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The subject of this thesis is the simulation study of the development of extensive air show ers produced by very high energy gamma-ray and hadronic cosmic rays with respect to the Cherenkov light they produce, and its imaging in ground based telescopes. Chapters 1-4 are introductory: Chapter 1 covers the mechanisms responsible for the production of very high energy gamma-rays, whereas, chapter 2 focusses on the development of extensive air showers and Cherenkov light production. Chapter 3 covers the instrumentation used to measure the Cherenkov light using the imaging atmospheric Cherenkov technique. Chapter 4 covers known and possible sources of very high energy gamma-rays. Chapters 5, 6 and 7 cover research performed by the author: Chapter 5 discusses some of the differences between three popular extensive air shower simulations codes, namely ALTAI, CORSIKA and MOCCA. Chapter 6 details the simulation of the response of two ground based imaging atmospheric Cherenkov telescope (the Durham Mark 6 and stand-alone H.E.S.S. telescopes), and in particular details the derivation of the flux of the x-ray selected BL-LAC PKS 2155-304 with the Durham Mark 6 telescope. This represents the refinement of a published measurement given an improved telescope simulation. The significance of the signal seen is 6.8o, and the integral flux derived above 1.5 TeV (assuming a differential spectral slope of-2.6) is {2.5±0.7stat ± (^0.5)(_1.6syst) x 10(^-7) photons m(^-2) s(^-1) Chapter 7 discusses the importance of the atmosphere, and the results of shower simulations under different atmospheric assumptions are presented, which indicate the importance of atmospheric calibration for the new generation of Cherenkov telescopes. The results of this chapter suggest that to first order large changes in the low level aerosol concentration have a much more significant effect on the trigger rate of a stand-alone H.E.S.S. telescope, than on the Hillas parameter distributions seen. Chapter 8 brings together the work done in this thesis, and highlights a final set of fluxes for the active galactic nuclei sources seen with the Durham Mark 6 telescope, many of which will form future sources to be measured with the H.E.S.S. system. The current status of the stand-alone H.E.S.S. system is also covered in chapter 8. The thesis concludes with a further brief discussion of the future prospects for imaging atmospheric Cherenkov astronomy.

Le domaine des sciences et technologies Térahertz gagne un intérêt international en raison de ses nombreuses applications allant des systèmes de transmission sans fils ultra rapide au diagnostique médical, au contrôle de qualité et à la sécurité industrielle. Aujourd’hui, cet intérêt pousse la recherche en électronique à se focaliser sur la réduction des composants dans le but d’augmenter leur fréquence de fonctionnement. Les nanotechnologies sont donc au cœur de cette course à la montée en fréquence. La gamme des longueurs d'onde THz ouvre une nouvelle ère de systèmes directement liés à l'information et aux technologies de communication, étendant considérablement ceux déjà existant qui reposent sur des dispositifs optiques et électroniques. Les dispositifs THz à base de semi-conducteurs sont une des voies possibles pour la réalisation de composants à l’interface entre les micro-ondes et la phonique. L'application à grande échelle du domaine des THz pour l'astronomie, l'environnement, les communications, l'imagerie, la sécurité, la biologie et la médecine pourraient conduire à définir la gamme THz comme un champ d’application spécifique pour les chercheurs et les ingénieurs. Par exemple la spectroscopie moléculaire très importante pour l'astronomie (analyse des gaz interstellaires, observations planétaires), l'environnement (surveillance de la pollution), les télécommunications, les communications locales sécurisées (à travers une forte atténuation en dehors de la zone ciblée) à très haut débit de données sera à terme possible. L'imagerie également un élément important contribuant à la sécurité (détection d’armes et de matériel illicite, analyse non destructive et non invasive d’objets).L'un des obstacles pour le développement d’applications pratique dans la gamme du THz est le manque de sources continues, compactes, accordables et puissantes (à faible coût, si possible). Ainsi, ce travail s’inscrit dans le cadre du projet européen ROOTHz et nous proposons d'exploiter les oscillations type Gunn sur un nouveau type de nanodispositifs fabriqués pour la première fois sur du nitrure de Gallium : La diode autocommutante, ou Self switching diode (SSD). Sur la base de simulations Monte-Carlo, la géométrie particulière des SSD favorise l'apparition d'oscillations Gunn à des fréquences Térahertz, en utilisant une dyssymétrie d'un canal de type transistor assez étroit, la SSD peut fournir un comportement de redressement. Cet effet, basé sur les effets de surface et les effets électrostatiques, utilisé sur le GaN permet le fonctionnement comme détecteurs THz à température ambiante. Au cours de cette thèse, des études sur les SSD à base de nitrure de gallium ont été menées afin d'évaluer leurs potentialités comme émetteurs et détecteurs dans la gamme de fréquence du THz. / The field of Terahertz Science and Technology is gaining international interest due to its numerous applications ranging from ultra high speed optical transmission systems to medical diagnosys, industrial quality control and security-screening tools. In this field, the efforts of electronics industry are centered on device scaling down to the nanometer range to increase the operational speed.The THz range is an intermediate range of wavelengths that will open a new area of systems directly related to information and communication technologies, significantly extending the present ones based on photonic and electronic devices. Thus, solid-state THz devices can be either considered as belonging to both fields or to none of them. Indeed the wide application area of THz for astronomy, environment, communications, imaging, security, biology and medicine could lead to define the THz range as a specific scientific, engineering and application field. Molecular spectroscopy is very important for astronomy (analysis of the interstellar gas, planetary observations), environment (pollution monitoring), etc. For telecommunications, secure local communications (through high attenuation outside the targeted area) with ultrahigh data rates will be possible. Imaging is an important application for security (weapon and illicit material detection), biology and medicine. The emergence of novel functional THz devices will be of immense interest for all those applications. One of the bottlenecks for the practical development of THz applications is the fabrication of room temperature (RT), continuous wave, compact, tunable and powerful sources (at low cost, if possible). For this sake, in the framework of the EU funded project ROOTHz, we propose to exploit THz Gunn oscillations in novel wide bandgap semiconductor nanodevices, which have been predicted by simulations but not experimentally confirmed yet, the Self Switching Diode (SSD). By breaking the symmetry of a narrow channel, SSD can provide a rectifying behaviour (based on surface and electrostatic effects) and using high-mobility material systems their operation frequency as detectors can approach the THz range at RT. Interestingly, the special geometry of SSDs also benefits the onset of Gunn oscillations.During this thesis, studies on Gallium nitride SSDs have been performed in order to evaluate their potentialities as emitters and detectors in the THz frequency range.

Contrairement à l’électronique conventionnelle à base de silicium, l’électronique organique offre de nouvelles possibilités telles que la production sur grande surface et à faible bilan thermique, ou encore l’utilisation de substrats flexibles et transparents. Afin d’améliorer la conductivité des semi-conducteurs ainsi que le contact polymère-électrode métallique, le dopage en électrons et en trous doit être développé dans les matériaux organiques. Dans cette thèse, des techniques de caractérisation électrique (courant tension à température variable, capacité, spectroscopie d’admittance), optique (spectroscopie UV-visible et de photoluminescence) et matériau (RMN, MEB, MET) ont été utilisées pour analyser l’influence de la concentration en dopant Mo(tfd-COCF3)3 sur les propriétés électriques du polymère PBDTTT-c et améliorer notre compréhension du mécanisme mis en jeu dans le dopage. La couche dopée a été intégrée avec succès dans un photodétecteur organique en utilisant une technique de laminage afin de remplacer la couche habituellement utilisée de PEDOT:PSS, connue pour générer des problèmes de stabilité dans le dispositif. Enfin, la technique de laminage et le savoir acquis sur le dopage des semi-conducteurs organiques ont permis d’étudier l’impact du dopage non intentionnel par l’oxygène sur les performances des photodétecteurs organiques. Bien qu’il soit encore nécessaire de renforcer notre compréhension sur le dopage des semi-conducteurs organiques, améliorer la technique de dépôt par laminage et introduire la couche dopée dans divers dispositifs imprimés, les résultats présentés dans cette thèse sont prometteurs pour le développement de l’électronique organique. / Organic electronics is a promising route for the next generation of electronic devices. With large area scalability, compatibility with flexible and semitransparent substrates, and low temperature processability, printed electronics offers an interesting alternative to conventional silicon-based electronics. On its way to achieve better performances, hole and electron doping needs to be developed to improve the material conductivity as well as the polymer-metal electrode contact. In this work, we have studied the electrical characteristics of the polymer PBDTTT-c upon addition of p-dopant Mo(tfd-COCF3)3. Complementary electrical (variable temperature current voltage, capacitance, admittance spectroscopy), optical (UV-visible absorption and photoluminescence spectroscopy) and material (NMR, SEM, TEM) characterization techniques have been used to analyze the impact of the doping concentration on the electrical properties of the polymer and improve our understanding of the doping mechanism involved. The doped layer was then successfully integrated in an organic photodetector using soft contact transfer lamination to replace the widely used PEDOT:PSS layer, known to be responsible for stability issues. Finally, both the lamination technique and the knowledge acquired on organic semiconductor doping were used to study the impact of unintentional oxygen doping on the organic photodetector performances.Although further works are necessary to complete our understanding of organic semiconductor doping, enhance the lamination processes and introduce doped layers in various solution printed devices, present results are promising for the improvement of organic electronic devices.

The aim of this thesis is to study Gamma-Ray Burst (GRB) progenitors and central engines, I begin by examining unexpected plateaus in GRB light curves and place constraints on the central engine, that are consistent with a proto-magnetar. Next I compare these to the normal plateaus seen in the light curve and expand my investigation to include flares. Finally I investigate whether some giant flares could actually be a GRB if the GRB in those light curves could be a progenitor.

Gamma-ray bursts are the most violent of known astrophysical events, with up to 10^53 ergs of energy released on the order of seconds. These extreme explosions, first observed in 1960s, form a fast moving field of research within astrophysics which relies on multi-wavelength observations of these transient events to probe the early- time (< 15 minutes) parameter space of these events. In the optical regime, follow-up observations to the prompt emission are ideally suited to the 2.0 metre Liverpool Tele- scope (LT), situated at an altitude of 2363m on the Observatorio Roque de las Mucha- chos (ORM), La Palma, Canary Islands. The LT is fully robotic and able to respond automatically to triggers of new gamma-ray burst (GRB) events, starting within 2-3 minutes of the detection of the prompt emission. The observed radiation from GRBs is released from relativistic jets. Launched from a black hole central engine, the energy within the jets is converted to the observed radi- ation predominantly through a synchrotron process, which can produce highly linearly polarised radiation. Polarimetric observations of this radiation are a key resource to infer the magnetic field structure of the emission region and distinguish between baryonic and magnetic models of jet physics. For this reason, the Liverpool Telescope has hosted the RINGO series of polarimeters which use a novel design to enable early-time polarimetric measurements of these highly variable optical sources. RINGO was mounted on the LT in 2005, and observed GRB 090102A providing a measure of 10.2 ± 1.3% average linear polarisation in the period of 160-220s post burst. RINGO2, mounted in June 2009 improved on the original design utilising a triggered electron multiplying CCD system. Both RINGO and RINGO2 were single band instruments. The development of RINGO3 extended the design of RINGO2 into a simultaneous 3 band polarimeter. This work focuses on the characterisation of RINGO2 and analysing the sample of GRB observations made during its lifetime. The observations of GRB 120308A provide measurements with a high confidence, inferring the existence of stable, or- dered magnetic fields within the jet. Analysis of other GRB afterglows observed with RINGO2 provides confidence in this result and confirm that jets can be highly magne- tised, with the majority of energy being contained in magnetic field recombination and not through kinetic energy of baryonic matter. RINGO3, a multi-band extension to RINGO2, was developed and tested within the Astrophysics Research Institute labs before being commissioned on the Liverpool Telescope in November 2012. Lab tests of instrument throughput with calculations of the signal to noise ratio across the operating wavelength defined the optimal cut-offs of the 3 wavelength bands. This instrument was then characterised using similar methods to RINGO2, and whilst not an ideal instrument was found to be within the required performance for the prime science goal of early-time GRB afterglow observations.

In this thesis I present the culmination of my research into the phenomenon of AGN winds in the X-ray regime using the Suzaku X-ray telescope. There are two studies described in this thesis. In the first I perform a deep broad-band spectral analysis of radio-quiet quasar MR2251-178 and outline the evidence which suggests that its X-ray continuum is partially-covered by ionised gas along the line of sight, possibly in the form of a disc-wind. In the second study I perform a survey for Fe xxv Heα and Fe xxvi Lyα absorption lines in a large sample of 51 Suzaku-observed AGN (99 observations). These absorption lines are unambiguously detected in 40% of the sample (20/51 AGN), consistent with recent results from XMM-Newton, with peak and mean absorber parameters of log(NH/cm⁻²) ≈ 23 and log(ξ/erg cm s⁻¹) ≈ 4.5. Their velocities cover a continuous range, from v_out < 1500kms⁻¹ to ~ 100,000kms⁻¹, with a median of ~ 0.056 c. The winds are located on sub-parsec scales ( < 0.1 pc, typically), placing them in the vicinity of the inner accretion disc. A correlation analysis shows that more powerful AGN launch more powerful winds which implies their being accelerated by radiation pressure. Indeed, the overall energetics of the outflows are quantitatively consistent with continuum-driving via Compton-scattering, although a magnetic origin cannot be ruled out on the basis of the available data. I find that the mean kinetic power of the outflows is ~ 1%L_bol, suggesting a possible feedback effect in their host galaxies could be important. Finally, I show that the measured outflow velocity distribution bares a striking resemblance to one inferred using the MBH - δ* relation for local quiescent galaxies which suggests that the observed MBH - δ* relation may be an artefact of wind-induced feedback. Overall, these results are consistent with the view that highly-ionised FeK winds may represent an important addition to the currently held AGN paradigm, and further enforce the hypothesis that they may be important for galaxy evolution.

We discuss the evolution of a relativistic outflow responsible for producing the emission associated with GRBs. We investigate how afterglows are produced in the inter- action between the outflow and the ambient medium. Understanding the properties of the outflow from afterglow emission can be coupled with information obtained from the prompt component to constrain the magnetisation of the outflow. We analytically and numerically evaluate the relative strength of the reverse shock emission as the out- flow propagates into either a wind or ISM -type environment. We find that previous estimates of magnetisation based on the relative strength of forward and reverse shock emission had been underestimated by up to a factor of 100. We then apply our revised magnetisation estimate to a sample of 10 GRBs and find that 5 of the 10 events can be described by the ISM model. As recent studies have indicated that the fraction of en- ergy stored in the magnetic fields are small, our findings would suggest that the ejecta is driven by thermal pressure. Finally we consider how inhomogeneities present in the outflow can lead to variations in the very early afterglow. Considering small gradi- ent in the ejecta density profile modifies the rising index of the afterglow and can be equivalent to changing the dimensionless parameter ξ by a factor of 2. Uncertainties in determining the width of the ejecta present difficulties in understanding the distribution of GRBs afterglow rising index.

Currently the Spitzer Space Telescope is the most reliable telescope for conducting secondary eclipse observations of exoplanets. The depth and the time of mid-eclipse are two important parameters that come from a secondary eclipse analysis. The eclipse depth gives information on the temperature of the atmosphere, and can provide evidence for the presence of molecules in the atmosphere of the planet. If multiple wavelengths have secondary eclipse depths measured then it is possible to constrain the spectral energy distribution (SED) of the atmosphere given some assumptions on, for example, the metallicity of the planet's atmosphere. The time of mid-eclipse gives e cos which, with an analysis including transit, radial velocity and secondary eclipse data, can strongly constrain the eccentricity of the planet's orbit. To fully understand the conclusions drawn from these two parameters realistic error bars must be quoted on the measurement of these parameters. It is generally understood that error bars that come from MCMC analyses of secondary eclipse observations are underestimated because the correlated noise in the data is not accounted for in the analysis. The goal of this thesis was to find a method to improve the estimates of the uncertainties on these two parameters as derived from Spitzer secondary eclipse lightcurves at 3.6 um and 4.5 um. This work was conducted through the generation and fitting of semi-synthetic Spitzer secondary eclipse light curves. I estimate the amount the uncertainties on these parameters need to be inflated by and show how my results compare with other similar work in the field. I show that the amount of inflation does affect the conclusions drawn when fitting these data with model atmospheres. This could also mean that for systems where complex chemistry is invoked to explain the observed data, simpler model can now fit the data due to the increase in the error bars. I also find that when multiple realisations of the same, simulated, secondary eclipse lightcurve are fit with the standard MCMC code, the amplitude and time of mid-eclipse can be recovered and found to be more than 3 away from the true value of the injected signal. This can mean that, because usually only 1 lightcurve is obtained per observation of the secondary eclipse, some detections of eccentricity and molecules may not be real detections but simply a result of noise in the data.

Patrick L. Van Hove. / Originally presented as author's thesis (Ph. D.--Massachusetts Institute of Technology), 1986. / Bibliography: p. 259-263. / Supported in part by the Advanced Research Projects Agency monitored by ONR under contract no. N00014-81-K-0742 Supported in part by the National Science Foundation under grant ECS-8407285