GRAVITATIONAL MICROLENSING, QUASAR VARIABILITY AND MISSING MATTER

QUASARs have long been known to vary in magnitude1, and it now appears that all quasars are to some extent variable2. In a few extreme cases, quasar luminosities have varied over several magnitudes on a timescale of months; this behaviour is normally accompanied by other phenomena (such as radio emission or enhanced polarization) indicative of processes intrinsic to the quasars. Long-term luminosity variations, while less dramatic, are much more common, but their origin remains poorly understood. Here I investigate this longer-term behaviour for a sample of approximately 300 quasars at redshifts ranging from 1 to 3, whose optical magnitudes have been measured periodically for 17 years. I show that there is a positive correlation between the timescale of variability and the average luminosity, but little evidence for an increase in timescale due to redshift (and hence time dilation). These findings are inconsistent with any known variability mechanism intrinsic to quasars, but can be explained by gravitational lensing of the quasar images by compact substellar objects along the lines of sight. If this interpretation is correct, the population of lensing objects must have a density of at least 0.1 of the critical cosmological density.