Photometric Variations in the Q2237+0305 System: First Detection of a Microlensing Event

We present new photometric data for the gravitationally lensed quasar system 2237 + 0305, which is unambiguous evidence for a change in brightness of one of the four component images. This represents the first detection of a microlensing event. For the case where the source intersects the cure of the microlens, observational constraints on the timescale for the event allows a limit of <~ 0.1 M_sun_ to be placed on the mass of the lensing object. The data illustrate the immense potential for obtaining information on the size of the quasar-continuum-emitting region, together with the mass function of compact objects in the lensing galaxy, through a comprehensive photometric monitoring program of the 2237 + 0305 system.