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Issue A&A Volume 416, Number 1, March II 2004 Page(s) 19 - 34 Section Cosmology (including clusters of galaxies) DOI 10.1051/0004-6361:20034284

A&A 416, 19-34 (2004)
DOI: 10.1051/0004-6361:20034284

Astrometric microlensing of quasars

Dependence on surface mass density and external shear

M. Treyer¹ and J. Wambsganss<sup>2

1</sup>  Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, 13376 Marseille, France
    e-mail: marie.treyer@oamp.fr
²  Universität Potsdam, Institut für Physik, Am Neuen Palais 10, 14467 Potsdam, Germany

(Received 5 September 2003 / Accepted 9 November 2003

)

Abstract
A small fraction of all quasars are strongly lensed and multiply imaged, with usually a galaxy acting as the main lens. Some, or maybe all of these quasars are also affected by microlensing, the effect of stellar mass objects in the lensing galaxy. Usually only the photometric aspects of microlensing are considered: the apparent magnitudes of the quasar images vary independently because the relative motion between source, lens and observer leads to uncorrelated magnification changes as a function of time. However, stellar microlensing on quasars has yet another effect, which was first explored by Lewis & Ibata (1998): the position of the quasar - i.e. the center-of-light of the many microimages - can shift by tens of microarcseconds due to the relatively sudden (dis-)appearance of a pair of microimages when a caustic is being crossed.

Here we explore quantitatively the astrometric effects of microlensing on quasars for different values of the lensing parameters and (surface mass density and external shear) covering most of the known multiple quasar systems. We show examples of microlens-induced quasar motion (i.e. astrometric changes) and the corresponding light curves for different quasar sizes. We evaluate statistically the occurrence of large "jumps" in angular position and their correlation with apparent brightness fluctuations. We also show statistical relations between positional offsets and time from random starting points. As the amplitude of the astrometric offset depends on the source size, astrometric microlensing signatures of quasars - combined with the photometric variations - will provide very good constraints on the sizes of quasars as a function of wavelength. We predict that such signatures will be detectable for realistic microlensing scenarios with near future technology in the infrared/optical (Keck-Interferometry, VLTI, SIM, GAIA). Such detections will show that not even high redshift quasars define a "fixed" coordinate system.

Key words: cosmology: observations -- gravitational lensing -- galaxies: quasars: general -- astrometry

Offprint request: J. Wambsganss, jkw@astro.physik.uni-potsdam.de

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