We find that the standard deviation for the observed magnitude of a large microlensed source is δm ≤ 2.17|κ|1/2θ∘/θs even in the presence of non-zero shear.

We present a quantitative analysis of the effect of microlensing caused by random motion of individual stars in a galaxy lensing a background quasar. We calculate a large number of magnification patterns for positions of the stars slightly offset from one frame to the next, and thus obtain light curves for fixed quasar and galaxy positions, only due to the change in the relative star positions. These light curves are analyzed to identify microlensing events, which are then classified with respect to height, duration, and slope. These random motion microlensing events are compared with the corresponding ones caused by the bulk motion of the galaxy.

We find that microlensing events produced by random motion of stars are shorter, steeper, and more frequent than bulk motion events, assuming the velocity dispersion of the stars equals the bulk velocity of the galaxy. The reason for this difference is that in the case of random motion, caustics can move with an arbitrarily high velocity, producing very short events, whereas in the comparison case for bulk motion a microlensing event can never be shorter than it takes a fold caustic, which moves with the velocity of the lensing galaxy projected onto the quasar plane, to cross the quasar. An accompanying video illustrates these results. For three different values of the surface mass density κ, it shows time sequences of 1000 magnification patterns for slowly changing lens positions, together with the positions and velocity vectors of the microlensing stars. The full paper including the video can be found in Wambsganss & Kundić (1995). A short version of the video is available as an MPEG movie under anonymous ftp at astro.princeton.edu, in the directory jkw/microlensing/moving_stars.

We find that the spectra of QSOs evolve: high redshift QSOs (z > 1.5) have lower equivalent width emission lines than low redshift QSOs (z < 0.5) with the same luminosities and radio properties.

We propose that microlensing by compact objects may account for the apparent evolution. If Ω ∼ 0.05 in compact objects, the continuum emission from many high redshift QSOs will be amplified, but not the line emission, leading to the observed decrease in the apparent equivalent widths.

Quasars are known to vary in brightness over a wide range of time scales. Short term intrinsic variability has been well documented, and a strong case can be made that long term variation is due to microlensing. In this paper the effect of time dilation as a means for distinguishing between intrinsic variation and gravitational lensing is discussed.

In order to look for an amplification bias (AB) by gravitational lensing caused by medium redshift (0.2≲ z ≲ 0.8) clusters or groups of galaxies, we compare galaxy counts in deep CCD images of highly luminous, high redshift QSOs with those in nearby control fields at a distance of 1 deg at the same galactic latitude. The total sample contains 37 objects up to now, from which one field had to be excluded because of a seeing difference between the QSO and control fields.

The observed strong UV/Optical excess in many Seyfert 1 galactic nuclei and quasars (QSOs) is well described as thermal radiation from the surface of an optically thick accretion disk surrounding a supermassive black hole (Malkan 1983, Sun and Malkan 1989). This scenario naturally leads to a radially symmetric temperature gradient with the innermost regions the hottest. Thus for a perturbation generated in the inner regions propagating outward, sequential variations from higher to lower frequencies should be expected, aside from the bolometric increase of brightness. However, the extremely intensive International AGN watch (Peterson 1993 and references therein) on NGC 5548 and UV/Optical monitoring campaign on Mkn 335 (Sun et al. 1995) point to opposite conclusions on the time lag between UV and Optical continua, with no lag for NGC 5548 and ∼ 50 days for Mkn 335. We thus began two monitoring programs at Lick Observatory and Mount Stromlo and Siding Spring Observatory (MSSSO) on a sample of Seyfert 1 galaxies with CCD direct imaging to search for more conclusive evidence on the sequential variabilities.

Following Paczyński's (1986) suggestion, many examples of gravitational lensing by Galactic objects have been recorded as a consequence of extensive photometric monitoring programs (e.g. Udalski et al. 1994). Now that such experiments have demonstrated the capability to discover microlensing events against large numbers of target sources, it behooves us to consider whether these techniques can be usefully applied elsewhere.

The extreme difficulty of many essential optical and infrared observations of lens systems has impeded progress and contributed to the popular but erroneous view that lensing is a curiosity rather than an important astrophysical tool. Keck I, with its unprecedented sensitivity for spectroscopy and infrared imaging, will have a major impact on lensing observations.

VLBI studies of small-scale structures in gravitationally lensed systems can provide important constraints on the lens mass distribution. We review the current status of VLBI observations of a number of lensed systems.

We present the results from VLBI observations at three frequencies of the gravitational lens system B0218+357. From the source double structure, seen in both the A and B images at 15 GHz, we have derived a relative magnification matrix, and we show that the lens mass distribution must be non-spherical. We investigate how far the matrix parameters derived from 15 GHz observations can be used to relate the A and B images at 1.7 and 5 GHz, where the image sizes are much larger.

The first phase of a large gravitational lens survey using the Very Large Array at a wavelength of 3.6 cm has been completed, yielding images for 3258 radio sources. The Cosmic Lens All-Sky Survey, or CLASS, is designed to locate gravitational lens systems consisting of multiply-imaged compact components with separations > 0.″2. From this first phase has come the discovery of 1608+656, a quadruply-imaged object with maximum separation of 2.″1. Images from the Palomar 5-m and Keck 10-m telescopes show the lensed images and the lensing galaxy. An optical spectrum obtained with the Palomar 5-m Telescope indicates a redshift of z = 0.63 for the lensing galaxy, and a newly-obtained Palomar spectrum indicates a redshift of z = 1.39 for the lensed source, which appears to be a galaxy. A simple single-galaxy lens model derived from the radio image reproduces the observed configuration and relative fluxes of the images, as well as the position, shape, and orientation of the lensing galaxy. Because a simple mass model is able to fit the observations, we argue that this lens system is promising for determining H0. CLASS has also yielded the new double image lens system 1600+434. The second phase of the survey is scheduled for August and September 1995 on the VLA, and should yield images for an additional 5000+ targets, bringing the CLASS total to over 8000.

We report the serendipitous discovery of quadruple gravitational lens candidates using the Hubble Space Telescope. We have so far discovered two good examples of such lenses, each in the form of four faint blue images located in a symmetric configuration around a red elliptical galaxy. The high resolution of HST has facilitated the discovery of this optically selected sample of faint lenses with small (∼1″) separations between the (I∼ 25–27) lensed components and the much brighter (I∼ 19–22) lensing galaxies. The sample has been discovered in the routine processing of HST fields through the Medium Deep Survey pipeline, which fits simple galaxy models to broad band filter images of all objects detected in random survey fields using WFPC2.

The discovery of gravitationally lensed radio-rings (Hewitt et al. 1988) opened up a new line of attack on the problem of dark matter in galaxies. High–resolution radio observations (Langston et al. 1989) resolve structure tangentially and radially within the rings, providing sophisticated analysis routines (Kochanek & Narayan 1992, Wallington, Narayan & Kochanek 1994) with enough constraints to compute realistic models of the mass distribution within the deflectors (Kochanek 1995). Five such systems are now known and extensive programs to identify further examples are underway.

B1422+231, a multiple component quasar at a redshift of 3.62, is believed to be a gravitationally lensed system (Patnaik et al. 1992). It has 4 components with maximum image separation of 1.3 arcsec. The three brighter components, A, B and C have similar polarization properties at 8.4 GHz. The radio spectra between 5 and 8.4 GHz of A, B and C are similar; since D is very weak its spectrum is not accurately determined.

We report the discovery of a new gravitational lens (GL) candidate for the quasar J03.13 (z = 2.55). The mean angular separation and magnitude difference between the A & B QSO images are found to be 0.84″ and 2.10 mag, respectively. A spatially unresolved medium resolution spectrum of J03.13 shows intervening absorption line systems at z = 2.34 and z = 1.085. The latter one is possibly associated with a σ = 206 km/s lens galaxy.

2016+112 was observed simultaneously with the European VLBI Network (EVN) and MERLIN arrays during the May 1993 joint EVN-MERLIN session at λ18 cm. Common elements to both arrays included the Jodrell Bank 76-m Lovell and 32-m Cambridge telescopes. In order to simultaneously map the entire 4 arcsec2 field of view, various wide-field mapping techniques were employed (see Garrett et al. 1994b).

PKS 1830-211 is a strong, flat-spectrum compact double source with a component spacing of 1 arcsecond. Observations of PKS 1830-211 were made with the Japanese domestic VLBI network at 2.3 GHz and 8.4 GHz bands in sessions between December 1991 and November 1994. The Usuda 64 m (ISAS) and Kashima 34 m (CRL) telescopes were used for all observations, and were used in conjunction with the Mizusawa 10 m (NAO) for observations in 1994. In addition, the total flux was measured with the Usuda 64 m at both bands. Data was recorded using K3 and K4 formatters and recorders, and correlated with NAOCO (the New Advanced One-unit COrrelator of the National Astronomical Observatory).

We report on observations indicating that the bright quadruple lens B1422+231 may be variable in the optical on a time scale of hours. We also find that a model can reproduce the optical positions and magnifications. Unfortunately, the system may be affected by possible microlensing and external shear. HST should provide a good model for the lens, the system should be monitored for several nights in superb seeing (0.3-0.4 arcsec FWHM), and the redshifts of the south-east galaxies should be determined. With this extra information, improved modeling will tell whether it is realistic to obtain H0 from the time delays of B1422+231.

The remarkably strong radio gravitational lens PKS 1830-211 consists of a one arcsecond diameter Einstein ring with two bright compact components located on opposite sides of the ring. We have obtained 22 GHz VLBA data on this source to determine the intrinsic angular sizes of the compact components. Previous VLBI observations at lower frequencies indicate that the brightness temperatures of these components are significantly lower than 1010 K (Jauncey et al. 1991), less than is typical for compact synchrotron radio sources and less than is implied by flux density variations. A possible explanation is that interstellar scattering is broadening the apparent angular size of the source and thereby reducing the observed brightness temperature. Our VLBA data support this hypothesis.

We have been observing the strong radio source and Einstein ring gravitational lens PKS 1830–211 as part of a flux density monitoring program at 2.3 and 8.4 GHz using the 26 m antenna at the Mt Pleasant Observatory. As can be seen from Figure 1 this source is variable at both frequencies. The 2.3 GHz flux density decreased from 10 Jy in 1990 November to a minimum of 9 Jy during 1992, and has since steadily increased to its present ∼12 Jy. Over the same interval, the flux density at 8.4 GHz has exhibited greater variability, more than doubling from an initial value of 5 Jy in late 1990 to a peak of ∼11 Jy early in 1992, before decreasing to a level of ∼7.5 Jy. VLA observations by van Ommen et al. (1995) show that the 15 GHz total flux density increased before the 8.4 GHz brightening seen at Mount Pleasant. Although the 15 GHz time series is not well sampled, it is clear from these data that the 15 GHz outburst precedes the 8.4 GHz feature by ∼400 days.