The multiple quasar Q2237+0305 under a microlensing caustic

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 - and -band photometry at the Apache Point Observatory 3.5 m telescope where we find that the event has a smaller amplitude in the -band than in the - and OGLE -bands. By comparing the light curves with microlensing simulations we obtain constraints on the sizes of the quasar regions contributing to the - and -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 1.20 × 10¹⁵ cm √M/0.1 M⊙ cm ≤ σd ≤ 7.96 × 10¹⁵ √M/0.1 M⊙ cm, where is the mean microlensing mass, and a ratio σ_r'/σ_d = 1.25^+0.45-0.15 With the limits on the velocity of the lensing galaxy from Gil-Merino et al. (2005) as our only prior, we obtain 0.60 × 10 ¹⁵ cm √M/0.1 M⊙ cm ≤ σd ≤ 7.96 × 10 ¹⁵ √M/0.1 M⊙ cm 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 percent confidence.