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 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 1.20$\times$10 15 $\sqrt{{M}/{0.1~M_\odot}}$ cm $\la \sigma_{g'} \la$ 7.96$\times$ 10$^{15} \sqrt{{M}/{0.1~M_\odot}}$ cm, where M is the mean microlensing mass, and a ratio $\sigma_{r'}/\sigma_{g'} = 1.25^{+0.45}_{-0.15}$. With the limits on the velocity of the lensing galaxy from Gil-Merino et al. (2005) as our only prior, we obtain 0.60$\times$10 15 $\sqrt{{M}/{0.1~M_\odot}}$ cm $\la \sigma_{g'} \la$ 1.57$\times$10 15 $\sqrt{{M}/{0.1~M_\odot}}$ cm and a ratio $\sigma_{r'}/\sigma_{g'} = 1.45^{+0.90}_{-0.25}$ (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.