Optimizing Long-term Variability of Active Galactic Nucleus Light Curves. I. A Case Study with ZTF Observations in the EGS Field

Abstract Optical variability is a key observational probe for studying the accretion dynamics and central engine physics of active galactic nuclei (AGNs). The quality and completeness of light curves have a direct impact on variability studies, particularly for faint AGNs and high-redshift AGNs. To improve the quality of long-term light curves for AGNs, we bin and stack multiepoch images balancing the image depths and temporal resolution. As a case study, we apply this method to Zwicky Transient Facility (ZTF) observations in the Extended Groth Strip (EGS) field, where the overlapping region covers an area of about 370 arcmin 2 and includes g -band and r -band data taken from 2018 March to 2024 December. The coadded images are approximately 2.0–2.5 mag deeper than the ZTF single-epoch images. With coadded images, we construct light curves for 73 AGNs in the EGS field. Compared to the traditional ZTF light curves, our light curves maintain consistent long-term variability trends but with higher photometric precision. Furthermore, this method can help detect AGNs with weak variability, which are missed from the traditional ZTF data due to the noisy light curves or being below the detection limit in ZTF’s single-epoch exposure. Among the 73 AGNs, the majority exhibit a bluer-when-brighter trend on long-term timescales, which is consistent with previous studies. This work offers insights for optimizing AGN light curves in both current and upcoming all-sky time-domain surveys.