@article{deFilippisSchindlerCastilloMoralesetal.2002,
  author    = {de Filippis, E. and Schindler, Sabine and Castillo-Morales, A. and Schwope, A. D and Wambsganß, Joachim},
  title     = {Discovery of holes in the core of the distant galaxy cluster RBS797 in a CHANDRA observation},
  isbn      = {1-583-81108-7},
  year      = {2002},
  language  = {en}
}
@article{GilMerinoWisotzkiWambsganss2002,
  author    = {Gil-Merino, Rodrigo and Wisotzki, Lutz and Wambsganß, Joachim},
  title     = {The Double Quasar HE 1104-1805 : a case study for time delay determination with poorly sampled lightcurves},
  year      = {2002},
  abstract  = {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of the time delay of the gravitational lens system HE 1104-1805 (``Double Hamburger'') based on a previously unpublished dataset.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.},
  language  = {en}
}
@article{GranotSchechterWambsganss2003,
  author    = {Granot, J. and Schechter, P. L. and Wambsganß, Joachim},
  title     = {The mean number of extra microimage pairs for macrolensed quasars},
  year      = {2003},
  abstract  = {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.},
  language  = {en}
}
@misc{HellingBergholzWalteretal.2004,
  author    = {Helling, Robert and Bergholz, Natalie and Walter, Kai and Bartram, Kristin and Humborg, Christian and Lieber, Silke and Armstrong, Stephen Andrew and Krotoschak, Kai-Uwe and Pohl, Franzisca and Wambsganß, Joachim and Gebhardt, Thomas},
  title     = {Portal alumni},
  series = {Das Ehemaligen-Magazin der Universit{\"a}t Potsdam},
  volume    = {2004},
  journal   = {Das Ehemaligen-Magazin der Universit{\"a}t Potsdam},
  number    = {1},
  organization    = {Stabsstelle Studierendenmarketing/Alumniprogramm Im Auftrag der Pr{\"a}sidentin der Universit{\"a}t Potsdam},
  doi       = {10.25932/publishup-48098},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-480981},
  pages     = {58},
  year      = {2004},
  abstract  = {More than 800 alumni have expressed an interest in staying in touch with the University of Potsdam by registering for the Alumni Programme since it was launched in May 2003. Among them are many international alumni like you. Whether you were here as a student, doing research or were employed at the University of Potsdam, we hope that you enjoyed your time with us and that you have good memories of it. Today, you have opened the first edition of our new, annual alumni magazine. lt is another important part of our Alumni Pogramme that will provide you with regular news from your form er University. The main feature of this edition is "weit weg", far away. When making contact again with the University's alumni in May of 200J, we found out that alumni have been scattered to the winds - some of you are in Australia, Africa and America. We were curious to hear how those far away had managed the transition of moving to live abroad, and we also wanted to learn about their experiences on the way. Apart from lots of exciting stories and recommendations for others, you will find information on the issues of careers and mobility. And next to what alumni with an international profile have to say, you will also read about how professors of the University of Potsdam are working with their alumni at the moment, and what they are planning to do in the future. We regret not being able to publish this magazine bilingually. However; we have added abstracts in English to these articles, hoping to help you with your reading comprehension. Also, we have launched an English website for those of you who only read English. You will find it at www.alumni.uni-potsdam.de. The website gives general information, and you can read this magazine online.},
  language  = {de}
}
@article{KoopmansBruynWambsganssetal.2001,
  author    = {Koopmans, L. V. E. and Bruyn, A. G. and Wambsganß, Joachim and Fassnacht, C. D.},
  title     = {A radio-microlensing caustic crossing in B1600+434?},
  isbn      = {1-538-81076-5},
  year      = {2001},
  abstract  = {First, we review the current status of the detection of strong `external' variability in the CLASS gravitational B1600+434, focusing on the 1998 VLA 8.5-GHz and 1998/9 WSRT multi-frequency observations. We show that this data can best be explained in terms of radio-microlensing. We then proceed to show some preliminary results from our new multi-frequency VLA monitoring program, in particular the detection of a strong feature (~30\%) in the light curve of the lensed image which passes predominantly through the dark-matter halo of the lens galaxy. We tentatively interpret this event, which lasted for several weeks, as a radio-microlensing caustic crossing, i.e. the superluminal motion of a micro-arcsec-scale jet-component in the lensed source over a single caustic in the magnification pattern, that has been created by massive compact objects along the line-of-sight to the lensed image.},
  language  = {en}
}
@article{KoopmansdeBruynWambsganss2000,
  author    = {Koopmans, L. V. E. and de Bruyn, A. G. and Wambsganß, Joachim},
  title     = {Radio Microlensing and dark-matter in high-z galaxies},
  year      = {2000},
  language  = {en}
}
@article{KoopmansWambsganss2001,
  author    = {Koopmans, L. V. E. and Wambsganß, Joachim},
  title     = {On the probability of microlensing in gamma-ray burst afterglows},
  year      = {2001},
  abstract  = {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.},
  language  = {en}
}
@article{LehmannHasingerGiacconietal.2000,
  author    = {Lehmann, Ingo and Hasinger, G{\"u}nther and Giacconi, R. and Gunn, J. E. and Schmidt, M. and Schneider, D. P. and Stanke, Thomas and Tr{\"u}mper, J. and Wambsganß, Joachim and Woods, D. and Zamorani, G.},
  title     = {One of the highest redshift X-Ray selected clusters of galaxies},
  isbn      = {3-540-67163-3},
  year      = {2000},
  language  = {en}
}
@book{PettersLevineWambsganss2001,
  author    = {Petters, Arlie O. and Levine, Harold and Wambsganß, Joachim},
  title     = {Singularity theory and gravitational lensing},
  series = {Progress in mathematical physics},
  volume    = {21},
  journal   = {Progress in mathematical physics},
  publisher = {Birkh{\"a}user},
  address   = {Boston},
  isbn      = {0-8176-3668-4},
  pages     = {XXIV, 603 S. : Ill., graph. Darst.},
  year      = {2001},
  language  = {en}
}
@article{PugaAlcaldeBarrenaetal.2001,
  author    = {Puga, E. and Alcalde, D. and Barrena, R. and Mediavilla, E. and Motta, V. and Munoz, J. A. and Oscoz, Alejandro and Serra-Ricart, M. and Gil-Merino, Rodrigo and Goicoechea, Luis Julian and Lewis, G. f and Libbrecht, C. and Moreau, O. and Surdej, J. and Wambsganß, Joachim},
  title     = {Daily monitoring of the gravitational lens QSO 2237+0305 at the Nordic Optical Telescope},
  isbn      = {0-7923-6974-2},
  year      = {2001},
  language  = {en}
}
@article{SackettAlbrowBeaulieuetal.2003,
  author    = {Sackett, Penny D. and Albrow, Michael D. and Beaulieu, Jean-Philippe and Caldwell, John A. R. and Coutures, C. and Dominik, M. and Greenhill, John and Hill, K. and Horne, Keith and Jorgensen, Uffe Grae and Kane, Stephen R. and Kubas, Daniel and Martin, Ralph and Menzies, J. W. and Pollard, K. R. and Sahu, K. C. and Wambsganß, Joachim and Watson, R. and Williams, A.},
  title     = {PLANET II : a microlensing and transit search for extrasolar planets},
  year      = {2003},
  abstract  = {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.},
  language  = {en}
}
@article{SchechterWambsganss2002,
  author    = {Schechter, P. L. and Wambsganß, Joachim},
  title     = {Quasar microlensing at high magnification and the role of dark matter : enhanced fluctuations and suppressed saddle points},
  year      = {2002},
  abstract  = {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.},
  language  = {en}
}
@article{SchindlerCastilloMoralesDeFilippisetal.2001,
  author    = {Schindler, Sabine and Castillo-Morales, A. and De Filippis, E. and Schwope, A. D. and Wambsganß, Joachim},
  title     = {Discovery of depressions in the X-ray emission of the distant galaxy cluster RBS797 in a CHANDRA observation},
  year      = {2001},
  abstract  = {We present CHANDRA observations of the X-ray luminous, distant galaxy cluster RBS797 at z=0.35. In the central region the X-ray emission shows two pronounced X-ray minima, which are located opposite to each other with respect to the cluster centre. These depressions suggest an interaction between the central radio galaxy and the intra-cluster medium, which would be the first detection in such a distant cluster. The minima are symmetric relative to the cluster centre and very deep compared to similar features found in a few other nearby clusters. A spectral and morphological analysis of the overall cluster emission shows that RBS797 is a hot cluster (T=7.7+1.2-1.0 keV) with a total mass of Mtot(r500)= 6.5+1.6-1.2 *E14Msun.},
  language  = {en}
}
@article{SchmidtKundicPenetal.2002,
  author    = {Schmidt, Robert W. and Kundic, T. and Pen, U.-L. and Wambsganß, Joachim and Bergeron, L. E. and Colley, W. and Corson, C. and Hastings, N. G. and Hoyes, T. and Long, D. C. and Loomis, K. A. and Malhotra, S. and Rhoads, J. E. and Stanek, K. Z.},
  title     = {Optical monitoring of the gravitationally lensed quasar Q2237+0305 from APO between June 1995 and January 1998},
  year      = {2002},
  abstract  = {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.},
  language  = {en}
}
@article{SchmidtWambsganssPenetal.2001,
  author    = {Schmidt, Robert W. and Wambsganß, Joachim and Pen, U.-L. and Turner, E. L.},
  title     = {APO monitoring of Q2237+0305 in 1995-97 : evidence for microlensing},
  isbn      = {1-583-81074-9},
  year      = {2001},
  language  = {en}
}
@article{TorresRomeroEiroaetal.2003,
  author    = {Torres, D. F. and Romero, G. E. and Eiroa, E. F. and Wambsganß, Joachim and Pessah, M. E.},
  title     = {Gravitational microlensing of gamma-ray blazars},
  year      = {2003},
  abstract  = {We present a detailed study of the effects of gravitational microlensing on compact and distant gamma-ray blazars. These objects have gamma-ray-emitting regions that are small enough to be affected by microlensing effects produced by stars lying in intermediate galaxies. We compute the gravitational magnification taking into account effects of the lensing and show that, whereas the innermost gamma-ray spheres can be significantly magnified, there is little magnification either for very high gamma-ray energies or for lower (radio) frequencies (because these wavelengths are emitted from larger regions). We analyse the temporal evolution of the gamma-ray magnification for sources moving in a caustic pattern field, where the combined effects of thousands of stars are taken into account using a numerical technique. We propose that some of the unidentified gamma-ray sources (particularly some of those lying at high galactic latitude with gamma-ray statistical properties that are very similar to detected gamma-ray blazars) are indeed the result of gravitational lensing magnification of background undetected active galactic nuclei (AGN). This is partly supported from a statistical point of view: we show herein as well, using the latest information from the third EGRET catalogue, that high-latitude gamma-ray sources have similar averaged properties to already detected gamma-ray AGN. Some differences between both samples, regarding the mean flux level, could also be understood within the lensing model. With an adequate selection of lensing parameters, it is possible to explain a variety of gamma-ray light curves with different time-scales, including non-variable sources. The absence of strong radio counterparts could be naturally explained by differential magnification in the extended source formalism.},
  language  = {en}
}
@article{Wambsganss2002,
  author    = {Wambsganß, Joachim},
  title     = {What gravitational microlensing can reveal about (the central parts of) AGNs},
  isbn      = {1-583-81135-4},
  year      = {2002},
  language  = {en}
}
@phdthesis{Wambsganss1998,
  author    = {Wambsganß, Joachim},
  title     = {Gravitational lensing as a universal astrophysical tool},
  pages     = {201 S.},
  year      = {1998},
  language  = {en}
}
@article{Wambsganss2002,
  author    = {Wambsganß, Joachim},
  title     = {Mimicking the most luminous objects with gravitational lensing},
  isbn      = {3-540-43769- x},
  year      = {2002},
  language  = {en}
}
@article{Wambsganss2002,
  author    = {Wambsganß, Joachim},
  title     = {The search for matter with gravitational lensing},
  year      = {2002},
  language  = {en}
}
@article{Wambsganss2001,
  author    = {Wambsganß, Joachim},
  title     = {Gravitational Lensing : a universal astrophysical tool},
  year      = {2001},
  abstract  = {In the roughly 20 years of its existence as an observational science, gravitational lensing has established itself as a valuable tool in many astrophysical fields. In the introduction of this review we briefly present the basics of lensing. Then it is shown that the two propagation effects, lensing and scintillation, have a number of properties in common. In the main part various lensing phenomena are discussed with emphasis on recent observations.},
  language  = {en}
}
@article{Wambsganss2001,
  author    = {Wambsganß, Joachim},
  title     = {Microlensing of Quasars},
  year      = {2001},
  abstract  = {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.},
  language  = {en}
}
@article{Wambsganss2001,
  author    = {Wambsganß, Joachim},
  title     = {Gravity's kaleidoscope : four consequences of gravitational light deflection},
  year      = {2001},
  language  = {en}
}
@article{Wambsganss2001,
  author    = {Wambsganß, Joachim},
  title     = {Quasar Microlensing},
  isbn      = {1-583-81074-9},
  year      = {2001},
  language  = {en}
}
@article{Wambsganss2001,
  author    = {Wambsganß, Joachim},
  title     = {Cosmological microlensing},
  isbn      = {1-583-81076-5},
  year      = {2001},
  language  = {en}
}
@article{Wambsganss2001,
  author    = {Wambsganß, Joachim},
  title     = {Gravitational lensing as a universal astrophysical tool},
  year      = {2001},
  language  = {en}
}
@article{Wambsganss2001,
  author    = {Wambsganß, Joachim},
  title     = {Gravitational lensing as a universal astrophysical tool},
  isbn      = {1-563-96986-6},
  year      = {2001},
  language  = {en}
}
@article{Wambsganss2000,
  author    = {Wambsganß, Joachim},
  title     = {Gravitationslinsen - universelle Werkzeuge der Astrophysik},
  year      = {2000},
  language  = {de}
}
@article{Wambsganss2000,
  author    = {Wambsganß, Joachim},
  title     = {N{\"u}tzliche Illusionen : Astrophysik mit Gravitationslinsen},
  year      = {2000},
  language  = {de}
}
@article{Wambsganss2000,
  author    = {Wambsganß, Joachim},
  title     = {Planeten um andere Sterne},
  year      = {2000},
  language  = {de}
}
@article{Wambsganss1999,
  author    = {Wambsganß, Joachim},
  title     = {Gravitational Lensing as a Universal Astrophysical Tool},
  year      = {1999},
  language  = {en}
}
@article{Wambsganss1999,
  author    = {Wambsganß, Joachim},
  title     = {Gravitational lensing : numerical simulations with a hierarchical tree code},
  year      = {1999},
  language  = {en}
}
@article{WambsganssBrunnerSchindleretal.1999,
  author    = {Wambsganß, Joachim and Brunner, H. and Schindler, Sabine and Falco, E.},
  title     = {The gravitationally lensed quasar Q2237+0305 in X-rays: ROSAT/HRI detection of the "Einstein Cross"},
  year      = {1999},
  abstract  = {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).},
  language  = {en}
}
@article{WambsganssCenXuetal.1997,
  author    = {Wambsganß, Joachim and Cen, Renyue and Xu, Guohong and Ostriker, Jeremiah P.},
  title     = {Effects of Weak Gravitational Lensing from Large-Scale Structure of the Determination of Q 0},
  year      = {1997},
  abstract  = {Weak gravitational lensing by large-scale structure affects the determination of the cosmological deceleration parameter q0. We find that the lensing induced dispersions on truly standard candles are 0.04 and 0.02 mag at redshift z = 1 and z = 0.5, respectively, in a COBE-normalized cold dark matter universe with Omega 0 = 0.40, Lamda 0 = 0.6, H = 65 km s-1 Mpc-1, and sigma 8 = 0.79. It is shown that one would observe q0 = -0.395^{+0.125}_{-0.095} and q0 = - 0.398^{+0.048}_{-0.077} (the error bars are 2 sigma limits) with standard candles with zero intrinsic dispersion at redshift z = 1 and z = 0.5, respectively, compared to the truth of q0 = -0.400. A standard COBE normalized Omega 0 = 1 CDM model would produce three times as much variance and a mixed (hot and cold) dark matter model would lead to an intermediate result. One unique signature of this dispersion effect is its non-Gaussianity. Although the lensing induced dispersion at lower redshift is still significantly smaller than the currently best observed (total) dispersion of 0.12 mag in a sample of type Ia supernovae, selected with the multicolor light curve shape method, it becomes significant at higher redshift. We show that there is an optimal redshift, in the range z ~ 0.5--2.0 depending on the amplitude of the intrinsic dispersion of the standard candles, at which q0 can be most accurately determined.},
  language  = {en}
}
@article{WambsganssHasingerGiacconietal.2000,
  author    = {Wambsganß, Joachim and Hasinger, G{\"u}nther and Giacconi, R. and Gunn, J. E. and Lehmann, Ingo and Schmidt, M. and Schneider, D. P. and Stanke, Thomas and Tr{\"u}mper, J. and Woods, D. and Zamorani, G.},
  title     = {A Distant X-ray Selected, Gravitationally-Lensing Galaxy Cluster},
  year      = {2000},
  language  = {en}
}
@article{WambsganssSchindler1999,
  author    = {Wambsganß, Joachim and Schindler, Sabine},
  title     = {ROSAT observations of clusters CL0500-24 \& CL0939+4713},
  year      = {1999},
  language  = {en}
}
@article{WambsganssSchmidtColleyetal.2000,
  author    = {Wambsganß, Joachim and Schmidt, Robert W. and Colley, W. and Kundic, T. and Turner, E. L.},
  title     = {Microlensing results from APO monitoring of the double quasar Q0957+561A,B between 1995 and 1998},
  year      = {2000},
  abstract  = {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.},
  language  = {en}
}