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

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