A&A 441, 443-450 (2005)
DOI: 10.1051/0004-6361:20052877

Photometric monitoring of the doubly imaged quasar UM 673: possible evidence for chromatic microlensing

Th. Nakos^{1, 2, 3}, F. Courbin⁴, J. Poels², C. Libbrecht^{2, 3}, P. Magain², J. Surdej², J. Manfroid², I. Burud⁵, J. Hjorth⁶, L. Germany¹, C. Lidman¹, G. Meylan⁴, E. Pompei¹, J. Pritchard^{7, 6, 1} and I. Saviane<sup>1

1</sup>  European Southern Observatory, Casilla 19001, Santiago, Chile
²  Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Sart Tilman, Bât. B5C, 4000 Liège, Belgium
    e-mail: nakos@astro.ulg.ac.be
³  Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
⁴  Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland
    e-mail: frederic.courbin@epfl.ch
⁵  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶  Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100, København Ø, Denmark
⁷  Mount John University Observatory and Department of Physics & Astronomy, University of Canterbury, Private Bag 4800, Christchurch, New Zealand

(Received 14 February 2005 / Accepted 3 June 2005)

Abstract
We present the results of two-band CCD photometric monitoring of the gravitationally lensed quasar Q 0142-100 (UM 673). The data, obtained at ESO-La Silla with the 1.54 m Danish telescope in the -band (October 1998-September 1999) and in the Johnson V-band (October 1998 to December 2001), were analyzed using three different photometric methods. The light-curves obtained with all methods show variations, with a peak-to-peak amplitude of 0.14 mag in V. Although it was not possible to measure the time delay between the two lensed QSO images, the brighter component displays possible evidence for microlensing: it becomes bluer as it gets brighter, as expected under the assumption of differential magnification of a quasar accretion disk.

Key words: gravitational lensing -- quasars: individual: Q 0142-100 (UM 673)

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