Predicting caustic-crossing high-magnification events in Q2237+0305

The central regions of the gravitationally lensed quasar Q2237+0305 can be indirectly resolved on nano-arcsecond scales if viewed spectrophotometrical ly during a microlensing high-magnification event (HME). Q2237+0305 is curr ently being monitored from the ground (e.g. OGLE collaboration, Apache Poin t Observatory), with the goal, among others, of triggering ground- and spac ecraft-based target-of-opportunity (TOO) observations of such an HME. In th is work we investigate the rate of change (trigger) in image brightness tha t signals an imminent HME and importantly, the separation between the trigg er and the event peak. In addition, we produce colour dependent model light curves by combining high-resolution microlensing simulations with a realis tic model for a thermal accretion disc source. We make hypothetical TOO spe ctroscopic observations using our determination of the appropriate trigger as a guide. We find that if the source spectrum varies with source radius, a three-observation TOO programme should be able to observe a microlensing change in the continuum slope following a light-curve trigger with a succes s rate of greater than or similar to 80 per cent.