2018/10/22 by N. Ding, B. Luo, W. N. Brandt +13
Physics and Astronomy · #Active galactic nucleus #Amplitude #Astrophysical Phenomena and Observations #Astrophysics and Cosmic Phenomena #Black hole (networking) #Chandra Deep Field South #Galaxies: Formation, Evolution, Phenomena #Galaxy #Luminosity #Supermassive black hole #astro-ph.GA #astro-ph.HE
paper · pdf · doi:10.3847/1538-4357/aaea60
18 pages, 11 figures, accepted for publication in ApJ
arxiv created 2018/10/22 · openalex created_date 2018/10/26 · openalex publication_date 2018/11/27 · arxiv updated 2018/12/05 · openalex updated_date 2026/08/05
Abstract In deep X-ray surveys, active galactic nuclei (AGNs) with a broad range of luminosities have been identified. However, cosmologically distant low-luminosity AGN (LLAGN, erg s −1 ) 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 40 erg s −1 . 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 42 erg s −1 , 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.