Brightness and colour variability in NLSy1s

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 two $\mathrm{log(L_{Bol})}-z$ matched samples, consisting of 2095 Narrow-line Seyfert 1 (NLSy1) galaxies and a control sample consisting of 2380 Broad-line Seyfert 1 (BLSy1) galaxies. 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 NLSy1s. To probe colour variability, we utilize quasi-simultaneous light curves, with half-hour epoch differences between $\mathrm{g}$- and $\mathrm{r}$-band measurements. We evaluated the colour index using both flux-flux space analysis and linear regression in magnitude-magnitude space. We find that large majorities of these sources -- 74% of NLSy1 and 79% of BLSy1 -- exhibit a clear ``bluer-when-brighter'' (BWB) trend, although part of this effect may arise from contamination by the non-varying, predominantly redder flux of the host galaxy. Furthermore, rest-frame structure function analysis reveals that BLSy1 galaxies are $1.44 \pm 0.06$ times more variable than NLSy1s. These results can provide valuable insights into the variability properties of AGN subclasses and their underlying physical drivers.