Abstract
Five years (1992-1997, ~2650 images) of optical observations of the
gravitational lens system QSO 0957+561 were analyzed using a new
photometry package developed by Jan Teuber and me. My supervisor was
Prof. Rolf Stabell, and the CCD images were kindly provided by Prof.
Rudy Schild.
The fully automated program was written in the IDL environment
and incorporated all operations needed to produce the final light
curves of the quasar images; from source detection, field star
photometry, calibration and finally, photometry of the quasar
images. We also corrected for the light-contamination by the lens
galaxy and for the crosstalk between the closely juxtaposed quasar
components. Both aperture photometry and PSF-fitting photometry were
performed.
From the brightness data, the time delay between the twin images was
determined utilizing two different methods: dispersion estimation
technique and chi-square minimization. We inferred a time delay of
~425 days, with an estimated 1-sigma formal error of about 1.2 days
(which is probably too optimistic).
The effects of gravitational microlensing on the measured brightness
of the two quasar images were also briefly investigated. Microlensing-induced
variations with a time scale of a year and amplitude of ~0.05
magnitudes were observed, and a few other tentative fluctuations in
the microlensing curve had shorter time scales.