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We discuss high-resolution, time-resolved spectra of the caustic exit of the binary microlensing event OGLE 2002-BLG-069 obtained with UVES on the VLT. The source star is a G5III giant in the Galactic Bulge. During such events, the source star is highly magnified, and a strong differential magnification around the caustic resolves its surface. Using an appropriate model stellar atmosphere generated by the PHOENIX v2.6 code we obtain a model light curve for the caustic exit and compare it with a dense set of photometric observations obtained by the PLANET microlensing follow up network. We further compare predicted variations in the Halpha equivalent width with those measured from our spectra. While the model and observations agree in the gross features, there are discrepancies suggesting shortcomings in the model, particularly for the Halpha line core, where we have detected amplified emission from the stellar chromosphere after the source star's trailing limb exited the caustic. This achievement became possible by the provision of the very efficient OGLE-III Early Warning System, a network of small telescopes capable of nearly-continuous round-the-clock photometric monitoring, on-line data reduction, daily near-real-time modelling in order to predict caustic crossing parameters, and a fast and efficient response of a 8 m class telescope to a "Target-of-Opportunity" observation request
The frequency of giant arcs - highly distorted and strongly gravitationally lensed background galaxies - is a powerful test for cosmological models. Previous comparisons of arc statistics for the currently favored concordance cosmological model ( lambda cold dark matter [LCDM]) with observations have shown an apparently large discrepancy in underpredicting cluster arcs. We present new ray-shooting results, based on a high-resolution (1024(3) particles in a 320 h(-1) Mpc box) large-scale structure simulation normalized to the Wilkinson Microwave Anisotropy Probe (WMAP) observations. We follow light rays through a pseudo - three-dimensional matter distribution approximated by up to 38 lens planes and evaluate the occurrence of arcs for various source redshifts. We find that the frequency of strongly lensed background galaxies is a steep function of source redshift: the optical depth for giant arcs increases by a factor of 5 when background sources are moved from redshift z(s) = 1.0 to 1.5. This is a consequence of a small decrease of the critical surface mass density for lensing, combined with the very steep cluster mass function at the high-mass end plus a modest contribution from secondary lens planes. Our results are consistent with those of Bartelmann et al. if we - as they did - restrict all sources to be at z(s) = 1. If we allow sources extending to or beyond z(s) greater than or equal to 1.5, the apparent discrepancy vanishes: the frequency of arcs increases by about a factor of 10 as compared to previous estimates, and results in roughly one arc per 20 deg(2) over the sky, in good agreement with the observed frequency of arcs
Microlensing is the only known direct method to measure the masses of stars that lack visible companions. In terms of microlensing observables, the mass is given by M (c(2)/4G)(r) over tilde (E)theta(E) and so requires the measurement of both the angular Einstein radius theta(E) and the projected Einstein radius (r) over tilde (E). Simultaneous measurement of these two parameters is extremely rare. Here we analyze OGLE-2003-BLG-238, a spectacularly bright (I-min 10.3), high-magnification (A(max) 170) microlensing event. Pronounced finite-source effects permit a measurement of theta(E) = 650 muas. Although the timescale of the event is only t(E) 38 days, one can still obtain weak constraints on the microlens parallax: 4.4 AU < <(r)over tilde>(E) < 18 AU at the 1 σ level. Together these two parameter measurements yield a range for the lens mass of 0.36 M-&ODOT; < M < 1.48 M-&ODOT;. As was the case for MACHO- LMC-5, the only other single star (apart from the Sun) whose mass has been determined from its gravitational effects, this estimate is rather crude. It does, however, demonstrate the viability of the technique. We also discuss future prospects for single-lens mass measurements
It has been conjectured that the distribution of magnifications of a point source microlensed by a randomly distributed population of intervening point masses is independent of its mass spectrum. We present gedanken experiments that cast doubt on this conjecture and numerical simulations that show it to be false
A small fraction of all quasars are strongly lensed and multiply imaged, with usually a galaxy acting as the main lens. Some, or maybe all of these quasars are also affected by microlensing, the effect of stellar mass objects in the lensing galaxy. Usually only the photometric aspects of microlensing are considered: the apparent magnitudes of the quasar images vary independently because the relative motion between source, lens and observer leads to uncorrelated magnification changes as a function of time. However, stellar microlensing on quasars has yet another effect, which was first explored by Lewis & lbata (1998): the position of the quasar - i.e. the center-of-light of the many microimages - can shift by tens of microarcseconds due to the relatively sudden (dis-)appearance of a pair of microimages when a caustic is being crossed. Here we explore quantitatively the astrometric effects of microlensing on quasars for different values of the lensing parameters kappa and gamma (surface mass density and external shear) covering most of the known multiple quasar systems. We show examples of microlens-induced quasar motion (i.e. astrometric changes) and the corresponding light curves for different quasar sizes. We evaluate statistically the occurrence of large "jumps" in angular position and their correlation with apparent brightness fluctuations. We also show statistical relations between positional offsets and time from random starting points. As the amplitude of the astrometric offset depends on the source size, astrometric microlensing signatures of quasars - combined with the photometric variations - will provide. very good constraints on the sizes of quasars as a function of wavelength. We predict that such signatures will be detectable for realistic microlensing scenarios with near future technology in the infrared/optical (Keck- Interferometry, VLTI, SIM, GAIA). Such detections will show that not even high redshift quasars define a "fixed" coordinate system
Portal alumni
(2004)
Liebe Leserin, lieber Leser, weit weg wollen wir Sie mit der ersten Ausgabe unseres Magazins Portal alumni entführen. Nach dem wir im Mai des vergangenen Jahres auf die Suche nach Ehemaligen der Universität Potsdam gingen, stellten wir fest, dass es Absolventen der Hochschule in alle Himmelsrichtungen verschlagen hat, nach Australien, Afrika oder Amerika. Aus ganz unterschiedlichen Gründen haben Ehemalige ihre Koffer gepackt, und uns hat interessiert, wie sie es geschafft haben, nach dem Studium ins Ausland zu gehen und welche Erfahrungen sie dabei gemacht haben. Herausgekommen sind neben spannenden Geschichten viele persönliche Empfehlungen zu den Themen Berufseinstieg und Mobilität. Und neben den Erfahrungen einiger unserer Absolventen mit einem "internationalen Berufseinstieg" interessierte es uns auch zu erfahren, wie sich Professoren der Universität Potsdam um ihre Ehemaligen kümmern und kümmern wollen. Von Seiten vieler Absolventen erhielten wir Signale, dass sie neben der Kommunikation untereinander Interesse daran haben, Neuigkeiten aus ihrer noch gar nicht so alten Alma mater zu erhalten. Mit unserem ersten Magazin fangen wir damit an, einen Rückblick über besondere Ereignisse des vergangenen Jahres zusammenzustellen. Und natürlich wollen wir mit den einzelnen Ausgaben von Portal alumni unseren Ehemaligen viele Tipps, Informationen und Links zu Weiterbildung, Jobs, Karrierestart und anderen Themen für den weiteren beruflichen Weg geben. Die Redaktion wünscht Ihnen nun viel Spaß bei der Lektüre.