New Aperture and PSF Photometry of QSO 0957+561A,B : Application to Time Delay and Microlensing

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.