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A&A 402, 891-904 (2003)
DOI: 10.1051/0004-6361:20030145
New aperture photometry of QSO 0957+561; application to time delay and microlensing
J. E. Ovaldsen1, J. Teuber2, R. E. Schild3 and R. Stabell11 Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway
2 Centre for Advanced Signal Processing, Copenhagen, Denmark
3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
(Received 1 October 2002 / Accepted 28 January 2003 )
Abstract
We present a re-reduction of archival CCD frames of the
doubly imaged quasar 0957+561 using a new photometry code. Aperture
photometry with corrections for both cross contamination between the
quasar images and galaxy contamination is performed on about 2650
R-band images from a five year period (1992-1997). From the
brightness data a time delay of
days is derived
using two different statistical techniques. The amount of
gravitational microlensing in the quasar light curves is briefly
investigated, and we find unambiguous evidence of both long term and
short term microlensing. We also note the unusual circumstance
regarding time delay estimates for this gravitational
lens. Estimates by different observers from different data sets or
even with the same data sets give lag estimates differing by
typically 8 days, and error bars of only a day or two. This probably
indicates several complexities where the result of each estimate
depends upon the details of the calculation.
Key words: gravitational lensing -- quasars: individual: QSO 0957+561 -- techniques: photometric -- methods: data analysis
Offprint request: J. E. Ovaldsen, j.e.ovaldsen@astro.uio.no
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