Short-term optical variability of high-redshift QSO's
  This paper presents results of a search for short-term variability in the optical band of selected high-luminosity, high-redshift radio-quiet quasars. Each quasar has been monitored typically for 2 - 4 hours with a time resolution of 2 - 5 minutes and a photometric accuracy of about 0.01 - 0.02 mag. Due to the significant redshift (z>2), the covered wavelength range falls into the UV region (typically 1500 - 2500A). We found no statistical evidence for any continuum variations larger than 0.01 - 0.02 for any of the monitored objects. Our results suggest that the presence of a short-term variability in radio-quiet quasars is unlikely even in the UV region, contrary to reports by other authors. This conclusion holds true at least for high-luminosity (large black hole mass and accretion rate?) objects. The results are consistent with the idea that significant short-term (less than 1 hour) variations in AGN, where observed, should be attributed primarily to processes in a relativistic jet. 
R. Bachev, A. Strigachev, E. Semkov