Intranight optical variability of blazars

Abstract

We present results of a multi-epoch intranight optical monitoring of 11 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 1998 November to 2002 May during a total of 47 nights with an average of 6.5 h per night, have enabled us to detect variability amplitudes as low as ~1 per cent on intranight time-scales. A distinction is found for the first time between the intranight optical variability (INOV) properties of the these two classes of relativistically beamed radio-loud active galactic nuclei (AGNs). BL Lacs are found to show a duty cycle (DC) of INOV of ~60 per cent, in contrast to CDQs, which show a much smaller INOV DC of ~20 per cent, 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 intranight 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.