Is there a caustic crossing in the lensed quasar Q2237+0305 observational data record?

Abstract

We re-investigate the gravitationally lensed system Q2237+0305 data record to quantify the probability of having a caustic crossing in the A component. Several works assume that this is the case, but no quantitative analysis is available in the literature. We combine the data sets from the OGLE and GLITP collaborations to accurately trace the prominent event in the light curve for the A component of the system. Then the observed event is compared with synthetic light curves derived from trajectories in magnification maps. These maps are generated using a ray-tracing technique. We take more than 109 trajectories and test a wide range of different physical properties of the lensing galaxy and the source quasar (lens transverse velocity, microlens mass, source intensity profile and source size). We found that around 75 per cent of our good trajectories (i.e. that are consistent with the observations) are caustic crossings. In addition, a high transverse velocity exceeding 300 km s−1, a microlens mass of about 0.1 M⊙ and a small standard accretion disc is the best parameter combination. The results justify the interpretation of the OGLE–GLITP event in Q2237+0305A as a caustic crossing. Moreover, the physical properties of the lens and source are in very good agreement with previous works. We also remark that a standard accretion disc is preferred to those simpler approaches, and that the former should be used in subsequent simulations.