Ding, N.N.DingLuo, B.B.LuoBrandt, W. N.W. N.BrandtPaolillo, MaurizioMaurizioPaolilloYang, G.G.YangLehmer, B. D.B. D.LehmerShemmer, O.O.ShemmerSchneider, D. P.D. P.SchneiderTOZZI, PaoloPaoloTOZZIXue, Y. Q.Y. Q.XueZheng, X. C.X. C.ZhengGu, Q. S.Q. S.GuKoekemoer, A. M.A. M.KoekemoerVignali, C.C.VignaliVito, F.F.VitoWang, J. X.J. X.Wang2020-12-282020-12-2820180004-637Xhttp://hdl.handle.net/20.500.12386/29246In deep X-ray surveys, active galactic nuclei (AGNs) with a broad range of luminosities have been identified. However, cosmologically distant low-luminosity AGN (LLAGN, {L}<SUB>{{X</SUB>}}≲ {10}<SUP>42</SUP> erg s<SUP>-1</SUP>) identification still poses a challenge because of significant contamination from host galaxies. Based on the 7 Ms Chandra Deep Field-South (CDF-S) survey, the longest-timescale (∼17 years) deep X-ray survey to date, we utilize an X-ray variability selection technique to search for LLAGNs that remain unidentified among the CDF-S X-ray sources. We find 13 variable sources from 110 unclassified CDF-S X-ray sources. Except for one source that could be an ultraluminous X-ray source, the variability of the remaining 12 sources is most likely due to accreting supermassive black holes. These 12 AGN candidates have low intrinsic X-ray luminosities, with a median value of 7 × 10<SUP>40</SUP> erg s<SUP>-1</SUP>. They are generally not heavily obscured, with an average effective power-law photon index of 1.8. The fraction of variable AGNs in the CDF-S is independent of X-ray luminosity and is only restricted by the total number of observed net counts, confirming previous findings that X-ray variability is a near-ubiquitous property of AGNs over a wide range of luminosities. There is an anticorrelation between X-ray luminosity and variability amplitude for high-luminosity AGNs, but as the luminosity drops to ≲10<SUP>42</SUP> erg s<SUP>-1</SUP>, the variability amplitude no longer appears dependent on the luminosity. The entire observed luminosity-variability trend can be roughly reproduced by an empirical AGN variability model based on a broken power-law power spectral density function.STAMPAenArticle10.3847/1538-4357/aaea602-s2.0-85057614512000451432300001https://arxiv.org/abs/1810.09465https://iopscience.iop.org/article/10.3847/1538-4357/aaea602018ApJ...868...88DFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation