Brightness and colour variability in NLSy1s versus BLSy1s and QSOs using the ZTF survey

Abstract: The Zwicky Transient Facility (ZTF), with its extensive optical monitoring capabilities, has provided an unprecedented opportunity to study the long-term variability of active galactic nuclei (AGNs). In this work, we present a comparative analysis of optical colour and brightness variability for $\mathrm{log(L_{Bol})}-z$ matched samples, consisting of 2095 Narrow-line Seyfert 1 galaxies similar to Broad-line Seyfert 1s (NLSy1_A) and 538 NLSy1_B galaxies matched with quasars (QSOs). The corresponding respective control samples of consist of 2380 Broad-line Seyfert 1 (BLSy1) galaxies and 741 QSOs. Using over six years of r-band and g-band light curves from the ZTF Data Release 22 (DR22), we characterize flux variability, fractional flux variability, and amplitude of temporal variability for each source in the samples. Our results indicate that BLSy1 galaxies exhibit significantly stronger variability compared to NLSy1_As, and similarly, QSOs show more variability than do NLSy1_Bs. To probe colour variability, we utilize quasi-simultaneous light curves, with half-hour epoch differences between $g$- and $r$-band measurements where colour index was evaluated using linear regression in magnitude-magnitude space. We find that large majorities of these sources -- 96% of NLSy1_A, 95% of BLSy1, 94% of NLSy1_B, and 91% of QSOs -- exhibit a clear "bluer-when-brighter" (BWB) trend. Furthermore, rest-frame structure function analysis reveals that BLSy1 galaxies are $1.42\pm0.06$ times more variable than NLSy1_A, while QSOs are $1.41\pm0.01$ times more variable than NLSy1_B. These results can provide valuable insights into the variability properties of AGN subclasses and their underlying physical drivers.