Intranight optical variability of blazars

Abstract

We present results of a multi-epoch intra-night optical monitoring of eleven blazars consisting of six BL Lac objects and five radio core-dominated quasars (CDQs). These densely sampled and sensitive R-band CCD observations, carried out from November 1998 through May 2002 during a total of 47 nights with an average of 6.5 hours/night, have enabled us to detect variability amplitudes as low as $\sim 1%$ on intra-night time scales. A distinction is found for the first time between the intra-night optical variability (INOV) properties of the these two classes of relativistically beamed radio-loud AGNs. BL Lacs are found to show a duty cycle (DC) of INOV of $\sim$60%, in contrast to CDQs, which show a much smaller INOV DC of $\sim$20%, the difference being attributable mainly to the weakly polarized CDQs. On longer time scales (i.e., between a week to a few years) variability is seen from all the CDQs and BL Lacs in our sample. The results reported here form part of our long-term programme to understand the intra-night optical variability characteristics of the four main classes of luminous AGNs, i.e., radio-quiet quasars (RQQs) and radio lobe-dominated quasars (LDQs), as well as CDQs and BL Lac objects.