TY - JOUR A1 - Wambsganß, Joachim A1 - Schmidt, Robert W. A1 - Colley, W. A1 - Kundic, T. A1 - Turner, E. L. T1 - Microlensing results from APO monitoring of the double quasar Q0957+561A,B between 1995 and 1998 N2 - 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. Y1 - 2000 ER - TY - JOUR A1 - Wambsganß, Joachim A1 - Schindler, Sabine T1 - ROSAT observations of clusters CL0500-24 & CL0939+4713 Y1 - 1999 ER - TY - JOUR A1 - Wambsganß, Joachim A1 - Hasinger, Günther A1 - Giacconi, R. A1 - Gunn, J. E. A1 - Lehmann, Ingo A1 - Schmidt, M. A1 - Schneider, D. P. A1 - Stanke, Thomas A1 - Trümper, J. A1 - Woods, D. A1 - Zamorani, G. T1 - A Distant X-ray Selected, Gravitationally-Lensing Galaxy Cluster Y1 - 2000 ER - TY - JOUR A1 - Wambsganß, Joachim A1 - Cen, Renyue A1 - Xu, Guohong A1 - Ostriker, Jeremiah P. T1 - Effects of Weak Gravitational Lensing from Large-Scale Structure of the Determination of Q 0 N2 - 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. Y1 - 1997 ER - TY - JOUR A1 - Wambsganß, Joachim A1 - Brunner, H. A1 - Schindler, Sabine A1 - Falco, E. T1 - The gravitationally lensed quasar Q2237+0305 in X-rays: ROSAT/HRI detection of the "Einstein Cross" N2 - 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). Y1 - 1999 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - What gravitational microlensing can reveal about (the central parts of) AGNs Y1 - 2002 SN - 1-583-81135-4 ER - TY - THES A1 - Wambsganß, Joachim T1 - Gravitational lensing as a universal astrophysical tool Y1 - 1998 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - Mimicking the most luminous objects with gravitational lensing Y1 - 2002 SN - 3-540-43769- x ER - TY - JOUR A1 - Wambsganß, Joachim T1 - The search for matter with gravitational lensing Y1 - 2002 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - Gravitational Lensing : a universal astrophysical tool N2 - 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. Y1 - 2001 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - Microlensing of Quasars N2 - 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. Y1 - 2001 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - Gravity's kaleidoscope : four consequences of gravitational light deflection Y1 - 2001 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - Quasar Microlensing Y1 - 2001 SN - 1-583-81074-9 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - Cosmological microlensing Y1 - 2001 SN - 1-583-81076-5 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - Gravitational lensing as a universal astrophysical tool Y1 - 2001 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - Gravitational lensing as a universal astrophysical tool Y1 - 2001 SN - 1-563-96986-6 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - Gravitationslinsen - universelle Werkzeuge der Astrophysik Y1 - 2000 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - Nützliche Illusionen : Astrophysik mit Gravitationslinsen Y1 - 2000 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - Planeten um andere Sterne Y1 - 2000 ER - TY - JOUR A1 - Wambsganß, Joachim T1 - Gravitational Lensing as a Universal Astrophysical Tool Y1 - 1999 ER -