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The incidence of a host galaxy in aperture photometry of active galactic nuclei is studied by means of actual and simulated CCD observations. Our goal is to evaluate the importance of spurious variations, introduced by seeing fluctuations during the observations, in the differential light curves used to study optical microvariability. Repeated CCD...
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... As seeing, conditions vary night by night, and within the night itself, it becomes imperative to understand the effect of seeing conditions on the variability of quasars. The seeing conditions also have some aperture effects, as is evident in (Cellone et al., 2000). Since seeing conditions can induce artificial variability in the light curves, we simulated images of the host galaxy and the AGN to understand and quantify the impact of seeing and the host galaxy's contribution to the AGN flux. ...
... As emphasized in Howell (1989), the SNR of a target recorded in a CCD is maximized for the aperture radius ∼ PSF. However, as suggested by Cellone et al. (2000) when the underlying host galaxy significantly contributes to the total optical flux, its contribution to the aperture photometry can vary significantly due to PSF variation, mimicking INOV. The possibility of such spurious INOV can be significant for the lower redshift NLSy1s in our sample, particularly, J032441.20+341045.0 (z = 0.06), J164442.53+261913.3 (z = 0.14), J170330.38+454047.3 (z = 0.06), J120014.08−004638.7 (z = 0.18) and J163323.59+471859.0 (z = 0.12) (Table 5.1). ...
... Also, we have paid particular attention to guarding against the possibility of spurious INOV claims arising from a varying flux contribution to the aperture photometry from the host galaxy of the AGN caused due to seeing disc variation during the session. As pointed out by Cellone et al. (2000), under such circumstances, false claims of INOV can result in low − z AGNs. Based on recent deep imaging studies of NLSy1 galaxies by Olguín-Iglesias et al. (2020), it can be inferred that any variable contamination arising from the host galaxy is very unlikely to matter when studying the variability of AGNs at least at z > 0.5. ...
The innermost regions of Active Galactic Nuclei (AGN) are critical for understanding galaxy evolution and the dynamics of matter near a Supermassive Black Hole (SMBH). Yet, due to smaller angular projections, it is very difficult to resolve these regions. This thesis explores indirect methods to understand these objects. We use the reverberation mapping technique to estimate accretion disk sizes for a sample of AGN, finding that the computed disk sizes are, on average, 3.9 times larger than the Shakura Sunyev (SS) standard disk model predictions. We also find a weak correlation between the obtained accretion disk sizes and the SMBH mass. We present initial results from a new accretion disk monitoring program to probe the accretion disk structure of Super Eddington Accreting AGN. We report that the disk sizes are about 4 times larger than the SS disk model. We calibrate the narrow-band photometric reverberation mapping (PRM) technique to develop tools for a large systematic narrow-band PRM project. We use simulations to test the effect of cadence, variability of the light curves, and the length of light curves in recovering the reverberation lags. We study the dichotomy between AGNs with and without detected jets using the method of microvariability observed in the accretion disk continuum. We find that AGNs with confirmed jets are about 3 times more variable on short time scales than the AGNs without a confirmed jet. By performing statistical analysis on a large sample of low luminosity AGNs, we find that the NLSy1 galaxies are more likely to have outflow signatures than their broad-line counterparts, hinting toward the disk wind origin of the material in BLR. We find that the principal components for NLSy1 galaxies differ from the BLSy1 galaxies, suggesting that the NLSy1 galaxies could be occupying their own parameter space.
... Additionally, caution about the point spread function (PSF) variation becomes very important when dealing with intra-night variability of nearby ( ≤0.4) AGNs. Because in such a situation a significant contribution to the total flux can come from the underlying host Galaxy that can mimic the INOV in the standard analysis of the differential light curves (DLCs) due to the significant relative contributions of the (point-like) AGN and the host Galaxy to the aperture photometry with the variation of PSF during the session (Cellone, Romero & Combi 2000 ). Therefore, the procedure of data reduction, PSF estimation for aperture photometry, selection of aperture, and caution for PSF variations (see Section 4 ) were followed from Ojha, Hum & Gopal-Krishna ( 2021 ). ...
... Thus for a genuine INOV detection from the current sample, we have first carefully inspected the seeing variations of all the variable (including probable variable cases) intra-night sessions, resulting from F p-enh -test. An RQNLSy1 is designated as V if either the FWHM of the session w as f airly steady during the time of RQNLSy1's flux variations or gradients in the FWHM of the session are anti-correlating with the systematic variations of differential magnitude of target RQNLSy1 and chosen comparison stars (see Cellone, Romero & Combi 2000 ). ...
Variability studies of active galactic nuclei are a powerful diagnostic tool in understanding the physical processes occurring in disk-jet regions, unresolved by direct imaging with currently available techniques. Here, we report the first attempt to systematically characterize intra-night optical variability (INOV) for a sample of seven apparently radio-quiet narrow-line Seyfert 1 galaxies (RQNLSy1s) that had shown recurring flaring at 37 GHz in the radio observations at Metsähovi Radio Observatory (MRO), indicating the presence of relativistic jets in them, but no evidence for relativistic jets in the recent radio observations of Karl G. Jansky Very Large Array (JVLA) at 1.6, 5.2, and 9.0 GHz. We have conducted a total of 28 intra-night sessions, each lasting ≥ 3 hrs for this sample, resulting in an INOV duty cycle ($\overline{DC} ~\sim$20 %) similar to that reported for γ-ray-NLSy1s (DC ∼25 % - 30 %), that display blazar-like INOV. This in turn infers the presence of relativistic jet in our sample sources. Thus, it appears that even lower-mass (MBH ∼ 106 M⊙) RQNLSy1 galaxies can maintain blazar-like activities. However, we note that the magnetic reconnection in the magnetosphere of the black hole can also be a viable mechanism to give rise to the INOV from these sources.
... For the purpose of (optical) differential aperture-photometry, the present sample of 6 TeV-HBLs (Table 1) has been drawn from the VLBI data published in Piner & Edwards (2018) for a sample of 38 TeV-HBLs. We imposed a limit of 0.3, in order to minimise the relative contribution from the host galaxy and thereby the possibility of claiming spurious INOV detection, in case the 'point spread function' (PSF) changes during the monitoring session (Cellone et al. 2000). This resulted in exclusion of 30 of the sources. ...
BL Lac objects detected at TeV energies preferentially belong to the subclass called 'high-frequency-peaked' BL Lacs (HBLs). Parsec-scale radio jets in these TeV-HBLs often show dominant, slow moving radio knots that are at most mildly superluminal. We report the first systematic campaign to characterise the Intra-Night Optical Variability (INOV) of TeV-HBLs using a representative sample of 6 such sources, all showing a fairly high degree of optical polarization. Our campaign consists of high-sensitivity monitoring of this sample in 24 sessions of more than 3 hour duration each. For these TeV-HBLs, we find a striking lack of INOV and based on this, we discuss the importance of superluminal motion of the radio knots vis-a-vis the optical polarization, as the key diagnostic for INOV detection.
... Next, since the surface-brightness profile of the host galaxy of an AGN does not generally behave like a point source when convolved with the 'point-spread function' (PSF), any intranight variation in PSF (i.e., 'seeing') can give rise to a changing ratio of light contributions from the blazar and its host galaxy. Such light-curves (LCs) can lead to spurious claims of INOV (Carini et al. 1991;Cellone et al. 2000). In order to reduce this possibility to a negligible level, we have only selected sources lying at redshifts > 0.3, leaving us with 1555 sources. ...
We explore the potential of the ongoing Zwicky-Transient-Facility (ZTF) survey for studying Intra-Night Optical Variability (INOV) of active galactic nuclei (AGN), in particular for picking rare events of large INOV amplitudes, whose detection may require extensive temporal coverage. For this, we have used the available high cadence subsets of the ZTF database to build a well-defined large sample of 53 blazars (BLs) and another sample of 132 radio-quiet quasars (RQQs), matched to the blazar sample in the redshift-magnitude plane. High-cadence ZTF monitoring of these two matched samples are available, respectively, for 156 and 418 intranight sessions. Median durations for both sets of sessions are 3.7 hours. The two classes of powerful AGN monitored in these sessions represent opposite extremes of jet activity. The present analysis of their ZTF light curves has revealed some strong INOV events which, although not exceptionally rare for blazars, are indeed so for RQQs, and their possible nature is briefly discussed.
... Since our targets are nearby AGN (z 0.1), their aperture photometry may have a significant contribution from the underlying host galaxy. It is therefore important that the PSF does not have any systematic drift through the session (see, Cellone et al. 2000;Nilsson et al. 2007). This is indeed the case for a large majority of our 36 sessions, as (2007) and Nyland et al. (2012), respectively; Col. 9: Calculated from , following Schmidt & Green (1983); Col. 10: Taken from Qian et al. (2018), except for the sources marked with ' †' (FIRST survey, Becker et al. 1995, White et al. 1997 and '*' (NVSS, Condon et al. 1998 seen from the bottom panel for each session (online Figs. ...
This study aims to characterise, for the first time, intranight optical variability (INOV) of low-mass active galactic nuclei (LMAGN) which host a black hole (BH) of mass $M_{BH} \sim 10^6 M_{\odot}$, i.e., even less massive than the Galactic centre black hole Sgr A* and 2-3 orders of magnitude below the supermassive black holes (SMBH, $M_{BH}$ $\sim$ $10^8 - 10^9 M_{\odot}$) which are believed to power quasars. Thus, LMAGN are a crucial subclass of AGN filling the wide gap between SMBH and stellar-mass BHs of Galactic X-ray binaries. We have carried out a 36-session campaign of intranight optical monitoring of a well-defined, representative sample of 12 LMAGNs already detected in X-ray and radio bands. This set of LMAGN is found to exhibit INOV at a level statistically comparable to that observed for blazars (M$_{BH} \gtrsim$ 10$^{8-9}$ M$_{\odot}$) and for the $\gamma$-ray detected Narrow-line Seyfert1 galaxies (M$_{BH}\sim 10^7$ M$_{\odot}$) which, too, are believed to have relativistic jets. This indicates that the blazar-level activity can even be sustained by central engines with black holes near the upper limit for Intermediate Mass Black Holes ($M_{BH}$ $\sim$ $10^3 - 10^6 M_{\odot}$).
... Variations were tested using a C-test (Romero et al. 1999;Cellone et al. 2000) and an F-test (de Diego 2010). If a variation occurred, then the variation amplitude Amp and the corresponding time interval were obtained; the time interval was taken as the timescale ΔT. ...
Variability is one of the main observational characteristics of blazars. Studying variability is an efficient method to reveal the nature of active galactic nuclei. In the present work, we report optical R -band photometry observations of a TeV blazar, 1ES 2344 + 514, carried out with a 70 cm telescope in the period of 1998 July–2017 November at Abastumani Observatory, Georgia. Based on the optical R -band observations, the optical variation behaviors on both short timescales and long timescales are investigated. Three methods (Jurkevich, discrete correlation function, and power spectrum analysis) are used to investigate periodicity in the light curve. In addition, combined with multiwavelength data, the jet physical properties are discussed. The following conclusions are drawn: (1) A variability of Δ R = 0.155 mag (15.356 − 15.201 mag) over a timescale of Δ T = 12.99 minutes is detected during our 628 days of monitoring. (2) According to the Kelvin–Helmholtz thermal instability, if the magnetic field intensity ( B ) for the source is greater than a critical value ( B c ), it will reduce the incidence of intraday variations in the light curves. (3) The physical parameters of the dissipation region are obtained by fitting the spectral energy distribution with a one-zone synchrotron self-Compton model for the average and flare states. (4) The three methods show that there are periods of P = 2.72 ± 0.47 yr, P = 1.61 ± 0.18 yr, P = 1.31 ± 0.17 yr, and P = 1.05 ± 0.07 yr. When a binary black hole system is adopted with a period of P = 2.72 ± 0.41 yr, we obtain the orbital parameters for the binary black hole system as follows: M = 8.08 × 10 ⁹ M ⊙ , the sum of the semiaxes is r = 7.18 × 10 ¹⁶ cm, and the lifetime of the binary black hole is τ merge = 6.24 × 10 ² yr.
... As emphasized in Howell ( 1989 ), the SNR of a target recorded in a CCD is maximized for the aperture radius ∼PSF. Ho we ver, as suggested by Cellone, Romero & Combi ( 2000 ) when the underlying host galaxy significantly contributes to the total optical flux, its contribution to the aperture photometry can vary significantly due to PSF variation, mimicking INOV. Possibility of such spurious INOV can be significant for the lower redshift NLSy1s in our sample, particularly, J032441.20 + 341045.0 ...
... Also, we have paid particular attention to guarding against the possibility of spurious INOV claims arising from a varying flux contribution to the aperture photometry from the host galaxy of the AGN caused due to seeing disc variation during the session. As pointed out by Cellone et al. ( 2000 ), under such circumstances, false claims of INOV can result in low − z AGNs. Based on recent deep imaging studies of NLSy1 galaxies by Olgu ín-Iglesias et al. ( 2020 ), it can be inferred that any variable contamination arising from the host galaxy is very unlikely to matter when studying the variability of AGNs at least at z > 0.5. ...
... A closer checkup of these two intranight sessions shows that either PSF remained fairly steady during the time of AGN's flux variations (Fig. 1 ) or the gradients in the DLCs of the target AGN are seen to be anticorrelated with systematic variations in the PSF (Fig. 2 ). This is opposite to what is expected in case the aperture photometric measurements were significantly contaminated by the underlying galaxy (see Cellone et al. 2000 ). Therefore, the possibility of a significant variation in the fractional contribution from the host galaxy can be safely discounted. ...
To quantify the role of radio jets for Intra-Night Optical Variability (INOV) in Radio-Loud Narrow-Line Seyfert 1 (RLNLSy1) galaxies, we report the first systematic comparative INOV study of 23 RLNLSy1 galaxies, with 15 RLNLSy1s having confirmed detection of jets (jetted) and the remaining 8 RLNLSy1s having no detection of jets (non-jetted) based on their Very Long Baseline Array observations. We have monitored these two samples, respectively, in 37 and 16 sessions of a minimum 3-hour duration each. Based upon Fη-test at 99% confidence level with a typical INOV amplitude (ψ) detection threshold of > 3%, we find the INOV duty cycles of 12% for the sample of jetted RLNLSy1s, however, none of the sources showed INOV in the sample of non-jetted RLNLSy1s. Among the jetted RLNLSy1s, we find that the Duty Cycle (DC) for jetted γ-ray detected (γ-ray) RLNLSy1s is found to be 34% in contrast to null INOV detection in the case of non-γ-ray RLNLSy1s. It suggests that instead of the mere presence of a jet, relativistic beaming plays a significant role for INOV in the case of low-luminous high accreting AGNs such as NLSy1s in which dilution of the AGN’s non-thermal optical emission by the (much steadier) optical emission contributed by the nuclear accretion disc is quite likely. Our study of jetted γ-ray RLNLSy1s shows more frequent INOV detection for sources with higher apparent jet speed. Further, our results also suggest that among the NLSy1s, only jetted γ-ray RNLSy1 galaxies DC approaches blazar like DC.
... As emphasized in Howell (1989), the SNR of a target recorded in a CCD is maximized for the aperture radius ∼ PSF. However, as suggested by Cellone et al. (2000) when the underlying host galaxy significantly contributes to the total optical flux, its contribution to the aperture photometry can vary significantly due to PSF variation, mimicking INOV. Possibility of such spurious INOV can be significant for the lower redshift NLSy1s in our sample, particularly, J032441.20+341045.0 (z = 0.06), J164442.53+261913.3 (z = 0.14), J170330.38+454047.3 (z = 0.06), J120014.08−004638.7 (z = 0.18) and J163323.59+471859.0 (z = 0.12) ( Table 1). ...
... Also, we have paid particular attention to guarding against the possibility of spurious INOV claims arising from a varying flux contribution to the aperture photometry from the host galaxy of the AGN caused due to seeing disc variation during the session. As pointed out by Cellone et al. (2000), under such circumstances, false claims of INOV can result in low − z AGNs. Based on recent deep imaging studies of NLSy1 galaxies by Olguín-Iglesias et al. (2020), it can be inferred that any variable contamination arising from the host galaxy is very unlikely to matter when studying the variability of AGNs at least at z > 0.5. ...
... A closer checkup of these two intranight sessions shows that either PSF remained fairly steady during the time of AGN's flux variations (Fig. 1) or the gradients in the DLCs of the target AGN are seen to be anticorrelated with systematic variations in the PSF (Fig. 2). This is opposite to what is expected in case the aperture photometric measurements were significantly contaminated by the underlying galaxy (see Cellone et al. 2000). Therefore, the possibility of a significant variation in the fractional contribution from the host galaxy can be safely discounted. ...
To quantify the role of radio jets for Intra-Night Optical Variability (INOV) in Radio-Loud Narrow-Line Seyfert 1 (RLNLSy1) galaxies, we report the first systematic comparative INOV study of 23 RLNLSy1 galaxies, with 15 RLNLSy1s having confirmed detection of jets (jetted) and the remaining 8 RLNLSy1s having no detection of jets (non-jetted) based on their Very Long Baseline Array observations. We have monitored these two samples, respectively, in 37 and 16 sessions of a minimum 3-hour duration each. Based upon F$^{\eta}$-test at 99\% confidence level with a typical INOV amplitude ($\psi$) detection threshold of $>$ 3\%, we find the INOV duty cycles of 12\% for the sample of jetted RLNLSy1s, however, none of the sources showed INOV in the sample of non-jetted RLNLSy1s. Among the jetted RLNLSy1s, we find that the Duty Cycle (DC) for jetted $\gamma$-ray detected ($\gamma$-ray) RLNLSy1s is found to be 34\% in contrast to null INOV detection in the case of non-$\gamma$-ray RLNLSy1s. It suggests that instead of the mere presence of a jet, relativistic beaming plays a significant role for INOV in the case of low-luminous high accreting AGNs such as NLSy1s in which dilution of the AGN's non-thermal optical emission by the (much steadier) optical emission contributed by the nuclear accretion disc is quite likely. Our study of jetted $\gamma$-ray RLNLSy1s shows more frequent INOV detection for sources with higher apparent jet speed. Further, our results also suggest that among the NLSy1s, only jetted $\gamma$-ray RNLSy1 galaxies DC approaches blazar like DC.
... Though we carried out galfit to remove the contribution of the host galaxy to the observed R-band brightness, it was not effective as the seeing was not stable during the observing period. As pointed by Cellone et al. (2000) seeing fluctuations during the observations can severely affect the observed light curves. According to them, degradation in the light curves due to seeing fluctuations are larger for AGN with brighter host. ...
We present the results of photometric reverberation mapping observations on the changing look active galactic nucleus Mrk 590 at z = 0.026. The observations were carried out from July to December, 2018 using broad band B, R and narrow band H{\alpha} and S II filters. B-band traces the continuum emission from the accretion disk, R-band encompasses both the continuum emission from the accretion disk and the redshifted H{\alpha} line from the broad line region (BLR), S II band contains the redshifted H{\alpha} emission and the H{\alpha} band traces the continuum emission underneath the S II band. All the light curves showed strong variation with a fractional root-mean-square variation of 0.132 (+/-) 0.001 in B-band and 0.321 (+/-) 0.001 in H{\alpha} line. From cross-correlation function analysis, we obtained a delayed response of H{\alpha} line emission to the optical B-band continuum emission of 21.44(+1.49/-2.11) days in the rest-frame of the source, corresponding to a linear size of the BLR of 0.018 pc. This is consistent with previous estimates using H{\beta}. By combining the BLR size with the H{\alpha} line full width at half maximum of 6478 (+/-) 240 km/s measured from a single-epoch spectrum obtained with the Subaru telescope, we derived a black hole mass of 1.96 (+0.15 / -0.21) X 10^8 Msun.
... To check the reality of a variability, we adopt a variability parameter, C, introduced by Romero et al. (1999) (see also Cellone et al. 2000), which is expressed as = ( ) , the average value of C 1 and C 2 , is greater than 2.576, then the nominal confidence level of a variability is greater than 99%. where m min and m max are the minimum and maximum magnitudes and s min and s max are the corresponding uncertainties. ...
In this work, we present 8523 pairs of R -band optical photometry observations for the quasar 3C 454.3 made during the period of 2006 October–2018 February on the 70 cm meniscus telescope at Abastumani Observatory, Georgia, to study its intraday variabilities (IDVs) and long-term variations, and we have come to the following results. (1) We detected 10 outbursts, a Δ R = 3.825 mag variation, and some IDVs. The IDV timescales are from 4.1 to 285 minutes, with the corresponding variability amplitude being A = 2.9%–43.67%. The amplitude increases with IDV timescale. (2) The largest variation over a 1 day timescale is Δ R = 1.38 mag. (3) The IDV timescales suggest that the emission sizes are from 8.9 × 10 ¹³ cm to 6.20 × 10 ¹⁵ cm, and the magnetic field strengths are B = 0.18–0.79 G. (4) Period analysis results show three possible long-term periods, p = 3.04 ± 0.02 yr, p = 1.66 ± 0.06 yr, and p = 1.20 ± 0.03 yr in the optical light curve. We adopted the accretion disk models and the lighthouse models to period p = 3.04 ± 0.02 yr: in the accretion disk models, the binary black holes have masses M = 1.17 × 10 ⁹ M ⊙ ; in the lighthouse models, we used two boosted jet flux densities to fit the observational light curve. (5) WWZ analysis gives some short-period (high-frequency) signals associated with strong bursts (JD 2,454,302 and JD 2,454,521) with variable frequencies and lasting for the entire observation time span (11.3 yr).
