The statistics of microlensing light curves – I. Amplification probability distributions

The passage of stars through the beam of a lensed quasar can induce violent fluctuations in its apparent brightness. The fluctuations observed in the Huchra lens (2237+0305) are taken to be the first evidence of this ‘microlensing’ occurring in lensing systems. Subsequent microlensing events observed in this system and in other gravitational lenses indicate that microlensing should be a common phenomenon in lensed quasars. The statistical properties of the component light curves will be defined to a large extent by the mass distribution of the microlensing objects and the internal light distribution of the lensed quasar. We present statistics of a large sample of hypothetical microlensing light curves, for a number of lensing situations, generated using an efficient numerical technique. These artificial light curves show that the amplification probability distributions are independent of the mass function of the compact lensing objects. The amplification probability distributions for the Huchra lens system are presented.