The nature and size of the optical continuum source in QSO 2237+0305
  From the peak of a gravitational microlensing high-magnification event in the A component of QSO 2237+0305, which was accurately monitored by the GLITP collaboration, we derived new information on the nature and size of the optical V-band and R-band sources in the far quasar. If the microlensing peak is caused by a microcaustic crossing, we firstly obtained that the standard accretion disk is a scenario more reliable/feasible than other usual axially symmetric models. Moreover, the standard scenario fits both the V-band and R-band observations with reduced chi-square values very close to one. Taking into account all these results, a standard accretion disk around a supermassive black hole is a good candidate to be the optical continuum main source in QSO 2237+0305. Secondly, using the standard source model and a robust upper limit on the transverse galactic velocity, we inferred that 90 per cent of the V-band and R-band luminosities are emitted from a region with radial size less than 1.2 10^{-2} pc (= 3.7 10^{16} cm, at 2*sigma confidence level). 
V. N. Shalyapin (IRA, Ukraine), L. J. Goicoechea (UC, Spain), D.   Alcalde, E. Mediavilla, J. A. Munoz (IAC, Spain), R. Gil-...