The multiple quasar Q2237+0305 under a microlensing caustic

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 1.20×10¹⁵√{{M}/{0.1 M_⊙}} cm ⪉ σ_g' ⪉ 7.96× 10¹⁵ √{{M}/{0.1 M_⊙}} 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. (2005) as our only prior, we obtain 0.60×10¹⁵ √{{M}/{0.1 M_⊙}} cm ⪉ σ_g' ⪉ 1.57×10¹⁵ √{{M}/{0.1 M_⊙}} 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.

Based on observations obtained with the Apache Point Observatory 3.5-m telescope, which is owned and operated by the Astrophysical Research Consortium.