Unlocking AGN Variability with Custom ZTF Photometry for High-Fidelity Light Curves and Robust Selection

Abstract

(Abridged)We explore the potential of optical variability selection methods to identify AGN, including those challenging to detect with conventional techniques. Using the unprecedented combination of depth, sky coverage, and cadence of the ZTF survey, we target even starlight-dominated AGN, known for their redder colours, weaker variability signals, and difficult nuclear photometry due to their resolved hosts. We perform aperture photometry on ZTF reference-subtracted images for 40 million sources across 8,000 deg^2, assemble light curves and classify objects employing an RF algorithm into 14 classes, including 341,938 candidate AGN. We compare variability metrics derived from our photometry to those obtained from ZTF Data Release light curves (DR11-psf), to assess the impact of our analysis. We find that the fraction of low-z quiescent galaxies exhibiting significant variability drops dramatically (from 98\% of the sample to 7\%) when replacing the DR11-psf light curves with our difference image, aperture photometry (DI-Ap) version. The overall number of variable low-z AGN remains high (99\% when using DR11-psf lightcurves, 83\% when using DI-Ap), however, implying that our photometry can detect the fainter variability in host dominated AGN. The classifier effectively distinguishes between AGN and other sources, demonstrating high recovery rates even for AGN in resolved nearby galaxies. AGN candidates in eROSITA's eFEDS field, detected in X-rays and bright enough for ZTF optical observations, were classified as AGN (79\%) and non-variable galaxies (20\%). These groups show a 2 dex difference in X-ray luminosity but not in X-ray flux. A significant fraction of X-ray AGN are optically too faint for ZTF, and conversely, a quarter of ZTF AGN in the eFEDS area lack X-ray detections, highlighting a wide range of X-ray-to-optical flux ratios in AGN.

Accepted for publication in A&A

References in corpus (29)

• The Sloan Digital Sky Survey: Technical Summary
• The Zwicky Transient Facility: Data Processing, Products, and Archive
• Exploring the Optical Transient Sky with the Palomar Transient Factory
• The SRG/eROSITA all-sky survey: First X-ray catalogues and data release of the western Galactic hemisphere
• The eROSITA Final Equatorial Depth Survey (eFEDS): The X-ray catalog
• The 5th edition of the Roma-BZCAT. A short presentation
• The Dark Energy Survey Data Release 2
• Reverberation mapping of Active Galactic Nuclei: from X-ray corona to dusty torus
• The SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Quasar Target Selection
• The Automatic Learning for the Rapid Classification of Events (ALeRCE) Alert Broker
• A Catalog of Quasar Properties from Sloan Digital Sky Survey Data Release 16
• A characteristic optical variability timescale in astrophysical accretion disks
• Selecting Quasars by their Intrinsic Variability
• Optimal Time-Series Selection of Quasars
• Alert Classification for the ALeRCE Broker System: The Light Curve Classifier
• A New Catalogue of Type 1 AGN and its Implication on the AGN Unified Model
• The eROSITA Final Equatorial-Depth Survey (eFEDS): Identification and characterization of the counterparts to the point-like sources
• The Extended Baryon Oscillation Spectroscopic Survey: Variability Selection and Quasar Luminosity Function
• A Morphological Classification Model to Identify Unresolved PanSTARRS1 Sources: Application in the ZTF Real-Time Pipeline
• The eROSITA Final Equatorial-Depth Survey (eFEDS): The AGN Catalogue and its X-ray Spectral Properties
• A Mexican Hat with holes: calculating low resolution power spectra from data with gaps
• Catalogues of Active Galactic Nuclei From Gaia and unWISE Data
• Variability-selected Active Galactic Nuclei in the VST-SUDARE/VOICE Survey of the COSMOS Field
• The QUEST-La Silla AGN Variability Survey: selection of AGN candidates through optical variability
• A Study of Quasar Selection in the Dark Energy Survey Supernova fields
• A random forest-based selection of optically variable AGN in the VST-COSMOS field
• Persistent and occasional: searching for the variable population of the ZTF/4MOST sky using ZTF data release 11
• Universality in the Random Walk Structure Function of Luminous Quasi-Stellar Objects
• The Success of Optical Variability in Uncovering AGNs in Low-stellar Mass Galaxies

Cited by in corpus (1)

• Mid-infrared Variability-based AGN Selection using the Multi-epoch Photometric Data from WISE