are also thought to play a key role in galaxy shaping and cluster formation. High efficiency and spectral resolution are required to measure ionization-velocities and density parameters from these sources to constrain the outflow structure. Beamline studies and theoretical modeling were carried out to characterize the throughput and spectral resolving power of off-plane gratings for use in future x-ray observatories which will make these measurements. Additionally, synchrotron measurements were carried out to test theoretical predictions of strong polarization response for off-plane diffraction gratings. The empirical results of this study are the first to demonstrate a lack of polarization sensitivity for grazing-incidence off-plane gratings and support more complex modeling results than used previously.

The main topic of this thesis is analysis of an unidentified Galactic TeV gamma-ray source, MGRO J1908+06, discovered by Milagro instrument in 2007. We analyzed 54 hours of observational data from the Very Energetic Radiation Imaging Telescope Array System (VERITAS), a ground-based gamma-ray observatory in southern Arizona comprised of an array of four Cherenkov Telescopes that reconstructs the energy and direction of astrophysical gamma-rays by imaging Cherenkov light emitted by energetic particles in air showers produced by the primary gamma-rays. MGRO J1908+06 is located between a supernova remnant SNR G40.5-0.5 and a young, energetic pulsar PSR J1907+0602. We studied the energy dependent morphology of the TeV emission from the source and measured the source extent and spectrum. The source extends well past the boundary of the SNR and is not correlated with strong radio continuum or molecular line emission which likely excludes an origin for the emission as solely due to the SNR. While emission in the 0.5-1.25 TeV band was centered around the pulsar, higher energy emission was observed near the supernova remnant. This morphology is opposite that observed in other pulsar wind nebulae. We proposed two models for the high energy emission located well away from the pulsar but close to the SNR: (1) shock acceleration at the shock front created by an interaction between the pulsar wind and the dense gas at the edge of the SNR or (2) molecular clouds around the SNR provides seed photons with energies higher than those from Cosmic Microwave Backgrounds for inverse Compton scattering. The former model can be tested by looking for molecular emission lines that trace shocks and by measuring the pulsar velocity. In addition, we investigated the gamma-ray emission from the nova explosion of V407 Cygni that occurred in March 2010. The Fermi-LAT observed this event in the energy range of E >100 MeV. The origins of the gamma-ray emission that the Fermi-LAT team proposed are either protons (hadronic model) or electrons (leptonic model), both of which were accelerated at the nova shock via the Fermi acceleration mechanism. We did not consider their leptonic model because no TeV gamma-ray emission is predicted. Their hadronic model can generate TeV gamma-rays with the modeled parameters. We found no evidence for TeV emission. We showed that with the flux upper limit calculated using the VERITAS data imposes constraints on the extension of the proton spectrum at high energies.

There has been an increasing interest in uncovering smuggled nuclear materials associated with the War on Terror. Detection of special nuclear materials hidden in cargo containers is a major challenge in national and international security. We propose a new physics-based method to determine the presence of the spectral signature of one or more nuclides from a poorly resolved spectra with weak signatures. The method is different from traditional methods that rely primarily on peak finding algorithms. The new approach considers each of the signatures in the library to be a linear combination of subspectra. These subspectra are obtained by assuming a signature consisting of just one of the unique gamma rays emitted by the nuclei. We propose a Poisson regression model for deducing which nuclei are present in the observed spectrum. In recognition that a radiation source generally comprises few nuclear materials, the underlying Poisson model is sparse, i.e. most of the regression coefficients are zero (positive coefficients correspond to the presence of nuclear materials). We develop an iterative algorithm for a penalized likelihood estimation that prompts sparsity. We illustrate the efficacy of the proposed method by simulations using a variety of poorly resolved, low signal-to-noise ratio (SNR) situations, which show that the proposed approach enjoys excellent empirical performance even with SNR as low as to -15db. The proposed method is shown to be variable-selection consistent, in the framework of increasing detection time and under mild regularity conditions. We study the problem of testing for shielding, i.e. the presence of intervening materials that attenuate the gamma ray signal. We show that, as detection time increases to infinity, the Lagrange multiplier test, the likelihood ratio test and Wald test are asymptotically equivalent, under the null hypothesis, and their asymptotic null distribution is Chi-square. We also derived the local power of these tests. We also develop a nonparametric approach for detecting spectra indicative of the presence of SNM. This approach characterizes the shape change in a spectrum from background radiation. We do this by proposing a dissimilarity function that characterizes the complete shape change of a spectrum from the background, over all energy channels. We derive the null asymptotic test distributions in terms of functionals of the Brownian bridge. Simulation results show that the proposed approach is very powerful and promising for detecting weak signals. It is able to accurately detect weak signals with SNR as low as -37db.

This work aims to contribute to the study of the origins of cosmic rays, and broadly, to the advancement of both data analysis methods and instrumentation for very-high-energy γ-ray astronomy. First, reviewing the state of γ-ray astronomy, we show how gains in sensitivity can be achieved through sophisticated data analyses and improved instrumental designs. We then develop such an improved analysis method for the Very Energetic Radiation Imaging Telescope Array System (VERITAS) by combining Image Template Method (ITM) with Boosted Decision Trees (BDT), and study its performance, attaining a 30-50% improvement in integral sensitivity over the instrument’s standard analysis. Systematic issues in spectral reconstruction that the analysis displays are resolved satisfactorily by imposing a more stringent condition on the selection of its energy threshold. We employ the newly developed analysis to measure the γ-ray energy spectrum of the starburst galaxy M82, and combining our result with a measurement from the Fermi Large Area Telescope (Fermi-LAT), we find that a single power law fits the spectrum well between 100 GeV and 10 TeV, with no evidence for a spectral break or a cutoff. We conclude that this is in line with the current understanding that M82 is not a good proton calorimeter. Finally, we detail the design, implementation, and performance of the optical alignment system of the prototypeSchwarzschild-Couder Telescope (pSCT) for the Cherenkov Telescope Array (CTA), a novel two-mirror design that addresses many shortcomings of current instruments.

No description available.

Graduation date: 2005

Graduation date: 1992