Near Infrared Variability of obscured and unobscured X-ray selected AGN in the COSMOS field

TL;DR

This study analyzes near-infrared variability of X-ray selected AGN in the COSMOS field, revealing differences in variability between obscured and unobscured AGN, and how these relate to luminosity, wavelength, and spectral features.

Contribution

It provides the largest NIR variability dataset for AGN, offering new insights into the variability behavior across different AGN types and obscuration levels.

Findings

BL AGN show higher variability than NL AGN.

Variability amplitude anti-correlates with wavelength and weakly with luminosity.

Obscuration affects optical and X-ray variability classifications differently.

Abstract

We present our statistical study of near infrared (NIR) variability of X-ray selected Active Galactic Nuclei (AGN) in the COSMOS field, using UltraVISTA data. This is the largest sample of AGN light curves in YJHKs bands, making possible to have a global description of the nature of AGN for a large range of redshifts, and for different levels of obscuration. To characterize the variability properties of the sources we computed the Structure Function. Our results show that there is an anti-correlation between the Structure Function $A$ parameter (variability amplitude) and the wavelength of emission, and a weak anti-correlation between $A$ and the bolometric luminosity. We find that Broad Line (BL) AGN have a considerably larger fraction of variable sources than Narrow Line (NL) AGN, and that they have different distributions of the $A$ parameter. We find evidence that suggests that most of the low luminosity variable NL sources correspond to BL AGN, where the host galaxy could be damping the variability signal. For high luminosity variable NL, we propose that they can be examples of "True type II" AGN or BL AGN with limited spectral coverage which results in missing the Broad Line emission. We also find that the fraction of variable sources classified as unobscured in the X-ray is smaller than the fraction of variable sources unobscured in the optical range. We present evidence that this is related to the differences in the origin of the obscuration in the optical and X-ray regimes.