京都大学 / 新制・課程博士 / 博士(理学) / 甲第24777号 / 理博第4971号 / 新制||理||1710(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 鶴 剛, 准教授 細川 隆史, 教授 田島 治 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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.

The main results of this work are the contribution to find the following results: <br />• The most general GRB is made by an early emission (P-GRB or Proper-GRB), with a time-scale not larger than 1 or 2 seconds and an afterglow, whose light curve is characterized by an increasing phase followed by a peak and a decreasing phase. This peak has been identified with the long GRBs prompt emission. In this scenario short GRBs are not but P-GRB, while long GRBs present both a peak and a decreasing late time emission, which is the observed afterglow.<br />• A possible GRB/SuperNova connection is based on the process of induced gravitational collapse of a companion star of the black hole originating the GRB. <br />• A thermal distribution in the comoving frame of the expanding system is assumed for X and γ bands of the spectrum. This assumption leads to a natural bending of the late time light curves making not necessary the hypothesis of a beamed emission within a collimated jet from the inner engine, which has been introduced in literature essentially to reduce the energy requirements.

Pulsar wind nebulae (PWNe), nebulae harbouring a rotation-powered neutron star that was born in a supernova, provide opportunities to study highly relativistic pulsar winds and their interaction with the surrounding medium. Particularly interesting are PWNe that do not show any sign of the expected surrounding SNR shell and were thought to be born in subenergetic explosions or with unusual progenitors. The detection of a shell around one such PWN suggested that shells are indeed produced but may be faint due to unseen shocked ejecta, a low density environment, and/or a young age that has not yet allowed the shell to brighten and become visible. Here, by using observational X-ray data from modern telescopes with excellent spatial and energy resolution (Chandra and XMM-Newton), we target PWNe that do not have prominent SNR shells, and are known to be in varied environments, to further explore the characteristics of this growing, but poorly explored, class of PWNe. By combining imaging and spectroscopic results, we study the morphology of the PWNe, search for thermal emission from shock-heated material, investigate the energetics of the nebulae, and search for candidates for the neutron stars powering the nebulae. We find that while the faint shell surrounding G21.5–0.9 can be explained as a young PWN evolving in a low density medium, CTB 87 (G74.9+1.2) appears to be in an advanced stage of evolution, and G63.7+1.1 appears to be both in an advanced stage of evolution and in a dense environment. By performing spatially resolved spectroscopy, we have shown how the spectral characteristics vary across the PWNe, and note that more data will place better constraints on possible thermal emission in these remnants. The imaging portion of these studies has revealed intriguing large-scale morphologies for CTB 87 and G63.7+1.1, as well as a torus-jet structure in CTB 87 and neutron star candidates in both CTB 87 and G63.7+1.1. We conclude that both CTB 87 and G63.7+1.1 are likely interacting with the supernova remnant reverse shock, and CTB 87 may be additionally influenced by the motion of its neutron star.

La découverte du rayonnement cosmique remonte à plus d'un siècle, mais son origine est encore incertaine aujourd'hui. Divers éléments théoriques et observationnels tendent à confirmer que les vestiges de supernova (SNRs) sont à l'origine de la majeure partie du rayonnement cosmique galactique jusqu'à des énergies de ~3 PeV, voire au-delà. Mais ce paradigme requiert encore une preuve irréfutable.Lors de l'expansion d'un SNR, les particules chargées du milieu circumstellaire/interstellaire sont accélérées par l'onde de choc et peuvent alors rayonner depuis le domaine radio jusqu'aux très hautes énergies (>100 GeV). Les mécanismes d'accélération et les processus d'émission des particules ont été étudiés et modélisés au cours de cette thèse.Les instruments d'astronomie Gamma permettent de détecter les rayonnements émis par les particules accélérées au choc des SNRs. En particulier, le télescope à création de paires Fermi-LAT et le réseau de télescopes au sol à imagerie Tcherenkov atmosphérique H.E.S.S. détectent, à eux deux, des photons Gamma d'énergie comprise entre ~30 MeV et plusieurs dizaines de TeV. Au cours de cette thèse, réalisée dans le cadre de l'expérience H.E.S.S., les données observationnelles de plusieurs SNRs ont été analysées et interprétées. Les études menées ont conduit à la détection d'une des sources les plus faibles encore jamais détectée et dont l'émission Gamma est associée à l'interaction du SNR G349.7+0.2 avec un nuage moléculaire adjacent. Des limites supérieures sur les flux intégrés de nombreux SNRs ont été extraites afin de contraindre les modèles d'accélération de particules. Le SNR Puppis A présente notamment un comportement spectral inattendu, difficile à expliquer dans le cadre des modèles d'accélération de particules actuels.Un cinquième télescope, de plus grande taille, a été ajouté mi-2012 au réseau de quatre petits télescopes H.E.S.S. afin d'abaisser le seuil en énergie et d'améliorer la sensibilité du réseau. Dans ce cadre, j'ai activement participé au développement, à l'intégration et à l'analyse des données du dispositif d'étalonnage de la caméra de ce cinquième télescope. / The discovery of Cosmic Rays (CRs) dates back more than one century ago, however their origin remains unclear. There is rather convincing evidence that the bulk of Galactic CRs, up to ~3 PeV, and possibly beyond, is accelerated in supernova remnants (SNRs). However this paradigm still needs a conclusive proof. While the SNR expands, charged particles from the circumstellar/interstellar medium are accelerated at the SNR shock wave and radiate from radio to very high energies (>100 GeV). Particles acceleration and broad band radiation mechanism were studied and modelled during this PhD. $gamma$-ray astronomy instruments enable to detect radiation from particles accelerated at the SNR shock wave. In particular the pair creation telescope Fermi-LAT and the array of imaging atmospheric Cherenkov telescopes H.E.S.S., enable together to detect Gamma-ray photons in the ~30 MeV-30 TeV energy range. As a member of the H.E.S.S. collaboration, I analysed and interpreted observational data from several Galactic SNRs. These studies led to the discovery of one of the faintest sources ever detected whose Gamma-ray emission is associated with the interaction of the SNR G349.7+02 with an adjacent molecular cloud. Upper limits on the integrated flux of many SNRs were extracted in order to constrain models of particle acceleration. In particular, the SNR Puppis A shows an unexpected spectral behavior difficult to explain with current models. A fifth bigger telescope was added mid 2012 to the H.E.S.S. array of four small telescopes to lower the energy threshold and to improve the sensitivity of the array. In this context I actively participated to the development, integration end data analysis of the calibration hardware of the camera of this fifth telescope.

<p dir="ltr"> This article-based dissertation provides an account of two distinct classes of expansive astrophysical outflows and techniques to interpret their observations using numerical modeling. The primary purpose of this dissertation is to provide an extensive description of the research projects I undertook during my tenure as a Graduate Research Assistant, under the guidance of my advisor Prof. Paul Duffell.</p><p dir="ltr">Chapter 1 provides a brief introduction to numerical hydrodynamics and techniques of modeling expanding flows numerically. I also introduce the aforementioned classes of astrophysical outflows, namely relativistic jets from Active Galactic Nuclei (AGN), and supernova remnants (SNRs). I provide a general overview of the theoretical picture, and the general strategy used in this work to model them.</p><p dir="ltr">Chapter 2 describes my investigation on the connection of kiloparsec scale AGN jet properties to their intrinsic parameters and surroundings, based on an article published in The Astrophysical Journal. Using a suite of over 40 relativistic hydrodynamic jet models, we find that the dynamics of relativistic jets can be described in terms of only two parameters, the jet to ambient medium energy density ratio, and the jet opening angle. The former is found to strongly control the Fanaroff-Riley (FR) morphological dichotomy, which was previously thought to be tied to the magnitude of the jet luminosity. We also suggest a purely hydrodynamical origin of bright spots observed in some AGN jets. Our models were tested against and found to be consistent with the observations of the jets in M87 and Cygnus A.</p><p dir="ltr">In chapter 3, I present my moving-mesh hydrodynamics code Sprout, also described in an article published in The Astrophysical Journal Supplements. Sprout solves the equations of ideal hydrodynamics on an expanding Cartesian mesh. The expanding mesh can follow fluid outflows for several orders of magnitude with very little numerical diffusion. This allows Sprout to capture expanding flows with very high dynamic range. Sprout is thus particularly suitable for studying expanding outflows such as supernova remnants and active galactic nuclei. Relative to other moving mesh codes, the simple mesh structure in Sprout is also convenient for implementing additional physics or algorithms. I discuss many code tests that were performed to test the accuracy and performance of the numerical scheme.</p><p dir="ltr">Chapter 4 details my study of hydrodynamic instabilities in supernova remnants (SNRs) as they expand against the circumstellar medium (CSM). This is based on an article published in The Astrophysical Journal. A suite of 3D hydrodynamical SNR models, generated using my hydro code \sprout, was used to study the impact of the stellar ejecta density profile and seed anisotropies in the ejecta and the CSM on formation of turbulent structures in the SNRs. We found that most of the turbulent power in these models resides at a typical angular mode or scale that is determined by the ejecta density structure. It was also found that clumps or anisotropies in either the ejecta or CSM do not imprint upon these turbulence structures unless they are massive and form large-scale coherent structures.</p><p dir="ltr">In chapter 5, I discuss the implementation of a technique to measure anisotropies in observed SNRs just using 2D high-resolution images. This technique is calibrated using 3D hydro SNR models and synthetic images derived from them. As seen in Chapter 4, we find a similar dominant angular scale of turbulent structures dictated by the ejecta density structure. Both the 3D models and the synthetic images yield the same value of this scale, which validates the image analysis technique used in this work. As an example of how this technique can be applied to observations, we analyze observations of a known supernova remnant (Tycho's SNR) and compare with our models. Our technique picks out the angular scale of Tycho's fleece-like structures and also agrees with the small-scale power seen in Tycho.</p><p dir="ltr">PhChapter 6 summarizes the results, conclusions, and future prospects of all the research work described so far. It is followed by a bibliography, my curriculum vita, and a list of publications.</p>

Understanding the asymmetry in core collapse supernova explosions is pointed out by various astrophysicists as it is the key factor in determining the observational properties of the pulsars. The initial kick given by the ex- plosion to the pulsar affects its spin period and space velocity. Up to now, although the observations do not show a direct relation between the observational features of the pulsar and its space velocity, they show a clear relation between the spin period and the magnetic field strength, hence its radiation processes. In this thesis, as the method, tracing the companions of progenitors if they were in close binaries, which becomes a runaway star after the supernova explosion was chosen. Over the candidates selected in Guseinov et al (2005), the spectral types of 11 runaway candidates from 7 supernova remnants determined through analyzing their spectroscopic observations. Radial velocity determination was applied to the discovered B6V type star GSC 03156-01430 inside the supernova remnant G78.2+2.1. Also by studying the proper motion data, we compared the motion of the runaway star and the related pulsar in order to determine the asymmetry in the supernova explosion. The neutron star PSR 2021+4026 is moving with a 2-D velocity of &sim / 580 km/s with respect to the rest frame of its birth association Cyg OB9. &sim / 550 km/s more than expected in the symmetric case. Re-constructing the pre-supernova binary shows that the asymmetry in the supernova explosion does not depend on the binarity.

Cette thèse présente le travail réalisé par l'auteur consacréà l'étude de l'influence des vents magnétosphériques des pulsars sur leur environnement. Le problème du vent magnétosphérique des pulsars est étudié dans le contexte des pulsars classiques, qui forment des nébuleuses de pulsar. L'observation de ces nébuleuses fournit des informations importantes sur leurs conditions physiques et dynamiques. Le vent magnétosphérique des pulsars milliseconde est également étudié. La contribution des pulsars millisecondeà l'émission gamma des amas globulaires est étudiée par des simulations numériques.Dans la première partie, les résultats des observations infrarouge du vestige de supernova G21.5-0.9 sont présentées. Les données utilisées comprennent des observations du Very Large Telescope de l'ESO, du télescope Canada-France-Hawaï (CFHT) et du télescope spatial Spitzer. La détection de la nébuleuse compacte autour du pulsar PSR J1833-1034, avec l'instrument CFHT/AOB-KIR (bande K') et la caméra IRAC/Spitzer (toutes les bandes), est présentée. La valeur moyenne de la fraction de polarisation linéaire de l'émission détectée est estiméeà $P_{rm L}^{avg} simeq 0.47$. Une oscillation du vecteur champ électrique dans la nébuleuse compacte peut être observée. Le spectre infrarouge de la nébuleuse compacte est bien décrit par une loi de puissance d'indice $alpha_{rm IR} = 0.7 pm 0.3$, et suggère un aplatissement spectral entre les domaines infrarouge et X. La détection de la raie d'émission [Fe II] à 1.64 $mu$m est présentée. La spectroscopieà moyenne résolution permet d'estimer l'extinction par le milieu interstellaire de l'émission infrarouge de l'objet, ainsi que la vitesse d'expansion de la matière émettant la raie du [Fe II], ce qui conduità une estimation de la distanceà G21.5-0.9 de $3.9 pm 1.2$ kpc.La deuxième partie présente une étude de l'activité magnétosphérique des pulsars milliseconde dans le contexte de l'émission gamma des amas globulaires. Une base de données des caractéristiques d'émission des pulsars milliseconde et des spectres d'éjection des électrons est créée sur la base du modèle pair starved polar cap de la magnétosphère des pulsars. Le concept de facteur de biais est introduit et étudié. Des spectres synthétiques d'amas globulaires sont simulés dans la gamme d'énergie allant du GeV au TeV. Ils consistent en une composante d'émission magnétosphérique des pulsars milliseconde résidant dans l'amas, et une composante Compton inverse résultant de la diffusion des photons ambiants (le champ stellaire des photons provenant des étoiles de l'amas, et le fond diffus cosmologique) par les leptons relativistes diffusant dans l'amas. Enfin, les spectres synthétiques des amas globulaires sont comparés et contrastés avec les observationsà haute et très haute énergie des amas globulaires sélectionnés: Terzan 5 et 47 Tucanae. / This PhD thesis presents the results of the studies on the influence of magnetospheric pulsar winds on the surroundings of these objects. The problem of the magnetospheric pulsar wind is studied in the context of classical pulsars, which power prominent pulsar wind nebulae. Observations of these nebulae yield important information on their physical and dynamical conditions. The magnetospheric winds of millisecond pulsars are also investigated. The contribution of millisecond pulsars to the gamma-ray emission of globular clusters is studied through numerical simulations. In the first part of the thesis, the results of infrared observations of the supernova remnant G21.5-0.9 are presented. The observational material includes data obtained with the ESO Very Large Telescope, the Canada-France-Hawaii Telescope and the Spitzer Space telescope. The detection of the compact nebula around the pulsar PSR J1833-1034, through imaging with both the CFHT/AOB-KIR instrument (K' band) and the IRAC/Spitzer camera (all bands), is reported. The average value of the linear polarisation fraction $P_{rm L}^{avg} simeq 0.47$ of the detected emission is estimated. A swing of the electric field vector across the compact nebula is observed. The infrared spectrum of the compact nebula is best described as a power law of index $alpha_{rm IR} = 0.7 pm 0.3$, and suggests its flattening between the infrared and X-ray bands. The detection of [Fe II] 1.64 $mu$m line emitting material is reported. Through medium resolution spectroscopy the infrared interstellar extinction to the object is estimated, and also the expansion velocity of the iron-line emitting material is determined, which in turn leads to estimating the distance of $d = 3.9 pm 1.2$ kpc to G21.5-0.9. The second part presents a study of the magnetospheric activity of millisecond pulsars in the context of the gamma-ray emission of globular clusters. Based on the pair starved polar cap model of the pulsar magnetosphere the database of the millisecond pulsar emission characteristics and the electron ejection spectra is created. The modelled electron ejection spectra are single-peaked for mildly inclined and fast rotating pulsars, while double-peaked for slowly rotating pulsars. The concept of the bias factor is introduced and studied. Synthetic spectra of globular clusters, stretching from MeV up to TeV energies, are simulated. They consist of the magnetospheric contribution from the millisecond pulsar population residing in the cluster, and an inverse Compton scattering (ICS) component resulting from up-scattering of the ambient photon fields (cosmic microwave background and optical photons from stellar population in the cluster) on the relativistic electrons diffusing through the cluster. The spectral characteristics of the ICS component depends on the composition of the ambient photon fields and also on the magnitude of the cluster magnetic field $B_{rm GC}$. For low $B_{rm GC} sim 1 mu$G the spectra are double-peaked. For the high $B_{rm GC} gtrsim 10 mu$G the ICS spectra are single-peaked. The level of ICS emission increases with the increase of $B_{rm GC}$, but it saturates for $B_{rm GC} sim 10 ~mu$G. Finally, the simulated synthetic spectra of globular clusters are confronted with the existing gamma-ray data for selected clusters: Terzan 5 and 47 Tucanae.

Le centre Galactique (GC) est une région très riche et complexe. Le taux de supernovae (SN) associé à la formation d'étoiles massives y est très élevée et devrait créer une injection continue de rayons cosmiques (CRs) dans le GC à travers les chocs qu'elles produisent. Cette région abrite également un trou noir supermassif (SMBH) de $4 \times 10^6 \, \rm{M_{\odot}}$, nommé Sgr A*. De nombreux arguments ont permis de montrer que le SMBH pouvait accélérer des particules à très haute énergie (VHE); son activité actuelle et passée pourrait donc également contribuer à la population de CRs. En 2006, la collaboration H.E.S.S. a révélé la présence d'une émission diffuse à VHE dans les 100 pc centraux de la Galaxie, très corrélée à la distribution de matière moléculaire répartie dans la zone moléculaire centrale (CMZ). Une partie importante de cette émission a donc très probablement une origine hadronique mais celle-ci reste toujours inconnue. Nous présentons une nouvelle analyse spectrale et morphologique détaillée de la région en utilisant 10 ans de prise de données de H.E.S.S. ainsi qu’une modélisation de l'émission $\gamma$ induite par les SNe. Nous étudions l'impact de la distribution temporelle et spatiale des SNe dans le CMZ sur la morphologie et le spectre de l'émission: nous construisons un model 3D d'injection de CRs à VHE et d'une propagation diffusive dans la région avec une distribution de gaz réaliste. La contribution des SNe ne peut pas être négligée. Nous montrons qu’un profil piqué de rayon $\gamma$ ainsi qu’un excès de CRs vers le GC peuvent être obtenus en utilisant une distribution spatiale réaliste de SNe prenant en compte les amas d'étoiles massives centraux. La morphologie de l'émission est très dépendante de l'énergie dans ce scénario. Le profil de densité de CRs peut également être reproduit avec une injection stationnaire unique au centre par Sgr A* mais cela implique alors une morphologie stable en énergie. L'utilisation d'une analyse 3D est donc nécessaire pour distinguer les modèles. Nous présentons les premiers résultats de cette analyse que nous avons développé dans la librairie Gammapy afin d'ajuster simultanément un spectre et une morphologie sur des données. Avec la prochaine génération d'instruments comme le Cherenkov Telescope Array, les observations de régions avec une morphologie complexe, avec une émission diffuse ou de multiples sources, vont devenir de plus en plus nombreuses. Elles nécessitent donc également le développement de cette technique. Nous détaillons les premières validations de cette méthode appliquée sur des sources ponctuelles avec un outil Monte Carlo. Pour l’émission diffuse, nous présentons le nouveau spectre obtenu en utilisant une méthode que nous avons développée pour l’extraction spectrale 1D classique. Nous réalisons par ailleurs une analyse morphologique dans différentes bandes en énergie indépendantes en utilisant de nouveaux modèles spatiaux. Pour l'instant, aucune variation significative n'est détectée mais des observations supplémentaires sont nécessaires ainsi qu'une vraie analyse 3D de la région du GC pour pouvoir donner une conclusion définitive. Les observations de CTA permettront de donner des réponses précises à ces questions. / The Galactic center (GC) is a very rich and complex astrophysical region. The high supernovae (SN) rate associated with the strong massive star formation should create a sustained cosmic rays (CR) injection in the GC via the shocks they produce. This region also harbors a Super-Massive Black Hole (SMBH) of $4 \times 10^6 \, \rm{M_{\odot}}$, named Sgr A*. Since it has been argued that the SMBH might also accelerate particles up to very high energies (VHE), its current and past activity could contribute to the CR population. In 2006, the H.E.S.S. collaboration revealed the presence of a VHE diffuse emission in the inner 100 pc of the Galaxy in close correlation with the molecular matter spread in the central molecular zone (CMZ). A major part of this emission is thus certainly of hadronic origin but it still remains mysterious. We report a new detailed spectral and morphological analysis of this region using 10 years of H.E.S.S. observations as well as a detailed modelling of the $\gamma$-ray emission induced by the SNe. We study the impact of the spatial and temporal distribution of SNe in the CMZ on the VHE emission morphology and spectrum: we built a 3D model of VHE CR injection and diffusive propagation with a realistic gas distribution. The contribution of SNe can not be neglected. We show that a peaked $\gamma$-ray profile and CR excess towards the GC, can be obtained using realistic SN spatial distribution taking into account the central massive star clusters. A strong dependence on the morphology of the emission with the energy is expected in this scenario. The CR density profile can also be reproduced by a unique stationary injection at the center by Sgr A* but it implies a stable morphology across the energy range. To distinguish the models, we need a 3D analysis. We present the first results of this analysis that we started to design in the software Gammapy to simultaneously fit a spectral and morphological model to the data. The observations of complex morphological regions with diffuse emission or multiple sources will become more and more numerous with the next generation instruments such as the Cherenkov Telescope Array. They will also require the development of this technique. We detail the first validations of this method on point sources using a Monte Carlo tool. For the ridge emission, we report the new spectrum using a method that we developed for the classical spectral fitting necessary for faint emission. By using new spatial templates to describe the complexity of the diffuse emission, we perform a morphological analysis in different energy bands independently. No significant variation is found but more observations are needed to give a conclusive statement as well as a real 3D analysis in the GC region. The observations of CTA will allow to give precise answers to these questions.

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>