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When a gravitationally lensed source crosses a caustic, a pair of images is created or destroyed. We calculate the mean number of such pairs of microimages <n> for a given macroimage of a gravitationally lensed point source due to microlensing by the stars of the lensing galaxy. This quantity was calculated by Wambsganss, Witt, and Schneider in 1992 for the case of zero external shear, ;=0, at the location of the macroimage. Since in realistic lens models a nonzero shear is expected to be induced by the lensing galaxy, we extend this calculation to a general value of ;. We find a complex behavior of <n> as a function of ; and the normalized surface mass density in stars, ;*. Specifically, we find that at high magnifications, where the average total magnification of the macroimage is <;>=|(1-;*)2- ;2|-1>>1, <n> becomes correspondingly large and is proportional to <;>. The ratio <n>/ <;> is largest near the line ;=1-;*, where the magnification <;> becomes infinite, and its maximal value is 0.306. We compare our semianalytic results for <n> with the results of numerical simulations and find good agreement. We find that the probability distribution for the number of extra microimage pairs is reasonably described by a Poisson distribution with a mean value of <n> and that the width of the macroimage magnification distribution tends to be largest for <n>~1.
The gravitationally lensed quasar Q2237+0305 in X-rays: ROSAT/HRI detection of the "Einstein Cross"
(1999)
We report the first detection of the gravitationally lensed quasar Q2237+0305 in X-rays. With a ROSAT/HRI exposure of 53 ksec taken in Nov./Dec. 1997, we found a count rate of 0.006 counts per second for the combined four images. This corresponds to an X-ray flux of 2.2*E(-13) erg/cm(2) /sec and an X-ray luminosity of 4.2*E(45) erg/sec (in the ROSAT energy window 0.1-2.4 keV). The ROSAT/HRI detector is not able to resolve spatially the four quasar images (maximum separation 1.8 arcsec). The analysis is based on about 330 source photons. The signal is consistent with no variability, but with low significance. This detection is promising in view of the upcoming X-ray missions with higher spatial/spectral resolution and/or collecting power (Chandra X-ray Observatory, XMM and ASTRO-E).
We present a new determination of the time delay of the gravitational lens system HE 1104-1805 (``Double Hamburger'') based on a previously unpublished dataset. We argue that the previously published value of Delta tA-B=0.73 years was affected by a bias of the employed method. We determine a new value of Delta tA-B=0.85+/-0.05 years (2sigma confidence level), using six different techniques based on non interpolation methods in the time domain. The result demonstrates that even in the case of poorly sampled lightcurves, useful information can be obtained with regard to the time delay. The error estimates were calculated through Monte Carlo simulations. With two already existing models for the lens and using its recently determined redshift, we infer a range of values of the Hubble parameter: H0=48+/-4 km s-1 Mpc-1 (2sigma ) for a singular isothermal ellipsoid (SIE) and H0=62+/-4 km s-1 Mpc-1 (2sigma ) for a constant mass-to-light ratio plus shear model (M/L+gamma ). The possibly much larger errors due to systematic uncertainties in modeling the lens potential are not included in this error estimate.
Contrary to naive expectation, diluting the stellar component of the lensing galaxy in a highly magnified system with smoothly distributed ``dark'' matter increases rather than decreases the microlensing fluctuations caused by the remaining stars. For a bright pair of images straddling a critical curve, the saddle point (of the arrival time surface) is much more strongly affected than the associated minimum. With a mass ratio of smooth matter to microlensing matter of 4:1, a saddle point with a macromagnification of ;=9.5 will spend half of its time more than a magnitude fainter than predicted. The anomalous flux ratio observed for the close pair of images in MG 0414+0534 is a factor of 5 more likely than computed by Witt, Mao, & Schechter, if the smooth matter fraction is as high as 93%. The magnification probability histograms for macroimages exhibit a distinctly different structure that varies with the smooth matter content, providing a handle on the smooth matter fraction. Enhanced fluctuations can manifest themselves either in the temporal variations of a light curve or as flux ratio anomalies in a single epoch snapshot of a multiply imaged system. While the millilensing simulations of Metcalf & Madau also give larger anomalies for saddle points than for minima, the effect appears to be less dramatic for extended subhalos than for point masses. Moreover, microlensing is distinguishable from millilensing because it will produce noticeable changes in the magnification on a timescale of a decade or less.
Quasar Microlensing
(2001)
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.
Planeten um andere Sterne
(2000)
Due to their extremely small luminosity compared to the stars they orbit, planets outside our own Solar System are extraordinarily difficult to detect directly in optical light. Careful photometric monitoring of distant stars, however, can reveal the presence of exoplanets via the microlensing or eclipsing effects they induce. The international PLANET collaboration is performing such monitoring using a cadre of semi-dedicated telescopes around the world. Their results constrain the number of gas giants orbiting 1-7 AU from the most typical stars in the Galaxy. Upgrades in the program are opening regions of ''exoplanet discovery space'' - toward smaller masses and larger orbital radii - that are inaccessible to the Doppler velocity technique.
We present a data set of images of the gravitationally lensed quasar Q2237+0305, that was obtained at the Apache Point Observatory (APO) between June 1995 and January 1998. Although the images were taken under variable, often poor seeing conditions and with coarse pixel sampling, photometry is possible for the two brighter quasar images A and B with the help of exact quasar image positions from HST observations. We obtain a light curve with 73 data points for each of the images A and B. There is evidence for a long (ga 100 day) brightness peak in image A in 1996 with an amplitude of about 0.4 to 0.5 mag (relative to 1995), which indicates that microlensing has been taking place in the lensing galaxy. Image B does not vary much over the course of the observation period. The long, smooth variation of the light curve is similar to the results from the OGLE monitoring of the system (Wozniak et al. cite{Wozniak00}). Based on observations obtained with the Apache Point Observatory 3.5-meter telescope, which is owned and operated by the Astrophysical Research Consortium.
The declining light curve of the optical afterglow of gamma-ray burst (GRB) GRB000301C showed rapid variability with one particularly bright feature at about t-t0=3.8d. This event was interpreted as gravitational microlensing by Garnavich, Loeb & Stanek and subsequently used to derive constraints on the structure of the GRB optical afterglow. In this paper, we use these structural parameters to calculate the probability of such a microlensing event in a realistic scenario, where all compact objects in the universe are associated with observable galaxies. For GRB000301C at a redshift of z=2.04, the a posteriori probability for a microlensing event with an amplitude of m>=0.95mag (as observed) is 0.7 per cent (2.7 per cent) for the most plausible scenario of a flat -dominated Friedmann- Robertson-Walker (FRW) universe with m=0.3 and a fraction f*=0.2 (1.0) of dark matter in the form of compact objects. If we lower the magnification threshold to m>=0.10mag, the probabilities for microlensing events of GRB afterglows increase to 17 per cent (57 per cent). We emphasize that this low probability for a microlensing signature of almost 1mag does not exclude that the observed event in the afterglow light curve of GRB000301C was caused by microlensing, especially in light of the fact that a galaxy was found within 2arcsec from the GRB. In that case, however, a more robust upper limit on the a posteriori probability of ~5 per cent is found. It does show, however, that it will not be easy to create a large sample of strong GRB afterglow microlensing events for statistical studies of their physical conditions on microarcsec scales.
Microlensing results from APO monitoring of the double quasar Q0957+561A,B between 1995 and 1998
(2000)
If the halo of the lensing galaxy 0957+561 is made of massive compact objects (MACHOs), they must affect the lightcurves of the quasar images Q0957+561 A and B differently. We search for this microlensing effect in the double quasar by comparing monitoring data for the two images A and B - obtained with the 3.5m Apache Point Observatory from 1995 to 1998 - with intensive numerical simulations. This way we test whether the halo of the lensing galaxy can be made of MACHOs of various masses. We can exclude a halo entirely made out of MACHOs with masses between 10-6 Msun and 10-2 Msun for quasar sizes of less than 3x 1014 h60-1/2 cm, hereby extending previous limits upwards by one order of magnitude.
Microlensing of Quasars
(2001)
Microlens-induced variability in multiple quasars can be used to study two cosmological issues of great interest, the size and brightness profile of quasars on one hand, and the distribution of compact (dark) matter along the line of sight on the other. Here a summary is given of recent theoretical progress as well as observational evidence for quasar microlensing, plus a discussion of desired observations and required theoretical studies.