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 spectrophotometrically during a microlensing high-magnification event (HME). Q2237+0305 is currently being monitored from the ground (e.g. OGLE collaboration, Apache Point Observatory), with the goal, among others, of triggering ground- and spacecraft-based target-of-opportunity (TOO) observations of such an HME. In this work we investigate the rate of change (trigger) in image brightness that signals an imminent HME and importantly, the separation between the trigger and the event peak. In addition, we produce colour dependent model light curves by combining high-resolution microlensing simulations with a realistic model for a thermal accretion disc source. We make hypothetical TOO spectroscopic 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 success rate of ≳ 80 per cent.