Thesis (M. Sc.)--University of Canterbury, 2008. / Typescript (photocopy). Includes bibliographical references (p. 138-182).

All active galactic nuclei (AGN) are known to vary in the rest-frame UV/optical. Typical variations are on the order of 30% or so and are stochastic in nature. Therefore, the discovery of a number of extreme AGN transients, which are smoothly evolving on year-long timescales and by a factor of four or more, is surprising and necessitates further analysis. Are these objects simply at the extreme end of the variability distribution seen in normal quasars or is there another mechanism which can explain their atypical behaviour? The primary focus for this work is on the possibility that a number of these extreme AGN transients are actually rare, high-amplitude microlensing events, caused by intervening stellar mass object(s). Not only do the microlensing models provide an explanation for the observed variability but they also allow constraints to be placed on the morphology of the emitting regions of the AGN, namely the accretion disc and broad line region (BLR). These transients have been monitored both photometrically and spectroscopically, since their discovery. The majority of spectroscopic observations have been conducted using the William Herschel Telescope. At time of writing (Sept. 2017), there are now 64 confirmed AGN and 235 individual spectra. The spectral reduction pipeline, calibration and initial measurements are described in Chapter 2. This chapter also details the microlensing models and procedures used in interpreting both the light curve information and spectral measurements. This includes: a comprehensive treatment of the simple point-source/point-lens model; quantitative point-lens models which allow for the use of extended sources; and also an initial exploration into more complex lensing morphologies involving multiple lensing objects and/or an external shear. Chapter 3 details the results of the spectroscopic monitoring campaign for the entire transient sample. A general classification scheme is developed which allows for a comparison of the evolutionary trends seen in objects exhibiting similar behaviour. A subset of transient AGN, the most extreme objects in the sample, is also discussed in detail, with a particular focus on the evolution of the continuum, line fluxes and equivalent widths. Chapter 4 details the results of the analysis of four key targets, selected for their suitability in addressing the microlensing hypothesis. For two targets the point-source point-lens model performs very well. Lens parameters for these objects are presented and in one particular case, the data is sufficient to allow constraints to be placed on the size of various components comprising the broad line region. Chapter 5 expands the microlensing analysis to include the entire AGN transient sample. Approximately 10% of objects are well matched by a simple point-source, point-lens microlensing model. In other objects, evidence is seen which requires a more complex lensing scenario to adequately explain. In one class of objects there is also evidence that the accretion disc is being resolved by the lens. Chapter 6 revisits a notable are seen in an AGN which lies behind M31. The analysis reaffirms that this event is well described by a simple microlensing model and provides an independent estimate that the most probable location for the lens is within M31 itself.

No description available.

<p>Stochastic microlensing is a central tool in probing dark matter on galactic scales. From first principles, we initiate the development of a mathematical theory </p><p>of stochastic microlensing. We first construct a natural probability space for stochastic microlensing and characterize the general behaviour of the random time </p><p>delay functions' random critical sets. Next we study stochastic microlensing in two distinct random microlensing scenarios: The uniform stars' distribution with</p><p> constant mass spectrum and the spatial stars' distribution with general mass spectrum. For each scenario, we determine exact and asymptotic (in the large number</p><p> of point masses limit) stochastic properties of the random time delay functions and associated random lensing maps and random shear tensors, including their </p><p>moments and asymptotic density functions. We use these results to study certain random observables, such as random fixed lensed images, random bending angles, </p><p>and random magnifications. These results are relevant to the theory of random </p><p>fields and provide a platform for further generalizations as well as analytical limits for checking astrophysical studies of stochastic microlensing.</p><p>Continuing our development of a mathematical theory of stochastic microlensing, we study the stochastic version of the Image Counting Problem, first considered </p><p>in the non-random setting by Einstein and generalized by Petters. In particular, we employ the Kac-Rice formula and Morse theory to deduce general formulas for </p><p>the expected total number of images and the expected number of saddle images for a general random lensing scenario. We further </p><p>generalize these results by considering random sources defined on a countable compact covering of the light source plane. This is done to introduce the notion of</p><p> global expected number of positive parity images due to a general lensing map. Applying the result to the uniform stars' distribution random microlensing </p><p>scenario, we calculate the asymptotic global expected number of minimum images in the limit of an infinite number of stars. This global expectation is bounded, </p><p>while the global expected number of images and the global expected number of saddle images diverge as the order of the number of stars.</p><p>Finally, we outline a framework for the study of stochastic microlensing in the neighbourhood of lensed images. This framework is related to the study of the </p><p>local geometry of a random surface. In our case, the surface is non-Gaussian, and therefore standard literature on the subject does not apply. We explore the case</p><p> of a random gravitational field caused by a random star.</p> / Dissertation

Gravitational lensing, initially a phenomenon used as a solid confirmation of General Relativity, has defined itself in the past decade as a standard astrophysical tool. The ability of a lensing system to produce multiple images of a luminous source is one of the aspects of gravitational lensing that is exploited both theoretically and observationally to improve our understanding of the Universe. In this thesis, within the field of multiple imaging we explore the case of maximal lensing, that is, the configurations and conditions under which a set of deflecting masses can produce the maximum number of images of a distant luminous source, as well as a study of the value for this maximum number itself. We study the case of a symmetric distribution of n-1 point-mass lenses at the vertices of a regular polygon of n-1 sides. By the addition of a perturbation in the form of an n-th mass at the center of the polygon it is proven that, as long as the mass is small enough, the system is a maximal lensing configuration that produces 5(n-1) images. Using the explicit value for the upper bound on the central mass that leads to maximal lensing, we illustrate how this result can be used to find and constrain the mass of planets or brown dwarfs in multiple star systems. For the case of more realistic mass distributions, we prove that when a point-mass is replaced with a distributed lens that does not overlap with existing images or lensing objects, an additional image is formed within the distributed mass while positions and numbers of existing images are left unchanged. This is then used to conclude that the maximum number of images that n isolated distributed lenses can produce is 6(n-1)+1. In order to explore the likelihood of observational verification, we analyze the stability properties of the symmetric maximal lensing configurations. Finally, for the cases of n=4, 5, and 6 point-mass lenses, we study asymmetric maximal lensing configurations and compare their stability properties against the symmetric case.

Gravitational lensing, initially a phenomenon used as a solid confirmation of General Relativity, has defined itself in the past decade as a standard astrophysical tool. The ability of a lensing system to produce multiple images of a luminous source is one of the aspects of gravitational lensing that is exploited both theoretically and observationally to improve our understanding of the Universe. In this thesis, within the field of multiple imaging we explore the case of maximal lensing, that is, the configurations and conditions under which a set of deflecting masses can produce the maximum number of images of a distant luminous source, as well as a study of the value for this maximum number itself. We study the case of a symmetric distribution of n-1 point-mass lenses at the vertices of a regular polygon of n-1 sides. By the addition of a perturbation in the form of an n-th mass at the center of the polygon it is proven that, as long as the mass is small enough, the system is a maximal lensing configuration that produces 5(n-1) images. Using the explicit value for the upper bound on the central mass that leads to maximal lensing, we illustrate how this result can be used to find and constrain the mass of planets or brown dwarfs in multiple star systems. For the case of more realistic mass distributions, we prove that when a point-mass is replaced with a distributed lens that does not overlap with existing images or lensing objects, an additional image is formed within the distributed mass while positions and numbers of existing images are left unchanged. This is then used to conclude that the maximum number of images that n isolated distributed lenses can produce is 6(n-1)+1. In order to explore the likelihood of observational verification, we analyze the stability properties of the symmetric maximal lensing configurations. Finally, for the cases of n=4, 5, and 6 point-mass lenses, we study asymmetric maximal lensing configurations and compare their stability properties against the symmetric case.

The presence of massive objects is detectable in observations via the gravitational lensing effect on light from more distant sources. From this effect it is possible to reconstruct the masses of clusters, and the distribution of matter within the cluster. However, further theoretical work needs to be done to properly contextualize any proposed projects involving, for instance, SALT data sets. Observational lensing studies use one of two techniques to recover the lens mass distribution: parametric (model dependent) techniques; and, a more recent innovation, non-parametric methods. The latter deserves further study as a tool for cluster surveys. To this end, we provide a comprehensive analysis of existing non-parametric algorithms and software, as well as estimates on the likely errors to be expected when used as an astronomical tool. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2009.

Thesis (Ph. D.)--Rutgers University, 2008. / "Graduate Program in Physics and Astronomy." Includes bibliographical references.

Free-floating planets (FFPs) are very difficult to observe directly since they are isolated and intrinsically faint. The gravitational microlensing effect is now major method to observe FFPs, but observing low-mass FFPs is still difficult due to their short duration. We compute simulations for FFP microlensing observations down to Earth-mass using the numerical Besancon Galactic model created by Robin et al. (2012a). These are the first detailed simulation of FFP microlensing using a population synthesis Galactic model incorporating a 3D extinction model, and we also take full account of finite source effects. Firstly, we simulate the microlensing event rate and spatial distribution using three different modes, and for each mode three FFP lens masses (Jupiter, Neptune, and Earth). For the target area of (l, b) =(1, -1.75) which corresponds to the centre of the proposed Euclid ExELS field, our simulations result in 184-920 Jupiter-mass FFPs during the 5 year Euclid mission depending on simulation assumptions. For the Earth-mass FFPs, the rate range is 9-49 FFPs assuming 100% detection efficiency. Next, we compute the rate of parallax detection using a 3D model of the observers. We consider parallax detection by Euclid and WFIRST-AFTA, and by Euclid and LSST. We found that 52 Jupiter-mass FFPs will be detected by a parallax between Euclid and WFIRST-AFTA for two 30-day continuous period around equinoxes if they observe simultaneously. The rate falls to 4 parallax events for Earth-mass FFPs. The parallax detection between Euclid and LSST would be affected by the observation time on the Earth, but it could provide 20 Jupiter-mass FFPs down to 1.4 Earth-mass FFPs.

Gravitational microlensing occurs when a massive lens (typically a star) deflects light from a more distant source, creating two unresolvable images that are magnified. The effect is transient due to the motions of the lens and source, and the changing magnification gives rise to a characteristic lightcurve. If the lensing object is a binary star or planetary system, more images are created and the lightcurve becomes more complicated. Detection of these lightcurve features allows the lens companion's presence to be inferred. Orbital motion of the binary lens can be detected in some microlensing events, but the expected fraction of events which show orbital motion has not been known previously. We use simulations of orbiting-lens microlensing events to determine the fraction of binary-lens events that are expected to show orbital motion. We also use the simulations to investigate the factors that affect this detectability. Following the discovery of some rapidly-rotating lenses in the simulations, we investigate the conditions necessary to detect lenses that undergo a complete orbit during a microlensing event. We find that such events are detectable and that they should occur at a low but detectable rate. We also derive approximate expressions to estimate the lens parameters, including the period, from the lightcurve. Measurement of the orbital period can in some cases allow the lens mass to be measured. Finally we develop a comprehensive microlensing simulator, MaBμLS, that uses the output of the Besançon Galaxy model to produce synthetic images of Galactic starfields. Microlensing events are added to the images and photometry of their lightcurves simulated. We apply these simulations to a proposed microlensing survey by the Euclid space mission to estimate its planet detection yield.