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

TL;DR

This work addresses the challenge of obtaining high-quality, long-term AGN light curves from irregular ground-based observations by binning and stacking multi-epoch ZTF images in the EGS field. The authors implement a cadence-aware co-addition pipeline with a weighted, sky-subtracted scheme, calibrate photometry to a common zero-point, and extract PSF-based light curves for 73 AGNs. They find that the optimized light curves preserve long-term variability while achieving substantially lower photometric uncertainties, enabling detection of variability in fainter AGNs and revealing a prevalent blue-when-brighter trend on long timescales. The approach offers a practical path to improved AGN variability studies in current and forthcoming time-domain surveys, with direct relevance to faint and high-redshift systems and to planning for LSST/CSST-like data.

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 multi-epoch 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 March 2018 to December 2024. The co-added images are approximately 2.0 to 2.5 magnitudes deeper than the ZTF single-epoch images. With co-added 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 below the detection limit in ZTF's single-epoch exposure. Among the 73 AGNs, the majority exhibit a bluer-when-brighter (BWB) 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.

Figures (9)

• Figure 1: Schematic diagram of the overlapping region between ZTF and the EGS field. The left panel shows the coverage of ZTF's three CCD-quadrants with the EGS field, along with the number of observations in the $g$, $r$, and $i$ bands from March 2018 to December 2024. The right panel illustrates the overlap between the blue CCD-quadrant (field 792 rcid 5) and the EGS field, with an overlapping area (marked by a solid red line) of approximately $10.2 \, \text{arcmin} \times 37.5 \, \text{arcmin}$. Yellow "x" symbols represent the 73 AGNs extracted in this study. Axes are in degrees.
• Figure 2: Schematic diagram of the data processing workflow.
• Figure 3: Observation cadence of ZTF $g$-band (blue) and $r$-band (orange) exposures in field 792 rcid 5 from March 2018 to December 2024. The gray vertical lines represent the range of each bin, the red numbers indicate the labels of each bin, and the black numbers represent the number of exposures in the g-bang and $r$-band for each bin.
• Figure 4: A small 5.3 arcmin × 3.7 arcmin cutout centered at $\alpha = 214.35^{\circ } ,\delta = 52.56^{\circ }$ compares single-epoch (top) and co-added (bottom) images in two bands. All images are displayed using the same scale, contrast, and stretch.
• Figure 5: The FWHM versus limiting magnitude for $g$-band (blue, left panel) and $r$-band (orange, right panel). Vertical bars represent the limiting magnitude ranges for single-epoch data. In the $g$-band, single-epoch medians are marked by squares and co-added values by stars. In the $r$-band, single-epoch medians are marked by circles and co-added values by diamonds. Gray arrows connect single-epoch to co-added median points. Each data point is labeled with its corresponding sample bin (Bin 1--19).