2008A&A...480..327A

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 3.5m 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 1.20x10¹⁵sqrt(M/0.1M_☉)cm≲σ_g'≲7.96x10¹⁵sqrt(M/0.1M_☉)cm, where M is the mean microlensing mass, and a ratio σ_r'/σ_g'=1.25^+0.45_–0.15. With the limits on the velocity of the lensing galaxy from Gil-Merino et al. (2005A&A...432...83G) as our only prior, we obtain 0.60x10¹⁵sqrt(M/0.1 M_☉)cm≲σ_g'≲1.57x10¹⁵sqrt(M/0.1M_☉)cm and a ratio σ_r'/σ_g'=1.45^+0.90_–0.25 (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.