Photometric monitoring of the gravitationally lensed ultraluminous broad absorption line quasar APM 08279+5255

We report on 1 year of photometric monitoring of the ultraluminous broad ab sorption line quasar APM 08279+5255. The temporal sampling reveals that thi s gravitationally lensed system has brightened by similar to 0.2 mag in 100 days. Two potential causes present themselves: either the variability is i ntrinsic to the quasar, or it is the result of microlensing by stars in a f oreground system. The data are consistent with both hypotheses, and further monitoring is required before either case can be conclusively confirmed. W e demonstrate, however, that gravitational microlensing cannot play a domin ant role in explaining the phenomenal properties exhibited by APM 08279 + 5 255. The identification of intrinsic variability, coupled with the simple g ravitational lensing configuration, would suggest that APM 08279 + 5255 is a potential "golden lens" from which the cosmological parameters can be der ived and is worthy of a monitoring program at high spatial resolution.