Abstract:
This thesis explores mainly the problem of the radio loudness dichotomy in quasars. This is addressed by carrying out an extensive search for intra-night optical variability (INOV) in seven sets of optically luminous radio-loud and radio-quiet quasars matched in the `optical luminosity - redshift' plane from z ˜ 0.2 to z ˜2.0. Each set consists of a radio-quiet quasar (RQQ), a radio lobe-dominated quasar (LDQ), a radio core-dominated quasar (CDQ) and/or a BL Lac object (BL Lac). This work has for the first time provided convincing evidence of intra-night optical variability in RQQs. Based on clear detection of INOV, the duty cycles (DCs) are about 17% and 9% for RQQs and LDQs respectively. Another new result is a clear distinction we find between the INOV properties of the two types of relativistically beamed AGNs, with BL Lacs showing high DC of INOV (72%) compared to CDQs (20%). Also, BL Lacs generally show higher variability amplitudes () compared to the other three AGN classes. However, we find that as well as the DC are similar for BL Lacs and the sub-set of CDQs with high (> 3%) optical polarization. This similarity in the DC and amplitude of INOV for RQQs, LDQs and CDQs suggests, firstly, that the radio loudness alone does not guarantee an enhanced INOV in QSOs and secondly, that as in LDQs, relativistic jets probably also exist in RQQs. We also demonstrate that the substantial differences in INOV properties of RQQs and LDQs as compared to BL Lacs can be explained in terms of special relativistic effects arising from our viewing the latter at small angles to the jet direction. Thus the mechanism producing the optical variations in radio-loud and radio-quiet AGNs can be very similar, and sub-parsec scale jets may well be present in all types of AGNs, obviating the need for any fundamental difference between the central engines of RQQs and radio-loud quasars.
