Seeing through the dust: Unraveling near-infrared variability in type 2 active galactic nuclei

¹ Instituto de Física y Astronomía, Universidad de Valparaíso, Gran Bretaña 1111 Valparaíso, Chile
² Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes (TITANS), Valparaíso, Chile
³ Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100 Providencia, Santiago, Chile
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁵ Department of Physics & Astronomy, Bishop’s University, 2600 rue College, Sherbrooke, QC J1M 1Z7, Canada
⁶ Departamento de Astronomía, Universidad de Chile, Casilla 36D, Santiago, Chile
⁷ Cosmic Dawn Center (DAWN)
⁸ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 January 2026
Accepted: 14 April 2026

Abstract

Context. Near-infrared (NIR) variability studies of active galactic nuclei (AGNs) are still limited, as long-term multiepoch monitoring in the NIR is observationally challenging. The depth, wavelength coverage, and 14-year temporal baseline of UltraVISTA make it one of the few surveys capable of providing a detailed characterization of AGN variability in this regime.

Aims. We aim to quantify the NIR variability of known AGNs in the COSMOS field and to investigate the physical origin of variability in type 2 AGNs. In particular, we examine how NIR variability can help clarify the discrepancies between optical and X-ray classifications.

Methods. Using the 14-year multiepoch UltraVISTA DR6 dataset in the YJHK_s bands, we constructed calibrated NIR light curves and quantified their variability through a set of metrics. Active galactic nucleus-like stochastic variability was identified by modeling the light curves with a damped random walk (DRW) process.

Results. We find that ∼7–17% of the 533 type 2 AGNs are variable in the NIR, with variability fractions increasing toward K_s, where the dusty torus dominates the emission. Based on the wavelength dependence of the DRW variability amplitude, we classify variable type 2 AGNs into disk-dominated, torus-dominated, and highly obscured groups. About one third of the X-ray unobscured (XR I) type 2 AGNs are variable in the NIR, consistent with misclassified weak type 1 or “true type 2” AGNs. On the other hand, 21.4% (30/140) of the X-ray obscured (XR II) type 2 AGNs show detectable variability in the NIR, most of them only in H or K_s, consistent with obscuration of the bluer (accretion disk) bands. Type 2 AGNs without X-ray counterparts (165) show the smallest fraction (3.6%) of variable objects.

Conclusions. NIR variability provides an effective and independent diagnostic for confirming optical classifications and for identifying weak or misclassified type 1 AGNs in deep extragalactic surveys.