Gravitational lensing and quasars
Gravitational lensing is an effect of Einstein's theory of general relativity - the presence of matter can curve spacetime and as a result, the path of a light ray will be deflected. In many cases gravitational lensing can be described in analogy to the deflection of light by glass lenses in optics and many useful applications for cosmology have come out of using this property of matter and light. In Fig. 1 an example of the effects of gravitational lensing for a certain system configuration is shown.
The most extreme bending of light rays occurs when the lens is a very compact massive object and the source is close to it. In this case light can take different paths to the observer and multiple images of the source can be seen. If the source varies with time, the multiple images will also vary with time and as the light does not travel the same distance to each image due to the bending of space, there will be time delays for the changes in the images. Up to now several examples of galaxies containing quasars have been discovered which act as gravitational lenses, amplifying and distorting images of galaxies aligned behind them. Quasars are compact regions in the center of massive galaxies and are among the brightest objects in the universe, far outshining the total starlight of their host galaxies.
At the Instituto de Astrofisica de Canarias' Teide Observatory the Quasar SDSS J1004+4112 was monitored between 2003 and 2010 and is split into at least four images by an intervening galaxy cluster. One of the goals is to study intrinsic quasar variations as well as microlensing variations and to estimate the accretion disk size of the quasar source. To do this, first of all the magnitudes of a reference star and the four quasar images are calculated using IRAF. As a next step the time delays between the four images are estimated and after correcting them, one can simulate a light curve that represents the quasar variability. To get the microlensing variability for each time, the intrinsic quasar variability has to be subtracted from each light curve. Once done this, one can create a magnification map for the lensed quasar (see Fig. 2), i.e. a map that shows the amount of magnification produced by a certain lens system in a region of the source plane. If the lens and source move with respect to each other, the source suffers different magnifications at different times, which are observed as variations in the brightness of the source (i.e. light curves). With the information obtained by studying microlensing one can estimate the accretion disk size of the quasar.