The multiple quasar Q2237+0305 under a microlensing caustic^*

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstrasse 12-14, 69120 Heidelberg, Germany e-mail: tanguita@ari.uni-heidelberg.de
² Princeton University Observatory, Peyton Hall, Princeton, NJ 08544, USA
³ School of Physics, The University of Melbourne, Parkville, Victoria 3052, Australia
⁴ Apache Point Observatory, 2001 Apache Point Rd, Sunspot NM 88349, USA

Received: 4 July 2007
Accepted: 22 November 2007

Abstract

We use the high magnification event seen in the 1999 OGLE campaign light curve of image C of the quadruply imaged gravitational lens Q2237+0305 to study the structure of the quasar engine. We have obtained g'- and r'-band photometry at the Apache Point Observatory 3.5 m telescope where we find that the event has a smaller amplitude in the r'-band than in the g'- and OGLE V-bands. By comparing the light curves with microlensing simulations we obtain constraints on the sizes of the quasar regions contributing to the g'- and r'-band flux. Assuming that most of the surface mass density in the central kiloparsec of the lensing galaxy is due to stars and by modeling the source with a Gaussian profile, we obtain for the Gaussian width 1.2010¹⁵ cm 7.96 10 cm, where M is the mean microlensing mass, and a ratio . With the limits on the velocity of the lensing galaxy from Gil-Merino et al. (2005) as our only prior, we obtain 0.6010¹⁵  cm 1.5710¹⁵  cm and a ratio (all values at 68 percent confidence). Additionally, from our microlensing simulations we find that, during the chromatic microlensing event observed, the continuum emitting region of the quasar crossed a caustic at ≥72 percent confidence.