1999PASP..111.1503L

We report on 1 year of photometric monitoring of the ultraluminous broad absorption line quasar APM 08279+5255. The temporal sampling reveals that this gravitationally lensed system has brightened by ∼0.2 mag in 100 days. Two potential causes present themselves: either the variability is intrinsic to the quasar, or it is the result of microlensing by stars in a foreground system. The data are consistent with both hypotheses, and further monitoring is required before either case can be conclusively confirmed. We demonstrate, however, that gravitational microlensing cannot play a dominant role in explaining the phenomenal properties exhibited by APM 08279+5255. The identification of intrinsic variability, coupled with the simple gravitational lensing configuration, would suggest that APM 08279+5255 is a potential ``golden lens'' from which the cosmological parameters can be derived and is worthy of a monitoring program at high spatial resolution.