The ongoing microlensing experiments have now discovered more than 70 candidate events (Alcock et al. 1993, Bennett et al. 1994, Aubourg et al. 1993, Udalski et al. 1994). These experiments have put important constraints on the dark matter content of the Galactic halo (Alcock et al. 1995a) and yielded many interesting results about Galactic structure (Paczyński et al. 1994, Stanek et al. 1994).

The limbs of hot stars are polarized due to electron scattering, but this polarization cancels out due to rotational symmetry. During microlensing, the star is amplified by different amounts across its surface so that the limb polarization no longer cancels out, but can be observable. Polarization can be much higher for a caustic crossing during microlensing by a binary lens than for a single lens.

We predict that chromatic and spectroscopic effects are likely to appear for a significant fraction of microlensing events. Differential amplification across a stellar disc produces a time and wavelength dependent signature which constrains the parameters of the lenses and can be used for 3-D mapping of stellar atmospheres.

Gravitational lensing and the time delay of a pulsar signal in the gravitational field of a mass are General Relativistic effects that may be used as a tool to detect the observational parameters of dark matter in our Galaxy. We propose to use observations of the time delay of pulses from pulsars to detect lensing objects located close to the line of the sight, to study the distribution of dark matter in our Galaxy. We discuss the possibility of finding such an event by measuring the delay of pulses from a pulsar, and apply it to data for PSR B0525+21.

The degree of microlensing induced amplification is dependent upon the size of a source. As quasar spectra consist of the sum of emission from different regions this scale dependent amplification can produce spectral differences between the images of a macrolensed quasar. This paper presents the first direct spectroscopic evidence for this effect, providing a limit on the scale of the continuum and the broad line emission regions at the center of a source quasar (2237+0305). Lack of centroid and profile differences in the emission lines indicate that substructure in the broad emission line region is > 0.05 parsecs.

We show that the z = 2.3 IRAS source F10214+4724 is gravitationally lensed by an intervening galaxy, as suggested by the observed near-IR structures. Its many anomalous properties can be explained if the source is an ordinary Seyfert 2 nucleus whose central regions are much more highly magnified than the surrounding host galaxy. Confirming expectations, we find a counterimage to the near-IR arc, and find spectral evidence for the lensing galaxy at z ∼ 1. We present new optical images which show that the optical source is compact and highly magnified. F10214+4724 may represent a population of lensed AGNs whose central engines are obscured.

During the ESO key-program on gravitational lenses, the light curves of two images (A and B) of the lensed quasar UM 425 were obtained (from 1987 to 1995). This poster presents a possible interpretation of the light curves in terms of a micro-lensing event in the faint B component of the system, without ruling out a possible intrinsic variation of the source.

Witt & Mao (1994) claim that the data reported by Racine (1992) contains “a quite well sampled M-shaped double event in image A of 0.3 and 0.4 mag, respectively”. They further state that the very low average mass scenario put forward by Refsdal & Stabell (1991) does not predict “well-resolved asymmetric events, as have been observed in image A”.

Determining the nature of spectral variability in gravitationally-lensed BAL (broad absorption line) quasars is important for time-delay and microlensing studies because the delay-times can be comparable with the time scales of intrinsic QSO ionization changes. Far-UV spectra of the candidate lens UM 425 = Q1120+019A and PG1115+080A (Triple Quasar) revealed the presence of strong O VI λ1033 emission. In both cases, absorption in the blue-wing of the O VI line profile indicated characteristic BAL outflow.

After some preliminary observations in 1990, a program was started in 1991 at the Nordic Optical Telescope (NOT) on the island of La Palma for monitoring mainly four well known gravitational lens systems: QSO0142-100, QSO0957+561, QS01413+117 and QSO2237+0305 (The Einstein Cross). Here we report results from the monitoring of the Einstein Cross. During the four year period the Monitor Program was active (1990-1993) we gathered 55 high quality observations of this well known lens system. In addition we have collected 32 more observations in 1994 and a further four so far in 1995, a total of 91 new observations of the Einstein Cross.

Here we report results from the monitoring of the Einstein Cross. During the four year period the Monitor Program was active (1990–1993) we gathered 55 high quality observations of this well known lens system. In addition we have collected 32 more observations in 1994 and a further four so far in 1995, a total of 91 new observations of the Einstein Cross.

Direct CCD images of the gravitationally lensed BAL quasar H1413+117 (Bessel B, V, R, I; Gunn r and i), obtained during 1989-1994 in the framework of photometric monitorings at ESO and with the NOT, have been carefully (re-)analyzed. Simultaneous fitting of 4 PSFs, fixing the relative positions of components B, C and D with respect to A, has been achieved successfully. The relative light curves of the 4 lensed QSO images, with respect to a stable nearby comparison star, confirm the general trend already presented by Arnould et al. (1993). The amplitude of the light curves is found to be about 0.5 magnitude, peak-to-peak in the V filter. The four lensed components display brightness variations quasi-simultaneously and in parallel. In addition to these intrinsic light variations due to the source, we find that component D shows extra light variations with respect to the other components. These are very likely caused by micro-lensing.

PG1115+080 (Weymann et al. 1980), the first quadruple lens system, remains the most promising for an optical measurement of the Hubble constant through a time delay. Factors contributing to this assessment include the well constrained geometry, the apparent magnitudes of the four components (Christian et al. 1987), the expected time delay (Narasimha & Chitre 1992), and the position of the lensing galaxy (Kristian et al. 1993). There are nonetheless many factors which combine to render this a difficult measurement, among them the small separation of the images and the small variability amplitude and long variability time scale of the “typical” optically selected quasar (Hook et al 1994). The measurement is further complicated by the likelihood of microlensing of the individual components (Witt et al. 1995), which depends upon the degree to which the quasar is resolved and the fraction of the lens mass in microlenses.

The BL-Lac object PKS 0537-441 has recently displayed strong intraday variability at 1.42 GHz on time scales of ∼ 104 s. Such kind of variability is hardly consistent with an intrinsic-to-source origin, since apparent brightness temperatures higher than 1021 K are implied for the emitting region in the source.

An alternative explanation for this ultra-rapid radio variability in PKS 0537-441 is developed based on gravitational microlensing of a superluminal component in the blazar caused by compact objects in the halo of an intervening galaxy. The mass of the microlenses is estimated and the blazar is proposed as a good candidate in a systematic search for extragalactic MACHOs.

The TwQSO microlensing shows rich structure with cusp-profiled features on time scales of weeks to months, and with periodic effects on time scales of a few years.

It is shown how the theory of scintillation may be applied to treat gravitational lensing. The theory is applied to microlensing by a system of N point masses. It is shown that scintillation theory reproduces a known result for the angular broadening due to multiple microlensing. Some unresolved differences between scintillation and microlensing theories for intensity fluctuations are pointed out.

The contouring methods described by Lewis et al. (1993) and Witt (1993) are very efficient for obtaining the magnification of a point source moving along a straight track in the source plane. For finite sources, however, the amplification must be computed for numerous parallel tracks and then convolved with the source profile. Rayshooting, on the other hand, is an efficient algorithm for relatively large sources, but the computing time increases with the inverse of the source area for a given noise level.

In gravitational lens systems with 3 or more resolved images of a quasar, the intrinsic variability may be unambiguously separated from the microlensing variability through parallax measurements from 3 observers when there is no relative motion of the lens masses (Refsdal 1993). In systems with fewer than 3 resolved images, however, this separation is not straightforward. For the purpose of illustration, I make the following simplifications for the one-dimensional case: The observations consist of well-sampled time series of the observed flux FA(ti) and FB(ti) at two points in the observer plane. The separation vector of the two points is parallel to the direction of the transverse motion of the source-lens-observer system, and the distance DAB between the observers is known. Furthermore, the distance DAB is small compared to the typical length scale of fluctuations in the magnification μ(x).

Since the image separations produced by microlensing are inaccessible to observation, we are left to observe either a change in total amplitude or the time delay between the images. Spillar (1993) has suggested that a time delay might be observed in the autocorrelation of a microlensed signal. The time delays are of order a few microseconds and are easily accessible to a sufficiently wide bandwidth system. We calculate expected observational results using the Green Bank Telescope with a VLBA recording system, and find that brightness temperatures exceeding the Compton limit are required for detection.

Consider a gravitational lens system with K planes. If light rays are traced back from the observer to the light source plane, then the points on the first lens plane where a light ray either terminates, or, passes through and terminates before reaching the light source plane, are “obstruction points.” More precisely, tracing rays back to the source plane induces a K-plane lensing map η : U ⊆ R2 → R2 of the form η(x1) = x1 −∑i=1k αi(xi(xi)). We then define an obstruction point of η to be a point a of U where limx1→a |αi(xi(x1))| = ∞ for some “deflection angle” αi.

Consider two point masses m1 and m2 on distinct planes with respective shears γ1, γ2 and continuous matter having densities κ1 and κ2. It is assumed that the lens equation is as follows: where