EDP Sciences
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Volume 402, Number 3, May II 2003
First Science with the ODIN satellite
Page(s) 891 - 904
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20030145

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. Stabell1

1  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 )

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  $424.9 \,\pm 1.2$ 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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