Galactic Microlensing beyond the Standard Model

Abstract

For most of the Galactic microlensing events as well as for the analysis of the results from microlensing surveys, a simplified model of pointlike source objects (observed stars) and lens objects (matter between source and observer that causes a light deflection) is being used. Here I discuss effects beyond this approximation, using observed events as examples if they exhibit such effects, prospects of observing such effects, and the influence of these effects on the interpretation of the data. Moreover, I investigate whether ambiguities show up in modeling the observed events, how well model parameters are constrained by the observations, and what implications follow for the physical quantities of the lens system, which are lens mass, distance, and velocity as well as rotation period and semimajor axis for binary lenses. The effects discussed include light from other sources (blending), extended and binary lenses and sources, and rotation effects in the observer (Earth around Sun), the lens, and the source. Taking into account these “nonstandard” effects, I discuss fits for the Galactic microlensing events MACHO LMC 1, EROS 1, OGLE 1–7, and DUO 2. For fitting model parameters of microlensing light curves to the observed data, I discuss an efficient numerical approach that has been used for the fits presented in this dissertation. I argue that some of the observed events that are claimed as being due to point sources and lenses are better explained by source or lens binaries, which are more general models and include the pointlike objects as a limit. In particular, MACHO LMC 1 is best explained by a binary lens. Several different binary lens models are presented for OGLE 7 and DUO 2, which are all consistent with the observed data. I show that the inclusion of blending in fits for the OGLE 1–6 events yields large uncertainties for the event timescales and therefore for the lens masses. Since “nonstandard” events show different statistics than point source–point lens events, one has to be careful with the interpretation of the results of ongoing microlensing searches. Since the Galactic halo is an unknown population, it is difficult to make assumptions about statistics of binary objects. Binarity of objects makes it difficult to obtain the mass spectrum of the lens objects. Since the distance and velocity of the lens are not known in general, the lens mass cannot be determined directly. I derive the probability density of the lens mass for a given observed event from assumed mass densities and velocity distributions. The inclusion of the revolution of the Earth around the Sun, the source size, and the lens rotation each give an additional constraint on the lens distance and velocity, so that the lens mass can be determined (up to a twofold degeneracy) if at least two of these effects are observed. If the lens consists of a star and a planetary companion, the presented methods of estimating physical quantities of the lens system and determining the uncertainties of the fit parameters as well as the discussion of ambiguities of the fit parameters are a necessary prerequisite to draw the right conclusions from an observed light curve.