Elmer, E.Almaini, O.Merrifield, M.Hartley, W. G.Maltby, D. T.Botti, IsmaelLawrence, A.Hirst, P.2021-08-242021-08-242019Monthly Notices of the Royal Astronomical Society, 2019:1-11https://doi.org/10.1093/mnras/staa381http://hdl.handle.net/11447/4458We present the first attempt to select AGN using long-term NIR variability. By analysing the K-band light curves of all the galaxies in the UKIDSS Ultra Deep Survey, the deepest NIR survey over ∼ 1 sq degree, we have isolated 393 variable AGN candidates. A comparison to other selection techniques shows that only half of the variable sources are also selected using either deep Chandra X-ray imaging or IRAC colour selection, suggesting that using NIR variability can locate AGN that are missed by more standard selection techniques. In particular, we find that long-term NIR variability identifies AGN at low luminosities and in host galaxies with low stellar masses, many of which appear relatively X-ray quiet.enGalaxies: activeInfrared: galaxiesSurveysX-rays: galaxiesArticle