Joël Poels

Flux and color variations of the quadruply imaged quasar HE 0435-1223

Aims. We present VRi photometric observations of the quadruply imaged quasar HE0435-1223, carried out with the Danish 1.54 m telescope at the La Silla Observatory. Our aim was to monitor and study the magnitudes and colors of each lensed component as a function of time. Methods. We monitored the object during two seasons (2008 and 2009) in the VRi spectral bands, and reduced the data with two independent techniques: difference imaging and point spread function (PSF) fitting. Results. Between these two seasons, our results show an evident decrease in flux by ≈ 0.2–0.4 magnitudes of the four lensed components in the three filters. We also found a significant increase ( ≈ 0.05–0.015) in their V − R and R − i color indices. Conclusions. These flux and color variations are very likely caused by intrinsic variations of the quasar between the observed epochs. Microlensing effects probably also affect the brightest “A” lensed component.

Photometric monitoring of the doubly imaged quasar UM 673: possible evidence for chromatic microlensing

We present the results of two-band CCD photometric monitoring of the gravitationally lensed quasar Q 0142−100 (UM 673). The data, obtained at ESO-La Silla with the 1.54 m Danish telescope in the Gunn i-band (October 1998−September 1999) and in the Johnson V-band (October 1998 to December 2001), were analyzed using three different photometric methods. The light-curves obtained with all methods show variations, with a peak-to-peak amplitude of 0.14 mag in V. Although it was not possible to measure the time delay between the two lensed QSO images, the brighter component displays possible evidence for microlensing: it becomes bluer as it gets brighter, as expected under the assumption of differential magnification of a quasar accretion disk.

The International Liquid Mirror Telescope (ILMT) as a Variability Time Machine

During the year 2012 the International Liquid Mirror Telescope (a collaboration between astronomical institutions in Belgium, Canada, India and Poland) wil see first light. The instrument will provide substantial, in-depth sky coverage and make an unprecedented number of nightly observations.

Gravitational lensing, dark matter and the optical gravitational lens experiment

After briefly reviewing the history of gravitational lensing, we recall the basic principles of the theory.We then describe and use a simple optical gravitational lens experiment which has the virtue of accounting for all types of image configurations observed so far among the presently known gravitational lens systems.Finally, we briefly present the 4m International Liquid Mirror Telescope project in the context of a photometric monitoring of multiply imaged quasars.