Speaker
Description
Accretion discs around supermassive black holes constitute extreme astrophysical environments in which ionized, radiatively efficient flows regulate mass transport and energy dissipation. Optical variability in active galactic nuclei (AGNs) therefore provides an important, though indirect, observational window into the dynamical behaviour of accretion flows on spatial and temporal scales inaccessible to direct imaging. In our recently published work, we conducted a comparative statistical analysis of long-term optical brightness and colour variability in two bolometric luminosity (LBol) – redshift (z) matched samples consisting of 2095 Narrow-line Seyfert 1 (NLSy1) galaxies and 2380 Broad-line Seyfert 1 (BLSy1) galaxies, using more than six years of quasi-simultaneous g- and r-band photometry from the Zwicky Transient Facility. Variability was quantified using flux variability, fractional variability, peak-to-peak amplitude variability, and ensemble structure function analysis, revealing systematically stronger variability in BLSy1 galaxies, with structure function amplitudes exceeding those of NLSy1s by a factor of 1.44 ± 0.06, while both subclasses exhibit statistically consistent structure function slopes. Our colour variability analysis indicated that a large fraction of sources show an apparent bluer-when-brighter (BWB) trend, observed in approximately 74 % of NLSy1 galaxies and 79 % of BLSy1 galaxies. However, we emphasize that these results are based on observed total flux light curves, which include an unavoidable contribution from the host galaxy. The presence of a relatively stable, predominantly red host component superposed on a variable, hotter nuclear component introduces significant ambiguity in interpreting the intrinsic variability behaviour of the accretion-powered nucleus. Consequently, the observed BWB trend cannot be robustly attributed to genuine spectral changes of the nuclear emission alone. Motivated by this limitation, we have developed an improved probabilistic flux variation gradient (PFVG) method to statistically disentangle host-galaxy and nuclear flux contributions in optical light curves. To assess the impact of host contamination on inferred variability properties, we apply this method to a sample of host-dominated BL Lac objects spanning the redshift range 0 ≤ z ≤ 1. We present preliminary results demonstrating how host subtraction alters the inferred brightness and colour variability behaviour, providing a clearer view of the intrinsic nuclear variability. This approach offers a promising pathway toward isolating accretion-flow–driven variability and placing more reliable observational constraints on the radiative and dynamical properties of accretion-disc plasma and hot AGN jet affects the variability, without over-interpreting host-contaminated observables.