The modern fields of 𝜸-ray astrophysics, multi-messenger astronomy, and particle astrophysics, have a shared history and future. This thesis will focus on the interconnectedness between them, and the role that measurements of very high-energy (VHE; 100 GeV < 𝐸 < 100 TeV) 𝜸 rays play in contributing to them. In particular, one confirmed and one putative source class for VHE 𝜸 rays will be explored: active galactic nuclei (AGN) and short gamma-ray bursts (sGRBs) (associated binary neutron star (BNS) merger progenitors), characterized by their variable and transient emission behavior respectively. The particle acceleration and radiative processes which can result in this energetic electromagnetic (EM) emission in these sources is described. Subsequently, the detection principles of VHE 𝜸-ray photons is detailed, highlighting the unique detection technique of imaging atmospheric Cherenkov telescopes (IACTs) to use the Earth’s atmosphere as an effective high-energy particle calorimeter. Advancements in the field of these detectors will be discussed, from the first-generation of detectors up to its current generation, which includes the VERITAS 𝜸-ray observatory, the principal instrument used to conduct the analyses in this thesis. Additionally, a prototype for the novel two-mirror Schwarzschild-Couder telescope (SCT) design will be introduced, which is proposed for inclusion in the future Cherenkov Telescope Array Observatory (CTAO), set to be the next-generation observatory for VHE 𝜸 rays. Finally, work from three projects, each demonstrating the contributions VHE ? rays can make to the initially listed fields, will be discussed. First, a multi-messenger search for VHE 𝜸-ray emission in VERITAS associated with sub-threshold gravitational wave (GW) signals. No VHE ?-ray emission is detected; the upper limit on the integral flux for all VERITAS observations is reported and compared to a sGRB model. Next, a search for the imprint of beyond-the-standard-model axion-like particles (ALPs) on the flaring VHE spectrum of the radio galaxy (RG) AGN NGC 1275, embedded in the cool-core Perseus cluster. No detection is made; limited exclusions on the mass and coupling strength to electromagnetism are reported. Finally, a VERITAS detection of a VHE ?-ray flare from the blazar AGN B2 1811+31 will be presented. Preliminary work from the broadband analysis of the source are shown, and the future of the project is discussed.

Astrophysical phenomena such as self-annihilation of primordial black holes (PBHs), gamma ray bursts (GRBs), and supernovae are expected to produce single dispersed pulses detectable in the low end of the radio spectrum. Analysis of these pulses could provide valuable information about the sources, and the surrounding and intervening medium. The Eight-meter-wavelength Transient Array (ETA) is a radio telescope dedicated to the search for these pulses in an 18 MHz bandwidth centered at 38 MHz. ETA consists of 10 dual-polarized active dipoles providing an all-sky field of view. This thesis describes the results of a search campaign using ETA, namely, a Crab giant pulse (CGP) search. CGPs are already known to exist, and thus provide an excellent diagnostic for system performance. We found 11 CGP candidates in 14 hours of data. Although there has not been a single compelling detection (signal-to-noise ratio > 6), our analysis shows that at least a few of these candidates may be CGPs. We also explain the analysis preparation for PBH and GRB searches. Additionally, we describe the instrument and a software "toolchain" developed for the analysis of data that includes calibration, radio frequency interference (RFI) mitigation, and incoherent dedispersion. A dispersed pulse simulation code was developed and used to test the toolchain. Finally, improvements are suggested. / Master of Science

Some of the brightest and most energetic events in the Universe are associated with the death of stars. These stellar deaths power transient electromagnetic emission which are routinely observed on Earth. This dissertation presents our research on various such transients. Its topics includes, supernova remnants, kilonovae, gamma-ray bursts (GRBs): The "long'' type produced from core-collapse supernovae and the "short'' type associated with neutron star merger events. It also focuses on the disruption of stars by the tidal forces of supermassive black holes i.e., tidal disruption events (TDEs). We model the emission from these transients and compare them to observations in order to draw a number of conclusions and make predictions for future detections. For example, we find that the non-thermal emission from supernovae and kilonovae associated with GRBs can produce long term emission which may be detected as a re-brightening in the overall emission. The sharp cut off observed in some TDE flares can be caused by a pre-existing accretion disk present around a supermassive black hole, which is expected in active galactic nuclei. Our work successfully predicted the nature of the very first electromagnetic detection from a neutron star merger, and was able to reproduce the emission that had been observed for more than one hundred days after the merger. This dissertation also provides frameworks on how the observable features of these transients can be leveraged to probe the properties of the progenitor system and their environment. <br>

Les explosions gamma (GRB) sont des évènements extrêmement violents. Ils sont sommairement classifiés en deux groupes par leur durée : les courts et les longs. Cette classification a permis de déterminer l'origine des GRBs : une collision entre deux objets compacts pour les courts ou l'explosion d'une étoile très massive pour les longs. Une meilleure classification des GRBs longs pourrait mieux contraindre leurs propriétés. Dans ma thèse, je présente des évidences de l'existence d'une sous-classification des GRBs basés sur la faible luminosité de leurs derniers reflets. Ces explosions sont appelées Low-Luminosity Afterflow (LLA). Je présente la technique de réduction des données, la méthode de sélection de ces GRBs ainsi que leurs principales propriétés. Leur lien avec les supernovæ (SN) est mis en évidence car 64 % de tous les GRBs associés à des Sns sont des LLA GRBs. Finalement, je présente d'autres propriétés comme leur fréquence, qui semble indiquer une nouvelle distincte classe, les propriétés de leurs galaxies hôte qui montrent que ces explosions ont pour origines des galaxies formant beaucoup d'étoiles. De plus, je montre qu'il est difficile de réconcilier les différences entre les GRBs normaux et les LLA GRBs en ne considérant que des effets instrumentaux et environnementaux, ou bien une géométrie différente. Donc je conclue que les deux classes de GRBs ont des propriétés différentes. En basant l'argumentation sur la fonction de masse initiale, sur la fréquence des LLA GRBs et sur le type de SNs qui les accompagnent, j'indique qu'un système binaire est favorisé pour leur origine. / Gamma-ray bursts (GRB) are extreme events. They are crudely classified into two groups based on their duration, namely the short and long bursts. Such a classification has proven to be useful to determine their progenitors: the merger of two compact objects for short bursts and the explosion of a massive star for long bursts. Further classifying the long GRBs might give tighter constraints on their progenitor and on the emission mechanism(s). In my thesis, I present evidence for the existence of a sub-class of long GRBs, based on their faint afterglow emission. These bursts were named low-luminosity afterglow (LLA) GRBs. I discuss the data analysis and the selection method, and their main properties are described. Their link to supernova is strong as 64\% of all the bursts firmly associated to SNe is LLA GRBs. Finally, I present additional properties of LLA GRBs: the study of their rate density, which seems to indicate a new distinct third class of events, the properties of their host galaxies, which show that they take place in young star-forming galaxies. Additionally, I show that it is difficult to reconcile all differences between normal long GRBs and LLA GRBs only by considering instrumental or environmental effects, different ejecta content or a different geometry for the burst. Thus, I conclude that LLA GRBs and normal long GRBs should have different properties. In a very rudimentary discussion, I indicate that a binary system is favored in the case of LLA GRB. The argument is based on the initial mass function of massive stars, on the larger rate density of LLA GRBs compared to the rate of normal long GRBs and on the type of accompanying SNe.

Gamma Ray Burst (GRB) are very bright cosmological explosions signalling the catastrophic formation of a black hole. Therefore, they act like real light beacons that could be detected through-out the Universe and be used as probes to study the contents and phases of the early Universe. However, only a handful sample is known so far. This is for two reasons: instrumental biases that may prevent their detection and the difficulty to find a near Infrared counterpart preventing their redshift measurements. The wide-field trigger camera ECLAIRs to-fly on-board the Space-based multi-band Variable Object Monitor (SVOM) mission will detect γ-/X-ray transients down to energies of 4 keV, as well as creating an alert for multi-wavelength/messenger follow-ups. My study focuses on analysing how ECLAIRs will detect GRBs, and more particularly high-redshift GRBs, based on a well-selected sample of GRBs with redshift measurement associated (see Section 2). Studying how ECLAIRs will see them may help identifying possible instrument biases as well as common observational characteristics for such GRBs that may be used in turn to recognise such special GRBs once SVOM will be launched. Using software tools developed within the ECLAIRs collaboration, I built an end-to-end simulator which I used to simulate the detection by ECLAIRs of the GRBs in the sample at their original redshift and higher redshifts (up to z = 15). I implemented a suited version of the count-rate trigger on-board ECLAIRs to assess the detectability of these bursts, and I retrieved their duration over the background when detected (see Section 2). The analysis shows good performance for detecting high-redshift GRBs in the centre of the Field of View (fully-coded), but significantly reduced, in comparison to other GRBs, for partially-coded detection. 5 of the GRBs with z &gt; 3.83 present a successful detection up to at least z = 15 (see Section 3). The retrieved rest-frame duration of a GRB remains constant for several redshifts in the simulations if the detected burst did not present a low-flux emission in their lightcurve, which is common for high redshift GRBs. On the other hand, if the original lightcurve of a burst presents this low-flux emission, it becomes buried in noise when simulating it at higher redshifts. This confirms the tip-of-the-iceberg detection bias which depends on the lightcurve burst morphology, and it may explain why the current sample seems to present lower burst durations at higher redshifts.

Gammastrahlenblitze (Gamma-Ray Bursts, GRBs) gehören zu den energiereichsten transienten Ereignissen im Universum und werden als mögliche Quellen von ultra-hochenergetischen kosmischen Strahlen (Ultra-High-Energy Cosmic Rays, UHECRs) gehandelt. Eine eindeutige Bestätigung durch UHECR-Messungen ist jedoch schwierig, da die Richtungsinformation der kosmischen Strahlen während ihrer Ausbreitung aufgrund von Ablenkung durch Magnetfelder teilweise verloren geht. In dieser Dissertation folgen wir einem alternativen multi-messenger Ansatz in welchem die Anwesenheit von kosmischen Strahlen in einem astrophysikalischen Objekt durch Neutrino- oder Photon-Signaturen angezeigt wird. Hierfür simulieren wir GRBs im Internal-Schock-Szenario, welches verschiedene Emissionszonen entlang des astrophysikalischen Jets erfasst, und berechnen nukleare Wechselwirkungen mit modernsten numerischen Codes. In diesem Rahmen diskutieren wir unter welchen Voraussetzungen die Quellklasse von GRBs UHECR-Daten beschreiben kann ohne Neutrinolimits. Letzere begründen sich im Mangel an gemessenen hoch-energetischen (High-Energy, HE) Neutrinos, die mit bekannten GRBs assoziert werden konnten. Die Neutrinolimits können alternativ in Objekten niedriger Leuchtkraft eingehalten werden, die typischerweise eine niedrige Neutrinoproduktionseffizienz haben. Wir präsentieren leptonische Strahlungsmodellierungen für die Unterklasse von GRBs niedriger Leuchktraft mit einem Fokus auf sehr hoch-energetischer (Very-High-Energy, VHE) Emission welche von aktuellen/zukünfitgen Instrumenten beobachtet werden könnte und bestimmen wir die maximalen Energien verschiedener Atomkerne. Die Präsenz von Hadronen kann alternativ durch Signaturen in verschiedenen Wellenlängen des Photonspektrums angezeigt werden. Wir erforschen diesen Ansatz in lepto-hadronischen Modellen für GRBs mit hoher Leuchtkraft, wobei wir kritisch diskutieren, welche Bedingungen erfüllt sein müssen damit typische GRB-Spektren reproduziert werden können. / Gamma-Ray Bursts (GRBs) are among the most energetic transients in the Universe and candidate sources of Ultra-High-Energy Cosmic Rays (UHECRs). A clear confirmation from UHECR measurements is however challenging, as the directional information of cosmic rays is partially lost due to deflection by (inter-)galactic magnetic fields. In this dissertation we follow an alternative multi-messenger approach, in which the presence of UHECRs in an astrophysical object is indicated by neutrino or photon signatures produced in nuclear interactions. For this, we simulate GRBs in the multi-zone internal shock model, which accounts for different emission zones along the astrophysical jet and calculate nuclear interactions with state-of-the-art numerical codes. In this framework we discuss under which conditions the population of GRBs can still account for UHECR measurements while obeying current neutrino limits that stem from the lack of detected High-Energy (HE) neutrinos which could be associated with known GRBs. These neutrino limits may alternatively be met in low-luminosity objects, which typically have low neutrino production efficiency. We present leptonic radiation models of the sub-class of low-luminosity GRBs, with a focus on Very-High-Energy (VHE) emission potentially observable by current/future instruments. Connecting to UHECRs, we determine maximal energies of different cosmic-ray nuclei. The presence of nuclei may also be indicated by multi-wavelength signatures in the photon spectrum. We explore this approach in lepto-hadronic models of high-luminosity bursts, where we also critically review the conditions necessary to reproduce typical GRB spectra within our model.

No description available.

In der Astronomie zeigen viele Quellen ein veränderliches Verhalten, das in manchen Fällen sehr kurz sein und im Bereich von Sekunden bis Minuten liegen kann. Beispiele dafür sind Gammablitze (GB) und aktive Galaxienkerne (AGK). Um die zugrundeliegenden physikalischen Prozesse besser zu verstehen, ist eine Messung der Photonenflussänderung notwendig. Allerdings kann die Entdeckung sehr kurzzeitiger Ausbrüche im sehr hochenergetischen Bereich des Spektrums, für welche Cherenkov Teleskope, wie VERITAS, zum Einsatz kommen, schwierig sein. Die Ursache dafür ist der hohe Untergrund und die relativ niedrige Signalrate. Diese Arbeit behandelt die Implementierung fortschrittlicher und für die Entdeckung signifikanter Ratenänderung optimierter, statistischer Methoden (exp-test and Bayesian-Blocks) innerhalb der VERITAS-Analyse. Das Verhalten dieser Methoden wird anhand von Daten minutenlanger Ausbrüche, die mittels Monte-Carlo-Technik für zwei unterschiedliche VERITAS Stadien simuliert wurden (vor und nach der Hardware-Aufrüstung), bewertet und miteinander verglichen. Die fortschrittlichen Methoden verbessern die Empfindlichkeit bei der Entdeckung von kurzzeitigen Ausbrüchen, wenn deren Fluss den des Krebsnebels übersteigt (Krebsnebel-Einheit = K.E.). Es sind nun Ausbrüche von 2 K.E. selbst dann nachweisbar, wenn deren Dauer nur 100 Sek. beträgt. Dies ist mit der Standardmethode nicht möglich. Im darauffolgenden Schritt dieser Arbeit werden das Nachglühen von 6 GB sowie die 450, ebenfalls mit VERITAS erzeugten, Aufnahmen von AGK-Quellen mittels der fortschrittlichen Methoden analysiert. In keiner der AGK-Aufnahmen wird eine signifikante Entdeckung kurzzeitiger Variabilität gemacht, was mit den anerkannten AGK-Modellen übereinstimmt. Auch die Untersuchung des Nachglühens der 6 GB offenbart keine kurzen Ausbrüche im GeV-TeV Bereich. Jedoch ist es für 2 von ihnen möglich, ein oberes Flusslimit von 1.25 K.E. (3,7 · 10^(−10) erg/(cm^2*s) [0,1;10 TeV]) abzuschätzen. / In astronomy, many of the observed sources show a transient behavior. Examples are gamma-ray bursts (GRBs) and active galactic nuclei (AGN). For those source types the variability can be very short, in the order of seconds to minutes. Measuring the flux variations is necessary to understand the underlying physical processes responsible for the emission. However, the detection of very short flares can be difficult in the very high-energy range, in which imaging atmospheric Cherenkov telescopes like VERITAS are operating. This is due to the large background and the comparable low signal rates. This thesis discusses the implementation of advanced statistical methods (exp-test and Bayesian-Blocks) into the VERITAS analysis framework, that are optimized for the detection of significant variations in the event rate. The performance of these methods is evaluated and compared by using Monte Carlo simulations of minute scale flares for two different VERITAS states, pre- and post-hardware-upgrade. It is shown that the advanced methods can improve the detection sensitivity for short flares with high fluxes of more than the Crab flux (Crab unit = C.U.). For example, flares at 2 C.U. with short durations down to 100 sec are now detectable, which is not possible with the standard method. In the next step of this thesis, 6 GRB afterglows and 450 runs of AGN data, observed by VERITAS, are analyzed with the advanced methods. In none of the AGN runs a significant detection of short time variability is made, which is in consistence with the canonical AGN models. The investigation of the 6 GRB afterglows also did not reveal any short flares in the GeV-TeV range. However, for two of them it was possible to estimate an upper flux limit of 1.25 C.U. (3,7 * 10^(−10) erg/(s*cm^2) [0,1;10 TeV]).

Les sursauts gamma (GRBs pour "Gamma-Ray Bursts" en anglais) sont de brèves bouffées très énergétiques de rayonnement de haute énergie qui sont émises sur de courtes échelles de temps (fraction de seconde à plusieurs minutes). L'émission intense des sursauts gamma à haute énergie est supposée provenir d'un trou noir de masse stellaire nouvellement formé, accompagné d'un vent collimaté (i.e. un jet) se propageant à vitesse relativiste. L'émission est observée suivant deux phases successives, la phase prompte très erratique, et la phase de rémanence, moins lumineuse. Les deux instruments embarqués sur le satellite Fermi, le "Gamma-ray Burst Monitor" (GBM) et le "Large Area Telescope" (LAT), permettent d'étudier l'émission prompte des sursauts gamma sur une grande plage d'énergie (de ~10 keV à ~100 GeV). L'objectif principal de ma thèse est l'analyse et l'interprétation des propriétés spectrales et temporelles de l'émission prompte des GRBs observés par Fermi, en particulier avec les nouvelles données du LAT (Pass 8) qui ont été rendues publiques en juin 2015.La première partie de mon travail est une analyse spectrale résolue en temps de la phase prompte du sursaut GRB 090926A avec les données du GBM et du LAT. Mes résultats confirment avec un meilleur niveau de confiance la présence d'une cassure spectrale à ~400 MeV, qui est observée en coincidence avec un pic d'émission très court. Ils révèlent que cette atténuation spectrale est présente durant toute l'émission prompte du sursaut, et que l'énergie de cassure augmente jusqu'au GeV. L'interprétation de la cassure spectrale en termes d'absorption gamma ou de courbure naturelle du spectre d'émission Compton inverse (CI) dans le régime Klein-Nishina fournit des contraintes fortes sur le facteur de Lorentz du jet. Mes résultats conduisent en outre à des rayons d'émission R ∼10^14 cm qui sont compatibles avec une origine interne de l'émission du keV au GeV au-dessus de la photosphère du jet.La seconde partie de mon travail est une exploration du modèle de chocs internes développé par des collaborateurs à l'Institut d'Astrophysique de Paris (IAP). Ce modèle simule la dynamique du jet et les processus d'émission (synchrotron et CI) d'une population d'électrons accélérés aux chocs. J'ai simulé la réponse instrumentale de Fermi à un sursaut synthétique fourni par ce code numérique, et j'ai construit une fonction paramétrique qui peut être utilisée pour ajuster le modèle aux spectres de sursauts du keV au MeV. J'ai appliqué cette fonction avec succès à un échantillon de 64 sursauts brillants détectés par le GBM. J'ai aussi confronté le modèle de l'IAP au spectre d'émission prompte de GRB 090926A. Mes résultats montrent un bon accord, et j'ai identifié quelques pistes pour les améliorer. Les spectres synthétiques sont plus larges que tous les spectres dans l'échantillon du GBM. En conséquence, je discute brièvement quelques pistes de développements théoriques qui pourraient améliorer l'accord du modèle avec les observations, ainsi que des avancées observationnelles attendues dans le futur. / Gamma-Ray Bursts (GRBs) are very energetic and brief flashes of high-energy radiations which are emitted in a short time scale (fraction of a second to several minutes). The GRB bright emission is thought to be powered by a newly formed stellar-mass black hole that is accompanied by a collimated outflow (i.e. a jet) moving at a relativistic speed. The emission is observed as two successive phases: the highly variable “prompt” phase and the late and less luminous “afterglow” phase. The two instruments on board the Fermi space telescope, the Gamma-ray Burst Monitor (GBM) and the Large Area Telescope (LAT), allow the study of GRB prompt emission over a broad energy range (from ~10 keV to ~100 GeV). In June 2015, a new set of LAT data (Pass 8) was publicly released, which were generated using improved algorithms of reconstruction and classification of gamma-ray events. The main goal of my thesis is the analysis and interpretation of the spectral and temporal properties of the prompt emission phase of the GRBs observed by Fermi, especially using LAT Pass8 data.In the first part of my work, I performed a detailed time-resolved spectral analysis of the prompt phase of GRB 090926A with GBM and LAT data. My results confirm with a greater significance the spectral break at ∼400 MeV that is observed during a fast variability pulse, and they also reveal the presence of a spectral attenuation throughout the GRB prompt emission, as well as an increase of the break energy up to the GeV domain. I interpreted the spectral break in terms of gamma-ray absorption or as a natural curvature of the inverse Compton (IC) emission in the Klein-Nishina regime. Strong constraints on the jet Lorentz factor were obtained in both scenarios. My results lead also to emission radii R ∼10^14 cm, which are consistent with an internal origin of both the keV-MeV and GeV prompt emissions above the jet photosphere.The second part of my work is an exploration of the internal shock model that has been developed by collaborators at the "Institut d'Astrophysique de Paris" (IAP). This model simulates the GRB jet dynamics and the radiations (synchrotron and IC processes) from a population of shock-accelerated electrons. I simulated the response of the Fermi instruments to the synthetic GRB spectra provided by this numerical code. From these simulations, I built a new parametric function that can be used to fit the keV-MeV spectra of GRBs with the model. I applied successfully this function to a sample of 64 GBM bright GRBs. I confronted also the IAP model to the prompt emission spectrum of GRB 090926A. I obtained a relatively good agreement and I identified a couple of solutions that may improve it. The synthetic spectra are wider than any GRB spectra in the GBM sample. I present some theoretical developments that could improve the data-model agreement in the future, and I discuss possible advances from future GRB missions as well.

Les sources transitoires sont souvent associées aux phénomènes les plus violents de l’Univers, où l’accélération de hadrons peut avoir lieu. Parmi ces sources, les sursauts gamma, les noyaux actifs de galaxie ou encore les supernovae à effondrement de coeur sont des candidats prometteurs pour la production de rayons cosmiques et de neutrinos de haute énergie. Le télescope ANTARES, situé au fond de la Méditerranée, a pour but de détecter ces neutrinos, qui pourraient révéler la présence d’une source de rayons cosmiques. Cependant, pour augmenter la sensibilité aux sources transitoires, une méthode basée sur le suivi multi-longueur d’onde d’alertes neutrino a été développée au sein de la collaboration ANTARES. Ce programme, TAToO, permet de déclencher un réseau de télescopes optiques et l’instrument XRT du satellite Swift seulement quelques secondes après la détection d’un neutrino par ANTARES. Les télescopes commencent un programme d’observation de la région du ciel correspondante pour tenter de détecter une contrepartie optique ou X à l’évènement neutrino. Les travaux présentés dans cette thèse portent sur le développement et la mise en place d’un programme d’analyse d’images optiques, ainsi que sur l’analyse de données optiques et X obtenues lors des observations par les différents télescopes, pour identifier des sources transitoires rapides, telles que les émissions rémanentes de sursauts gamma, ou lentes, telles que les supernovae à effondrement de coeur. / Transient sources are often associated with the most violent phenomena in the Universe, where the acceleration of hadrons may occur. Such sources include gamma-ray bursts (GRBs), active galactic nuclei (AGN) or core-collapse supernovae (CCSNe), and are promising candidates for the production of high energy cosmic rays and neutrinos. The ANTARES telescope, located in the Mediterranean sea, aims at detecting these high energy neutrinos, which could reveal the presence of a cosmic ray accelerator. However, to enhance the sensitivity to transient sources, a method based on multi-wavelength follow-up of neutrino alerts has been developed within the ANTARES collaboration. This program, denoted as TAToO, triggers a network of robotic optical telescopes and the Swift-XRT with a delay of only few seconds after a neutrino detection. The telescopes start an observation program of the corresponding region of the sky in order to detect a possible electromagnetic counterpart to the neutrino event. The works presented in this thesis cover the development and implementation of an optical image analysis pipeline, as well as the analysis of optical and X-ray data to search for fast transient sources, such as GRB afterglows, and slowly varying transient sources, such as CCSNe.