Optical microvariability of southern AGNs

We present results of a search for optical microvariability in a selected s ample of 23 southern AGNs, which includes both radio-quiet and radio-loud o bjects. Microvariations were clearly detected in 60% of the radio-loud sour ces, with amplitudes from 2.2% up to 8% within a single night. Night-to-nig ht variability with amplitudes of similar to 20% was also observed in the B L Lac object 0537 - 441. On the contrary, no rapid variability was detected at all in 8 radio-quiet quasars. We have used microvariability data previo usly reported for northern objects along with our new results for southern AGNs in order to estimate duty cycles of each class of objects from the lar gest possible sample. Most of the microvariability in radio-loud objects could be originating in interactions between relativistic shocks and features in the inner jets, al though contributions from superluminal microlensing and accretion disk inst abilities can be present in some sources. It is possible that the latter ph enomenon is responsible for the microvariability observed in northern radio -quiet quasars. We suggest that the difference in the microvariability beha viour of radio and X-ray selected BL Lacs could be due to the effect of str onger magnetic fields in the latter group of objects, fields that call prev ent the formation of features like density inhomogeneities and bends in the base of the jets by Kelvin-Helmholtz macroscopic instabilities.