Probing jet-induced optical variability across timescales in radio-loud NLSy1 galaxies

TL;DR

This study investigates optical variability across timescales in radio-loud NLSy1 galaxies to determine if jet-driven short-term fluctuations extend into coherent long-term trends. Using ZTF $r$-band light curves, it combines $F_{ m var}$ and rest-frame SF analyses across γ-ray-detected jetted RL-NLSy1s, γ-ray-undetected jetted RL-NLSy1s, non-jetted RL-NLSy1s, and HPQs (52). The results show that jet-dominated sources exhibit long-term coherence with rising SFs, while disc-dominated systems display stochastic, DRW-like variability, revealing that temporal coherence (SF) is a more robust jet-diagnostic than amplitude alone. Colour–magnitude trends further separate populations, with RWB predominating in jet-dominated sources and BWB in disc-dominated ones, supporting a two-component jet–disc emission model. The findings have implications for interpreting AGN variability in upcoming time-domain surveys and highlight the SF as a key tool for jet–disc coupling studies in large-scale datasets like LSST.

Abstract

We investigate optical variability across multiple timescales in a sample of radio-loud narrow-line Seyfert~1 (RL-NLSy1) galaxies, including $γ$-ray detected, $γ$-ray undetected, and non-jetted systems along with a comparison set of highly polarised core-dominated quasars (HPQs). Using Zwicky Transient Facility light curves, we measure fractional variability ($F_{\rm var}$) and rest-frame structure functions (SFs) to test whether short-term jet-linked variability is reflected in long-term behaviour. $γ$-ray detected RL-NLSy1s and HPQs show steeply rising SFs, revealing strong long-term coherence despite modest $F_{\rm var}$, consistent with Doppler-boosted synchrotron emission from relativistic jets. Non-jetted RL-NLSy1s exhibit the highest $F_{\rm var}$ but plateauing SFs, indicative of stochastic, disc-driven fluctuations lacking long-term coherence. $γ$-ray undetected RL-NLSy1s show the lowest $F_{\rm var}$ and nearly flat SFs, consistent with weak or absent jet activity across all timescales. Colour-magnitude trends show that jet-dominated sources exhibit redder-when-brighter behaviour, whereas disc-dominated systems exhibit bluer-when-brighter trends. These results show that SF-derived temporal coherence, rather than variability amplitude alone, is a promising diagnostic of jet dominance and orientation, offering a framework for interpreting AGN variability in forthcoming time-domain surveys.

Figures (3)

• Figure 1: The ZTF r-band light curves for 4 representative sources studied in this work. The SDSS name for the source, along with their subclass, are noted on the top of each panel. The light curves of all the sources are available as supplementary material.
• Figure 2: Comparison of fractional variability amplitude ($F_{\mathrm{var}}$) in the $g$- and $r$-bands for different AGN classes. Blue circles represent jetted RL-NLSy1 sources without $\gamma$-ray detection, green triangles denote $\gamma$-ray detected jetted sources, and red squares correspond to non-jetted NLSy1, while pink diamonds represent the HPQs. The dotted line with 45$^{\circ}$ slope refering to $F_{var(g)}=F_{var(r)}$ is drawn to guide the eye.
• Figure 3: Left: Structure function (SF) comparison including the three NLSy1 subclasses of $\gamma$-ray detected RL-NLSy1s (blue pentagons), $\gamma$-ray undetected RL-NLSy1s (orange circles), and non-jetted NLSy1s (red triangles), and the highly polarised core-dominated quasars (HPQ) as an additional jet-dominated class (purple diamonds). Both the $\gamma$-RL-NLSy1 and HPQ groups show rising SFs with higher variability amplitude and long-term coherence, while the other two classes plateau after a certain duration. Right: This panel shows the ensembled SF in log–log space with best-fitting power-law slopes (SF $\propto \Delta t^\beta$). The best-fit slopes for different subclasses are listed in the second-to-last column of Table \ref{['tab:sample']}. Error bars in all plots are smaller than the symbol sizes and are therefore not visible.