Interband Lag Variability in Active Galactic Nuclei across ZTF Data from Multiple Years

TL;DR

This study systematically analyzes interband lag variability in 94 AGN over multiple years, revealing significant seasonal changes and supporting the idea that AGN lags are inherently stochastic and influenced by variability baseline.

Contribution

First comprehensive analysis of interband lag variability in a large AGN sample using multi-year ZTF data, highlighting the stochastic nature of AGN lags.

Findings

Over half of the AGN show seasonal lag variations.

Short-term lags are smaller than long-term lags.

Lag variability aligns with theoretical models of stochastic AGN behavior.

Abstract

Interband lags in the optical continua of active galactic nuclei (AGN) have been observed over years of monitoring, yet their physical origins remain unclear. While variable interband lags have been found in a few individual AGN potentially, the temporal behavior of interband lags of an AGN sample has not been explored systematically. Here, we analyze the interband lags of 94 bright AGN at $z<0.8$, using both seasonal one-year and full six-year $gri$-band light curves from Zwicky Transient Facility Data Release 22. We find that more than half of 94 AGN show significant seasonal variations in the interband lags. Besides, the short-term lags, derived by averaging lags inferred from multiple seasonal light curves, are consistently smaller than the long-term lags, which are inferred from the full six-year light curves. This supports recent theoretical simulations where the lag measurement is sensitive to the baseline of light curve and the lag variation could be simply attributed to the inherent randomness of AGN variability. Our findings suggest that the interband lags of AGN are more complex and stochastic than commonly thought, and highlight the importance of high-precision time-domain surveys in uncovering the properties of AGN variability as well as the associated accretion physics.