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

Abstract

We present results of a programme of multi-epoch, intra-night optical monitoring of a sample of non-blazar type AGN, which includes seven radio-quiet QSOs (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 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 $\sim 1$% variation over the night. Unambiguous detection of intra-night variability(INOV) amplitudes in the range 1$-$3% 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% for RQQs and LDQs, respectively; inclusion of the two cases of probable variations of LDQs would raise the duty cycle to 15% for LDQs. The similarity in the duty cycle and amplitude of INOV for the RQQs and LDQs suggests, firstly, 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.