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Blazars, the peculiar class of active galactic nuclei, are known to show flux variations across the accessible electromagnetic spectrum. Though they have been studied extensively for their flux variability characteristics across wavelengths, information on their ultraviolet (UV) flux variations on timescales of hours is very limited. Here, we present the first UV flux variability study on intraday timescales of a sample of ten blazars comprising two flat-spectrum radio quasars (FSRQs) and eight BL Lacertae objects (BL Lacs). These objects, spanning a redshift (z) range of 0.034 ≤ z ≤ 1.003, were observed in the far-UV (1300−1800 Å) and near-UV (2000−3000 Å) wavebands using the ultraviolet imaging telescope on board AstroSat. UV flux variations on timescales of hours were detected in nine sources out of the observed ten blazars. The spectral variability analysis showed a bluer-when-brighter trend with no difference in the UV spectral variability behavior between the studied sample of FSRQs and BL Lacs. The observed UV flux and spectral variability in our sample of both FSRQs and BL Lacs revealed that the observed UV emission in them is dominated by jet synchrotron process. [ABSTRACT FROM AUTHOR]

We investigate the torus size–luminosity relation of Type 1 active galactic nuclei (AGNs) based on the reverberation-mapping analysis using the light curves of the optical continuum and the IR continuum obtained with the W1 and W2 bands of the Wide-field Infrared Survey Explorer survey. The final sample consists of 446 and 416 AGNs, respectively, for W1- and W2-band light curves, covering a large dynamic range of bolometric luminosity from 1043.4 to 1047.6 erg s−1, which show reliable lag measurements based on our quality assessment analysis. After correcting for the accretion disk contamination in the observed IR flux, we constrain the torus size (R dust) and AGN bolometric luminosity (L bol) relationship with the best-fit slope of 0.39 (0.33) for the W1 (W2) band, which is shallower than expected from the dust radiation equilibrium model. By combining the previous K -band lag measurements, we find that the measured torus size depends on the observed wavelength of the dust radiation, as R dust, K : R dust,W1: R dust,W2 = 1.0:1.5:1.8 (R dust ∝ λ 0.80) at L bol = 1046 erg s−1, confirming a stratified structure of the torus, where wavelength-dependent emissions originate from distinct regions of the torus. By investigating the deviation from the best-fit torus size–luminosity relation, we find a moderate correlation between the offset from the R dust– L bol relation and Eddington ratio. This suggests a possible influence of the Eddington ratio on the observed flattening of the R dust– L bol relationship. [ABSTRACT FROM AUTHOR]

We present multiwavelength observations and a model for flat-spectrum radio quasar (FSRQ) NVSS J141922−083830, originally classified as a blazar candidate of unknown type (BCU II object) in the Third Fermi - LAT AGN Catalog. Relatively bright flares (>3 magnitudes) were observed on 2015 February 21 (MJD 57074) and 2018 September 8 (MJD 58369) in the optical band with the MASTER Global Robotic Net telescopes. Optical spectra obtained with the Southern African Large Telescope on 2015 March 1 (MJD 57082), during outburst, and on 2017 May 30 (MJD 57903), during quiescence, showed emission lines at 5325 Å and at ≈3630 Å that we identified as the Mg  ii 2798 Å and C  iii ] 1909 Å lines, respectively, and hence derived a redshift |$z$| = 0.903. Analysis of Fermi - Large Area Telescope (LAT) data was performed in the quiescent regime (5 yr of data) and during four prominent flaring states in 2014 February–April, 2014 October–November, 2015 February–March, and 2018 September. We present spectral and timing analysis with Fermi - LAT. We report a hardening of the gamma-ray spectrum during the last three flaring periods, with a power-law spectral index Γ = 2.0–2.1. The maximum gamma-ray flux level was observed on 2014 October 24 (MJD 56954) at (7.57 ± 1.83) × 10−7 ph cm−2 s−1. The multiwavelength spectral energy distribution (SED) during the 2015 February–March flare supports the earlier evidence of this blazar to belong to the FSRQ class. The SED can be well represented with a single-zone leptonic model with parameters typical of FSRQs, but also a hadronic origin of the high-energy emission cannot be ruled out. [ABSTRACT FROM AUTHOR]

To understand the mass distribution and co-evolution of supermassive black holes with their host galaxy, it is crucial to measure the black hole mass of AGN. Reverberation mapping is a unique tool to estimate the black hole masses in AGN. We performed spectroscopic reverberation study using long-term monitoring data with more than 100 spectra of a radio-loud quasar PKS 0736 + 017 to estimate the size of the broad-line region (BLR) and black hole mass. The optical spectrum shows strong H β and H γ emission lines. We generated the light curves of 5100 Å continuum flux (f 5100), H β, and H γ. All the light curves are found to be strongly variable with fractional variability of 69 per cent, 21 per cent, 30 per cent for V-band, H β, and H γ light curves, respectively. Along with the thermal contribution, non-thermal emission contributes to the estimated continuum luminosity at 5100 Å. Using different methods, e.g. CCF, JAVELIN, von-neumann, we estimated the size of the BLR, which is found to be 66.4 |$^{+6.0}_{-4.2}$| light days in the rest frame. The BLR size combined with the line width of H β provides a black hole mass of 7.32 |$^{+0.89}_{-0.91} \times 10^{7}M_{\odot }$|⁠. The source closely follows the BLR size–luminosity relation of AGN. [ABSTRACT FROM AUTHOR]

Study of the polarization behaviour in blazars is a powerful tool to discern the role of the magnetic field in the variable emission process in their relativistic jets. We present here the results of our systematic investigation on the correlation between optical flux and polarization variations for eight flat-spectrum radio quasars on various time-scales using data from the Steward Observatory that covers a period of ∼10 years. On long time-scales (∼several months), from a total of 79 observing cycles, we found a significant positive correlation between optical flux and optical polarization degree (PD) in 34 observing cycles, a negative correlation in three cycles, and no correlation in 42 cycles. On short time-scales (∼few days), in 47 out of a total of 55 epochs we found a positive correlation between optical flux and PD, while for the remaining eight epochs an anticorrelation was detected between the two quantities. Moreover, we noticed a significant positive correlation between optical and γ-ray fluxes in 14 epochs, and a negative correlation between the two in one epoch. While the observed optical flux changes fit the shock-in-jet model well, the observed changes in PD are not explainable by changes in the power-law spectral index of the relativistic electrons in the jet. Instead, the observed varied correlations between optical flux and PD could be due to multizone emission regions or the enhanced flux coinciding with the emergence of a new emission knot with its magnetic field either aligned or misaligned with the large-scale magnetic field. [ABSTRACT FROM AUTHOR]

Blazars are known to show flux variations over a range of energies from low-energy radio to high-energy γ-rays. Cross-correlation analysis of the optical and γ-ray light curves in blazars shows that flux variations are generally correlated in both bands, however, there are exceptions. We explored this optical–GeV connection in four flat spectrum radio quasars by a systematic investigation of their long-term optical and γ-ray light curves. On analysis of the four sources, namely 3C 273, 3C 279, PKS 1510−089, and CTA 102, we noticed different behaviours between the optical and GeV flux variations. We found instances when (i) the optical and GeV flux variations are closely correlated, (ii) there are optical flares without γ-ray counterparts, and (iii) γ-ray flares without optical counterparts. To understand these diverse behaviours, we carried out broad-band spectral energy distribution (SED) modelling of the sources at different epochs using a one-zone leptonic emission model. The optical–UV emission is found to be dominated by emission from the accretion disc in the sources PKS 1510−089, CTA 102, and 3C 273, while in 3C 279, the synchrotron radiation from the jet dominates the optical–UV emission. Our SED analysis indicates that (i) correlated optical and γ-ray flux variations are caused by changes in the bulk Lorentz factor (Γ), (ii) γ-ray flares without optical counterparts are due to increase in Γ and/or the electron energy density, and (iii) an optical flare without γ-ray counterpart is due to increase in the magnetic field strength. [ABSTRACT FROM AUTHOR]

The majority of the γ-ray sources detected by the Fermi Gamma-ray Space Telescope belong to the blazar class of active galactic nuclei (AGNs). In addition to blazars, a minority of non-blazar AGNs is also found to be γ-ray emitters. A class of AGNs with large jet to line-of-sight angle are termed as misaligned AGNs (MAGNs). A significant fraction of γ-ray emitting non-blazar AGNs belong to this class. The detection of these sources in γ-rays suggests that most of their emission originate in the jet. Considering jet origin of intra-night optical variability (INOV), one thus can expect to observe strong presence of INOV in these γ-ray detected MAGN. However, no observations exist on their INOV nature. We therefore, have carried out a first search for INOV on a sample of six MAGN, that comprises three FR I, two Steep Spectrum Radio Quasars (SSRQs)/Compact Steep Spectrum (CSS) and one FR II galaxy. Our observations carried out on a total of eight nights for a duration of ∼29 h, reveal that MAGNs show large amplitude (between 2 and 10 per cent) INOV with a duty cycle of about 50 per cent, including both variable and probably variable sources, which is similar to that shown by blazars. Though INOV has been detected in MAGNs, differences if any between the INOV characteristics of FR I, SSRQs/CSS, and FR II types of MAGNs could not be unambiguously established owing to limited observations. Nevertheless, the first detection of INOV in MAGNs points to their jet based origin. [ABSTRACT FROM AUTHOR]

A small subset of optically selected radio-quiet quasars showing weak or no emission lines may turn out to be the elusive radio-quiet BL Lac objects, or simply be radio-quiet QSOs with a still-forming/shielded broad line region (BLR). High polarization (p > 3–4 per cent), a hallmark of BL Lacs, can be used to test whether some optically selected ‘radio-quiet weak emission line quasars’ (RQWLQs) show a fractional polarization high enough to qualify as radio-quiet analogues of BL Lac objects. Out of the observed six RQWLQs candidates showing an insignificant proper motion, only two are found to have p > 1 per cent. For these two RQWLQs, namely J142505.59 + 035336.2 and J154515.77+003235.2, we found polarization of 1.03 ± 0.36 per cent and 1.59 ± 0.53 per cent, respectively, which again is too modest to justify a (radio-quiet) BL Lac classification. We also present here a statistical comparison of the optical spectral index, for a set of 40 RQWLQs with redshift-luminosity matched control sample of 800 QSOs and an equivalent sample of 120 blazars. The spectral index distribution of RQWLQs is found to differ, at a high significance level, from that of blazars and is consistent with that of the ordinary QSOs. Likewise, a structure–function analysis of photometric light curves presented here suggests that the mechanism driving optical variability in RQWLQs is similar to that operating in QSOs and different from that of blazars. These findings are consistent with the common view that the central engine in RQWLQs, as a population, is akin to that operating in normal QSOs and the primary differences between them might be related to differences in the BLR. [ABSTRACT FROM AUTHOR]

For a new sample of eight weak-line quasars (WLQs) we report a sensitive search in 20 intranight monitoring sessions, for blazar-like optical flux variations on hour-like and longer time-scale (day/month/year-like). The sample consists exclusively of the WLQs that are not radio-loud and either have been classified as ‘radio-weak probable BL Lac candidates’ and/or are known to have exhibited at least one episode of large, blazar-like optical variability. Whereas only a hint of intranight variability is seen for two of these WLQs, J104833.5+620305.0 (z = 0.219) and J133219.6+622715.9 (z = 3.15), statistically significant internight variability at a few per cent level is detected for three of the sources, including the radio-intermediate WLQ J133219.6+622715.9 (z = 3.15) and the well-known bona fide radio-quiet WLQs J121221.5+534128.0 (z = 3.10) and WLQ J153259.9−003944.1 (z = 4.62). In the rest frame, this variability is intraday and in the far-ultraviolet band. On the time-scale of a decade, we find for three of the WLQs large brightness changes, amounting to 1.655 ± 0.009, 0.163 ± 0.010 and 0.144 ± 0.018 mag, for J104833.5+620305.0, J123743.1+630144.9 and J232428.4+144324.4, respectively. Whereas the latter two are confirmed radio-quiet WLQs, the extragalactic nature of J104833.5+620305.0 remains to be well established, thanks to the absence of any feature(s) in its available optical spectra. This study forms a part of our ongoing campaign of intranight optical monitoring of radio-quiet WLQs, in order to improve the understanding of this enigmatic class of active galactic nuclei and to look among them for a possible tiny, elusive population of radio-quiet BL Lacs. [ABSTRACT FROM AUTHOR]

We present results of a systematic study of flux variability on hourly tim-scales in a large sample of active galactic nuclei (AGN) in the 3-79 keV band using data from Nuclear Spectroscopic Telescope Array. Our sample consists of four BL Lac objects (BL Lacs), three flat spectrum radio quasars (FSRQs) 24 Seyfert 1, 42 Seyfert 2 and eight narrow line Seyfert 1 (NLSy1) galaxies. We find that in the 3-79 keV band, about 65 per cent of the sources in our sample show significant variations on hourly time-scales. Using the Mann-Whitney U-test and the Kolmogorov-Smirnov test, we find no difference in the variability behaviour between Seyfert 1 and 2 galaxies. The blazar sources (FSRQs and BL Lacs) in our sample are more variable than Seyfert galaxies that include Seyfert 1 and Seyfert 2 in the soft (3-10 keV), hard (10-79 keV) and total (3-79 keV) bands. NLSy1 galaxies show the highest duty cycle of variability (87 per cent), followed by BL Lacs (82 per cent), Seyfert galaxies (56 per cent) and FSRQs (23 per cent).We obtained flux doubling/halving time in the hard X-ray band less than 10 min in 11 sources. The flux variations between the hard and soft bands in all the sources in our sample are consistent with zero lag. [ABSTRACT FROM AUTHOR]

In this work, we present a homogeneous curve-shifting analysis using the di erence-smoothing technique of the publicly available light curves of 24 gravitationally lensed quasars, for which time delays have been reported in the literature. The uncertainty of each measured time delay was estimated using realistic simulated light curves. The recipe for generating such simulated light curves with known time delays in a plausible range around the measured time delay is introduced here. We identified 14 gravitationally lensed quasars that have light curves of sufficiently good quality to enable the measurement of at least one time delay between the images, adjacent to each other in terms of arrival-time order, to a precision of better than 20% (including systematic errors). We modeled the mass distribution of ten of those systems that have known lens redshifts, accurate astrometric data, and sufficiently simple mass distribution, using the publicly available PixeLens code to infer a value of H0 of 68.1 ± 5.9 km s1 Mpc1 (1σuncertainty, 8.7% precision) for a spatially flat universe having m = 0.3 and Ω = 0.7. We note here that the lens modeling approach followed in this work is a relatively simple one and does not account for subtle systematics such as those resulting from line-of-sight e ects and hence our H0 estimate should be considered as indicative. [ABSTRACT FROM AUTHOR]

This paper presents optical R-band light curves and the time delay of the doubly imaged gravitationally lensed quasar SDSS J1001+5027 at a redshift of 1.838. We have observed this target for more than six years, between March 2005 and July 2011, using the 1.2-m Mercator Telescope, the 1.5-m telescope of the Maidanak Observatory, and the 2-m Himalayan Chandra Telescope. Our resulting light curves are composed of 443 independent epochs, and show strong intrinsic quasar variability, with an amplitude of the order of 0.2 magnitudes. From this data, we measure the time delay using five different methods, all relying on distinct approaches. One of these techniques is a new development presented in this paper. All our time-delay measurements are perfectly compatible. By combining them, we conclude that image A is leading B by 119:3 ± 3:3 days (1σ, 2.8% uncertainty), including systematic errors. It has been shown recently that such accurate time-delay measurements offer a highly complementary probe of dark energy and spatial curvature, as they independently constrain the Hubble constant. The next mandatory step towards using SDSS J1001+5027 in this context will be the measurement of the velocity dispersion of the lensing galaxy, in combination with deep Hubble Space Telescope imaging. [ABSTRACT FROM AUTHOR]

Variable γ -ray emission has been discovered in five radio-loud narrow-line Seyfert 1 (NLSy1) galaxies by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. This has clearly demonstrated that these NLSy1 galaxies do have relativistic jets similar to two other cases of γ -ray-emitting active galactic nuclei (AGNs), namely, blazars and radio galaxies. We present here our results on the multi-band analysis of two γ -ray-emitting NLSy1 galaxies, namely, PKS 1502+036 (z = 0.409) and PKS 2004-447 (z = 0.240), using archival data. We generate multi-band long-term light curves of these sources, build their spectral energy distribution (SED), and model them using a one-zone leptonic model. They resemble more the SEDs of the flat spectrum radio quasar (FSRQ) class of AGNs. We then compare the SEDs of these two sources with two other Fermi-detected AGNs along the traditional blazar sequence, namely, the BL Lac Mrk 421 (z = 0.03) and the FSRQ 3C 454.3 (z = 0.86). The SEDs of both PKS 1502+036 and PKS 2004-447 are found to be intermediate to the SEDs of Mrk 421 and 3C 454.3. In the γ -ray spectral index versus γ -ray luminosity plane, both these NLSy1 galaxies occupy a distinct position, wherein they have luminosity between Mrk 421 and 3C 454.3; however, their steep γ -ray spectra are similar to 3C 454.3. Their Compton dominance as well as their X-ray spectral slope also lie between Mrk 421 and 3C 454.3. We argue that the physical properties of both PKS 1502+036 and PKS 2004-447 are generally similar to blazars and intermediate between FSRQs and BL Lac objects and these sources thus could fit into the traditional blazar sequence. [ABSTRACT FROM AUTHOR]

Aims. Within the framework of the COSMOGRAIL collaboration we present 7- and 8.5-year-long light curves and time-delay estimates for two gravitationally lensed quasars: SDSS J1206+4332 and HS 2209+1914. Methods. We monitored these doubly lensed quasars in the R-band using four telescopes: the Mercator, Maidanak, Himalayan Chandra, and Euler telescopes, together spanning a period of 7 to 8.5 observing seasons from mid-2004 to mid-2011. The photometry of the quasar images was obtained through simultaneous deconvolution of these data. The time delays were determined from these resulting light curves using four very different techniques: a dispersion method, a spline fit, a regression difference technique, and a numerical model fit. This minimizes the bias that might be introduced by the use of a single method. Results. The time delay for SDSS J1206+4332 is ΔtAB = 111:3 ± 3 days with A leading B, confirming a previously published result within the error bars. For HS 2209+1914 we present a new time delay of ΔtBA = 20:0 ± 5 days with B leading A. Conclusions. The combination of data from up to four telescopes have led to well-sampled and nearly 9-season-long light curves, which were necessary to obtain these results, especially for the compact doubly lensed quasar HS 2209+1914. [ABSTRACT FROM AUTHOR]

We study the X-ray and optical properties of 16 broad absorption line (BAL) quasars detected in a 3 deg region common to the wide synoptic (W-1) component of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) and the XMM Large Scale Structure Survey (XMM-LSS). The BAL fraction is found to be 10 per cent in the full sample, 7 per cent for the optical colour-selected quasi-stellar objects (QSOs) and as high as 33 per cent if we consider QSOs selected from their infrared colours. The X-ray-detected non-BAL and BAL quasars have a mean observed X-ray-to-optical spectral slope () of and , respectively. We also find that the BAL QSOs have systematically smaller than what is expected from the relationship between optical luminosity and as derived from our sample. Based on this, we show, as already reported in the literature for quasars with high optical luminosities, that our new sample of BAL QSOs has X-ray luminosity a factor of 3 smaller than what has been found for non-BAL QSOs with similar optical luminosities. Comparison of hardness ratios of the BAL and non-BAL QSOs suggests a possible soft X-ray weakness of BAL QSOs. Combining our sample, of relatively fainter QSOs, with others from the literature we show that larger balnicity index (BI) and maximum velocity () of the C absorption are correlated with steeper X-ray-to-optical spectral index. We argue that this is most likely a consequence of the existence of a lower envelope in the distribution of BI (or ) values versus optical luminosity. Our results thus show that the previously known X-ray weakness of BAL QSOs extends to lower optical luminosities as well. [ABSTRACT FROM AUTHOR]

In recent years, the Lyman- α forest in quasar spectra has been used, together with N-body simulations, to determine the underlying matter distribution in the intergalactic medium (IGM). One of the key parameters to be known in order to compare observations and numerical simulations is the mean HI absorption in the IGM. To derive the latter, one has first to fit the quasar continuum. We have observed 20 high redshift and highly luminous QSOs ( m ≤ 17.5 and 2.40 ≤ z ≤ 3.91) at intermediate spectral resolution, with either EMMI (ESO Multi-Mode Instrument) on the ESO-NTT telescope or CARELEC at the OHP (Observatoire de Haute-Provence), and applied different methods of determining the QSO continuum to this QSO sample. We have measured the amount of absorption, known as the flux decrement, DA, in the Lyman- α forest for these different methods and compared the results. In addition, we have compared DA values measured along the same lines of sight observed at high and intermediate spectral resolutions. We discuss the systematics resulting from the use of automatic continuum fitting methods. [ABSTRACT FROM AUTHOR]

We present optical spectroscopic identifications of X-ray sources in ∼3 deg2 of the XMM–Large Scale Structure survey ( XMM–LSS), also covered by the Canada–France–Hawaii Telescope Legacy Survey (CFHTLS), obtained with the AAOmega instrument at the Anglo-Australian Telescope. In a flux-limited sample of 829 point-like sources in the optical band with mag and the 0.5–2 keV flux , we observed 693 objects and obtained reliable spectroscopic identification for 487 sources, approximately 59 per cent of the overall sample. We therefore increase the number of identifications in this field by a factor close to 5. Galactic stellar sources represent about 15 per cent of the total (74/487). About 54 per cent (265/487) are broad-line active galactic nuclei (AGN) spanning redshifts between 0.15 and 3.87 with a median value of 1.68. The optical-to-X-ray spectral index of the broad-line AGN is 1.47 ± 0.03, typical of optically selected type I quasars, and is found to correlate with the rest-frame X-ray and optical monochromatic luminosities at 2 keV and 2500 Å, respectively. Consistent with previous studies, we find not to be correlated with z. In addition, 32 and 116 X-ray sources are, respectively, absorption- and emission-line galaxies at . From a line ratio diagnostic diagram, it is found that in about 50 per cent of these emission-line galaxies, the emission lines are powered significantly by the AGN. 30 of the XMM sources are detected at one or more radio frequencies. In addition, 24 sources have ambiguous identification: in eight cases, two XMM sources have a single optical source within 6 arcsec of each of them, whereas two and 14 XMM sources have, respectively, three and two possible optical sources within 6 arcsec of each of them. Spectra of multiple possible counterparts were obtained in such ambiguous cases. [ABSTRACT FROM AUTHOR]

We report optical CousinsR- andI-band monitoring observations of the high-redshiftquasi-stellar object (QSO) SDSS J153259.96−003944.1 which does not show detectable emission lines in its optical spectrum. We show this object varies with a maximum amplitude of∼0.4 mag over 1 yr and 3 months of monitoring. Combined with two other epochs of photometric data available in the literature, we show the object has gradually faded by∼0.9 mag during the period 1998 June–2001 April. A linear least-squares fit to all available observations gives a slope of∼0.35 mag yr−1 which translates to∼1.9 mag yr−1 in the rest frame of the quasar. Such a variability is higher than that typically seen in QSOs but consistent with that of BL Lacs, suggesting that the optical continuum is Doppler boosted. Alternatively, within photometric errors, the observed light curve is also consistent with the object going through a microlensing event. Photoionization model calculations show the mass of the broad line region to be a few tens ofsimilar to that of low-luminosity Seyfert galaxies, but∼2 orders of magnitude less than that of luminous quasars. Further frequent photometric/spectroscopic monitoring is needed to support or refute the different alternatives discussed here about the nature of SDSS J153259.96−003944.1. [ABSTRACT FROM AUTHOR]

We report monitoring observations of 20 high-luminosity active galactic nuclei (AGN), 12 of which are radio-quiet quasars (RQQs). Intranight optical variability (INOV) was detected for 13 of the 20 objects, including 5 RQQs. The variations are distinctly stronger and more frequent for blazars than for the other AGN classes. By combining these data with results obtained earlier in our programme, we have formed an enlarged sample consisting of 9 BL Lacs, 19 RQQs and 11 lobe-dominated radio-loud quasars (RLQs). The moderate level of rapid optical variability found for both RQQs and radio lobe-dominated quasars (LDQs) argues against a direct link between INOV and radio loudness. We supplemented the present observations of 3 BL Lacs with additional data from the literature. In this extended sample of 12 well observed BL Lacs, stronger INOV is found for the EGRET detected subset. [ABSTRACT FROM AUTHOR]

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– MB) 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 (ψ) compared to the other three AGN classes. However, we find that ψ as well as the DC are similar for BL Lacs and the sub-set of CDQs with high (gt; 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 subparsec 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. [ABSTRACT FROM AUTHOR]

In order to address a currently much debated issue, we ate carrying out a systematic search for intra-night optical micro-variability in major classes of powerful (M[subv] ≥ -23.0 mag) active galactic nuclei (AGNs). Our sample includes 7 sets of AGNs, covering narrow redshift bins across the range 0.18 ≤ z ≤ 2.2 . Each set consists of a radio-quiet quasar, a radio lobe-dominated quasar, a BL Lac object and/or a radio core-dominated quasar (blazar), all having m[subv] ∼ 16 mag. The observations were made primarily using the 104-cm telescope of State Observatory, Nainital. The monitoring completed so far (∼ 90 nights) clearly indicates that, compared to the other AGN types, BL Lacs are distinctly more prone to exhibit intra-night optical variations. [ABSTRACT FROM AUTHOR]