Variability-selected Low-luminosity Active Galactic Nuclei Candidates in the 7 Ms Chandra Deep Field-South

N. Ding, B. Luo, W. N. Brandt, M. Paolillo, G. Yang, B. D. Lehmer, O. Shemmer, D. P. Schneider, P. Tozzi, Y. Q. Xue, X. C. Zheng, Q. S. Gu, A. M. Koekemoer, C. Vignali, F. Vito, J. X. Wang

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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, LX ≲ 10 42 erg s-1) identification still poses a challenge because of significant contamination from host galaxies. Based on the 7Ms 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 × 1040 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 ≲1042 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.

Original languageEnglish (US)
Article number88
JournalAstrophysical Journal
Volume868
Issue number2
DOIs
StatePublished - Dec 1 2018

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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