Intranight variability properties of different classes of active galactic nuclei

Abstract

We present results of a multi-epoch intranight optical monitoring of a set of 25 AGN, comprised of 11 blazars (6 BL Lac objects and 5 radio core-dominated quasars [CDQs]) and 14 non-blazars (7 radio-quiet QSOs [RQQs], and 7 radio-loud, lobe-dominated quasars [LDQs]) with 0.2 < z< 2.0. These densely sampled R-band CCD observations allowed us to clearly detect variability amplitudes ~0.01 mag on intra-night timescales and covered a total of 108 nights with an average of 6.3 hours/night. For the first time we find a distinction between the intra-night optical variability (INOV) properties of the two classes of blazars: BL Lacs have a high duty cycle (DC) of INOV of about 60%, in contrast to CDQs, which show a much smaller INOV DC of about 20%, the difference arising from the weakly polarized CDQs. Similarly small DCs were found for both RQQs (17%) and LDQs (9%). On longer timescales (a week to a few years) variability is seen from all the CDQs and BL Lacs and from the great majority of RQQs and LDQs. The similarity in DC and amplitude of INOV for the RQQs, LDQs and CDQs implies that the radio loudness alone does not guarantee enhanced INOV in QSOs. The milder, rarer and slower INOV of RQQs, LDQs and (low-polarization) CDQs, as compared to BL Lacs, can be understood in terms of their all having optical synchrotron jets which are modestly misaligned from us, but are otherwise intrinsically as relativistic and active as the jets in BL Lacs and high-polarization CDQs. This points toward an orientation-based unification scheme for the INOV of RL and RQ quasars. The relativistic electrons in optically emitting compact (microarcsec) jets may be quenched through collisions with CMB photons before reaching the scale at which radio emission can emerge.