Intranight optical variability of radio-quiet and radio lobe-dominated quasars

Abstract

We present results of a programme of multi-epoch, intranight optical monitoring of a sample of non-blazar-type active galactic nuclei (AGN), which includes seven radio-quiet quasars (RQQs) and an equal number of radio-loud, lobe-dominated quasars (LDQs), covering a redshift range from about 0.2 to 2.0. These two sets of optically bright and intrinsically luminous quasi-stellar objects (QSOs) are well matched in the redshift-optical luminosity (z-M_B) plane. Our CCD monitoring covered a total of 61 nights with an average of 6.1 hours of densely sampled monitoring of just a single QSO per night, thereby achieving a typical detection threshold of ~1 per cent variation over the night. Unambiguous detection of intranight optical variability (INOV) amplitudes in the range 1-3 per cent on day-like or shorter time-scales were thus made for both RQQs and LDQs. Based on these clear detections of INOV, we estimate duty cycles of 17 and 9 per cent for RQQs and LDQs, respectively; inclusion of the two cases of probable variations of LDQs would raise the duty cycle to 15 per cent for LDQs. The similarity in the duty cycle and amplitude of INOV for the RQQs and LDQs suggests, first, that the radio loudness alone does not guarantee an enhanced INOV in QSOs and, secondly, that as in LDQs, relativistic jets may also be present in RQQs. We argue that, as compared to BL Lacs, the conspicuously milder, rarer and possibly slower INOV of RQQs and LDQs can in fact be readily understood in terms of their having optical synchrotron jets which are modestly misaligned from us, but are otherwise intrinsically as relativistic and active as the jets in BL Lacs. This points toward an orientation-based unifying scheme for the INOV of radio-loud and radio-quiet quasars. Variability of up to ~0.3 mag on month- to year-like time-scales is seen for nearly all those RQQs and LDQs in our sample for which sufficient temporal coverage is available. These data have revealed an interesting event that seems most likely explained as an occultation, lasting less than six months, of much of the nuclear optical continuum source in an RQQ. The observations reported here form part of a larger ongoing project to study the intranight optical variability of four major classes of powerful AGN, including blazars.