Während seiner normalen Funktion generiert das Gehirn starke elektrische Signale, die technisch gemessen werden können. Das schon seit über einem Jahrhundert bekannte Phänomen ermöglicht es die Signalverarbeitung im Gehirn räumlich und zeitlich zu beobachten. Heute versteht man die zellulären Prozesse die zur Generierung der elektrischen Signale in einzelnen Neuronen führen. Jedoch rekrutieren die meisten neuronalen Ereignisse große Populationen von Zellen, dessen Aktivität zeitlich und räumlich koordiniert ist. Diese Koordinierung führt dazu, dass ihre elektrische Aktivität auch weit von den Quellen gemessen werden kann, sodass die Beobachtung des Gehirns auch nicht invasiv auf der Schädeloberfläche mittels dem sogenannten Elektroenzephalogramm (EEG) möglich ist. Der zeitliche Verlauf des Signals hängt nicht nur von den Eigenschaften einzelner Zellen ab sondern auch von ihrer Wechselwirkung mit anderen Neuronen, die oft komplex oder gar nicht bekannt ist. Diese Komplexität verhindert die Auswertung der gemessen Signale im Bezug auf die Anzahl von aktiven Neuronen, die Art der Antwort (Inhibition, Exzitation), die Synchronisationsstärke und den Einfluss anderer aktiver Prozesse (wie zum Beispiel: Lernen, Aufmerksamkeit usw.). In dieser Arbeit werden die Zusammenhänge zwischen diesen mikroskopischen Parametern (einzelne Neurone) und ihrer makroskopischen Wirkung (EEG) experimentell, datenanalytisch und theoretisch untersucht. / During its normal function the brain generates strong and measurable electric signals. This phenomenon, which has been known for more than a century, makes it possible to investigate the signal processing in the brain. Nowadays the cellular processes taking part in the generation of the electric signals are well understood. However, most of the neuronal events recruit large populations of cells, whose activities are coordinated spatially and temporally. This coordination allows for summation of activities generated by many neurons leading to extracellular electric signals that can be recorded non-invasively from the scalp by means of electroencephalography (EEG). The temporal structure of the EEG signal does not depend only on the properties of single neurons, but also on their interactions that may be very complex. The complexity hinders the evaluation of the recoded signal with respect to the number of active neurons, the type of response, the degree of synchronisation and the contribution of other processes (such as, learning and attention). In the thesis, the relations between the microscopic (single-neuron) and their macroscopic (EEG) properties will be investigated by means of experimental, data-analytic and theoretical approaches.

published_or_final_version / abstract / Physics / Master / Master of Philosophy

We present axisymmetric hydrodynamical simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a "collapsar," onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in two spatial dimensions and with an explicit shear viscosity. The evolution of the central accretion rate exhibits phases reminiscent of the long GRB [gamma]-ray and X-ray light curve, which lends support to the proposal by Kumar et al. (2008a,b) that the luminosity is modulated by the central accretion rate. In the first "prompt" phase, the black hole acquires most of its final mass through supersonic quasiradial accretion occurring at a steady rate of [scientific symbols]. After a few tens of seconds, an accretion shock sweeps outward through the star. The formation and outward expansion of the accretion shock is accompanied with a sudden and rapid power-law decline in the central accretion rate Ṁ [proportional to] t⁻²̇⁸, which resembles the L[subscript x] [proportional to] t⁻³ decline observed in the X-ray light curves. The collapsed, shock-heated stellar envelope settles into a thick, low-mass equatorial disk embedded within a massive, pressure-supported atmosphere. After a few hundred seconds, the inflow of low-angular-momentum material in the axial funnel reverses into an outflow from the thick disk. Meanwhile, the rapid decline of the accretion rate slows down, which is potentially suggestive of the "plateau" phase in the X-ray light curve. We complement our adiabatic simulations with an analytical model that takes into account the cooling by neutrino emission and estimate that the duration of the prompt phase can be ~ 20 s. The model suggests that the steep decline in GRB X-ray light curves is triggered by the circularization of the infalling stellar envelope at radii where the virial temperature is below 10¹⁰ K, such that neutrino cooling is inefficient and an outward expansion of the accretion shock becomes imminent; GRBs with longer prompt [gamma]-ray emission should have more slowly rotating envelopes. / text

Les sursauts gamma sont des sources astrophysiques parmi les plus brillantes du ciel. Dans le modèle standard actuel, leur émission prompte (X et gamma) est due à des particules chargées accélérées au sein de jets relativistes émis à la formation de trous noirs de masses stellaire. L'émission rémanente observée de la radio aux X serait due à l'interaction de ces jets avec le milieu interstellaire. Le LAT, détecteur à création de paire du télescope spatial Fermi, permet depuis juin 2008 l'étude du ciel gamma de 20 MeV à plus de 300 GeV avec des performances inégalées. Le GBM, détecteur de sources transitoires de Fermi (8 keV à 40 MeV) a observé ~450 sursauts gamma, dont ~18 ont été observés jusqu'au domaine du GeV. Une localisation précise de ces sursauts et la synergie de Fermi avec les autres observatoires permettent l'étude des rémanences associées et une meilleure interprétation des observations. L'étude de sursauts gamma de 8 keV au domaine du GeV est présentée. Les localisations obtenues avec le LAT sont étudiées ainsi que leurs erreurs. Des analyses spectrales des émissions promptes combinant les données du GBM et du LAT sont exposées, ainsi que leur interprétation. Une analyse alternative basée sur une sélection relâchée des données LAT est présentée et caractérisée. L'utilisation des événements d'énergies inférieures à 100 MeV améliore l'analyse temporelle et spectrale des émissions promptes. La recherche d'émission gamma prolongée est présentée, ainsi que l'étude de l'émission rémanente de GRB 090510 observé des UV au GeV par Fermi et Swift. Enfin, un modèle d'émission prompte par les chocs internes, développé à l'IAP, est comparé aux observations de Fermi. / Gamma-Ray Bursts (GRB) are among the brightest gamma-ray sources in the sky. The current standard framework associates their prompt gamma-ray emission to charged particles accelerated in relativistic jets issued by newly-formed stellar-mass black holes. The radio to X-ray afterglow emission is due to the interaction between these jets and the interstellar medium.The LAT, pair-creation instrument onboard Fermi gamma-ray space telescope, performs unprecedented observation of the gamma-ray sky at energies of 20 MeV to over 300 GeV since its launch in june 2008. Fermi's transient sources detector, GBM, observed prompt emissions of ~450 GRB between 8 keV and 40 MeV. ~18 of these GRB were also studied up to GeV energies with the LAT. Accurate GRB localizations and Fermi's synergy with other observatories allows the study of GRB afterglows, and therefore a better interpretation of these observations.The analyses of GRB emissions between 8 keV to GeV energies is presented here. Localizations based on LAT data and their biases are studied. Spectral analyses of combined GBM and LAT data are shown, and their theoretical interpretations explained.An alternative analysis based on a relaxed selection of LAT data is presented and fully characterized. It allows to recover and use low-energy LAT statistics in temporal and spectral analyses of GRB prompt emission.Searches for long-lived high-energy emission from GRB are presented. The analysis of GRB 090510 afterglow emission from eV to GeV energies is described.Finally, Fermi bright GRB prompt emissions are compared to an internal shock model developed at IAP.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

Les expériences d’astroparticules n’ont cessé de fleurir ces dix dernières années dans le but de connaître l’origine des rayons cosmiques et les procédés mis en œuvre dans leur accélération à des ultra-hautes énergies. Les télescopes Cherenkov tels que ANTARES détectent la lumière Cherenkov issue des muons se propageant dans l’eau de mer méditerranée. Les muons sont les produits d’interaction de neutrinos cosmiques (signal) ou atmosphériques (bruit) avec les noyaux de la terre. Pour offrir les meilleures conditions de détection de la lumière Cherenkov, et par conséquent bien reconstruire les traces des muons détectés, les détecteurs de lumière i.e les photomultiplicateurs doivent satisfaire plusieurs conditions. Le premier volet de cette thèse porte sur le développement de photomultiplicateurs utilisant le cristal comme préamplificateur de signal optique. L’avantage d’un tel procédé est la possibilité de séparer plusieurs pics photo-électrons et d’améliorer la résolution tem- porelle grâce à un fort champ électrique. La conception d’un tel prototype et la possibilité de son industrialisation à grande échelle sont développés pour la première fois dans cette thèse. Le deuxième volet s’inscrit dans la nouvelle thématique multi-messagers d’ANTARES.Un suivi avec six télescopes optiques répartis sur chaque hémisphère est déclenché lorsque ANTARES détecte un neutrino de haute énergie ou un ensemble de neutrinos coïncidents en temps et en direction. Un tel système permet d’augmenter la sensibilité de détection de neutrinos provenant de sources transitoires tels que les sursauts gamma et les Supernovæ à effondrement de cœur. Un système d’alertes neutrino est maintenant implémenté et fonctionne continument et de manière autonome sur ANTARES. Le système offre de bonnes performances; le suivi optique se déclenche dans la minute suivant la détection neutrino et l’incertitude sur la reconstruction de la trajectoire du neutrino est de 0.4◦ (valeur médiane). Une fois l’alerte reu çe par un des télescopes, celui-ci se repositionne sur la direction du neutrino et entame une série d’observations réparties sur le mois suivant l’alerte. Cette thèse a porté dans un premier temps sur l’aspect neutrino en développant le filtre sélectionnant les évènements haute énergie et multi-neutrinos. Des outils d’analyse d’images optiques ont ensuite été implémentés pour la recherche de sources transitoires. Les premiers résultats de recherche de contreparties optiques de sursauts gamma en corrélation avec un signal neutrino sont présentés. / Astroparticle experiments are being developed with the aim of resolving the mystery of cosmic rays. Questions like from where cosmic rays originate or how do they get accelerated up to ultra high energies are still unresolved. Cherenkov telescopes like ANTARES detect the Cherenkov light emitted by muons propagating in the sea water. Muons are produced by the interaction of cosmic neutrinos (signal) or atmospheric neutrinos (noise) with the Earth nuclei.In order to offer the best detection conditions of Cherenkov light, and subsequently ensure a good reconstruction quality of the muons trajectory, the light sensors i.e the photomultipliers must satisfy various conditions.The first part of this thesis focuses on the development of photomultipliers using crystals as a preamplifier of the light signal. The advantage of such a method is the possibility to separate individual photo-electrons and to enhance the temporal resolution by applying a high electric field.The design and conception as well as the possibility to produce such devices at large industrial scales is for the first time developed in this thesis.The second part of the thesis is inspired from the new multi-messenger approach in ANTARES. An optical follow-up with six telescopes spread over the two hemispheres is triggered by the detection of a high energy neutrino or a set of neutrinos coincident in time and direction. Such a system enhances the sensitivity to transient sources such as Gamma Rays Bursts (GRB) or Core Collapse Supernovae. The neutrino alert system is now fully operational. The system offers good performances; the optical follow-up starts within one minute from the neutrino detection and the median angular accuracy of the reconstructed neutrinos is 0.4$^circ$.Upon the reception of an alert, the telescopes point at the neutrino direction and start the acquisition of image series almost each night of the month following the neutrino alert. Image analysis tools have been implemented to search for optical transients; First results on the search of GRB optical counterparts correlated with a neutrino signal are presented.

A Thesis presented to the Department of Geophysics of the University of the Witwatersrand, Johannesburg / Violent failure of the rock surrounding under ground excavations forms a major hazard and obstacle in deep-level mining. (Abbreviation abstract) / AC 2018

Cette thèse présente les résultats de la recherche de signaux impulsionnels d’ondes gravitationnelles associés aux sursauts gamma dans les données 2009-2010 des interféromètres LIGO-Virgo. L’étude approfondie des mécanismesd’émission d’ondes gravitationnelles par les progéniteurs de sursauts gamma, ainsi que des mécanismes d’émission de rayons gamma eux-mêmes, permet de déterminer les caractéristiques essentielles du signal à détecter : polarisation, délai temporel, etc ... Cette connaissance de l’émission conjointe permet alors de construire une méthode d’analyse qui inclut les a priori astrophysiques. Cette méthode est de plus robuste vis-à-vis des bruits transitoires présents dans les données. L’absence de détection nous permet de placer des limites observationnelles inédites sur la population des sursauts gamma. / In this thesis we present the results of the search for gravitational wave bursts associated with gamma-ray bursts in the 2009-2010 data from the LIGOVirgo gravitational wave interferometer network. The study of gamma-ray bursts progenitors, both from the gamma-ray emission and the gravitational wave emission point of view, yields the characteristic of the sought signal: polarization, time delays, etc ... This knowledge allows the construction of a data analysis method which includes the astrophysical priors on joint gravitational wave and gamma-ray emission, and moreover which is robust to non-stationary transient noises, which are present in the data. The lack of detection in the analyzed data yields novel observational limits on the gamma-ray burst population.

Esta tese apresenta a caracterização de desempenho dos serviços executados em um ambiente em nuvem, quando são consideradas rajadas de diferentes origens, intensidades e variabilidades nas cargas de trabalho. Os resultados mostram que a presença de rajadas no processo de chegada das requisições e/ou nas demandas de serviço, ocasiona uma considerável degradação no desempenho dos serviços e, portanto, devem ser consideradas nos modelos de cargas de trabalhos e nas atividades voltadas para avaliação de desempenho em computação em nuvem. Considerando-se a grande influência das rajadas, é proposta e validada uma metodologia que permite monitorar uma carga de trabalho e determinar a ocorrência de rajadas tanto nas taxas de chegadas de requisições quanto nas demandas de serviços. A metodologia utilizada na condução deste trabalho consta de diferentes modelos de cargas de trabalho com rajadas de diferentes variabilidades e intensidades, desenvolvidos e integrados à arquitetura CloudSim-BEQoS proposta nesta tese. Utilizando-se essa arquitetura é possível executar um conjunto de experimentos que possibilitam a obtenção dos resultados que caracterizam o desempenho dos serviços quando são criadas condições de rajadas nas cargas de trabalho submetidas à nuvem. / This thesis presents the performance characterization of the services executed in a cloud environment, when bursts are considered from different sources, intensity and variability in the workload. The results show that the presence of bursts in the arrival process of requests and/or in service demands, causes a significant degradation in the performance of services and therefore should belong to the models of workloads and in the activities considered for performance evaluation in cloud computing. Considering the great influence of bursts, a methodology to monitor a workload and predict the occurrence of bursts in both the rates of request arrivals and the service demands is proposed and validated. The methodology used in the conduction of this work consists in different types of workloads with bursts of different variability and intensity, developed and integrated into the CloudSim-BEQoS architecture proposed in this thesis. By using this architecture it is possible to execute a set of experiments that enable the achievement of the results that characterize the performance of services when bursts conditions are created in the workload submitted to the cloud.

To gain understanding of the physical and structural events in the turbulent bursting process, an effort to generate artificial bursts in a turbulent boundary layer was made. Turbulent bursts, which are both random in time and in space, and cause a large portion of drag of a turbulent boundary layer. Control of the bursts could yield a decrease in skin friction and hence drag. Data were taken in a turbulent boundary layer developed over a flat plate in a low-speed wind tunnel with an array of eight hot-wire probes. The turbulent burst like events were created by pitching a rectangular shaped piezoelectric bimorph actuator out into the flow. The actuator effect is proposed which models the production of counter-rotating vortices in a rectangular vortex filament configuration. The results are compared with naturally occurring bursts and data from previous studies. Knowing more about the turbulent bursting process gives us more opportunities to control the turbulent bursts and therefore reduce drag over airfoils.