Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. I. X-Ray Spectral and Long-term Variability Analyses

Junyao Li, Yongquan Xue, Mouyuan Sun, Teng Liu, Fabio Vito, William N. Brandt, Thomas M. Hughes, Guang Yang, Paolo Tozzi, Shifu Zhu, Xuechen Zheng, Bin Luo, Chien Ting Chen, Cristian Vignali, Roberto Gilli, Xinwen Shu

Research output: Contribution to journalArticle

Abstract

We present a detailed X-ray spectral analysis of 1152 active galactic nuclei (AGNs) selected in the Chandra Deep Fields (CDFs), in order to identify highly obscured AGNs (NH > 1023 cm-2). By fitting spectra with physical models, 436 (38%) sources with LX > 10 erg s-42 1 are confirmed to be highly obscured, including 102 Comptonthick (CT) candidates. We propose a new hardness ratio measure of the obscuration level that can be used to select highly obscured AGN candidates. The completeness and accuracy of applying this method to our AGNs are 88% and 80%, respectively. The observed log N?log S relation favors cosmic X-ray background models that predict moderate (i.e., between optimistic and pessimistic) CT number counts. Nineteen percent (6/31) of our highly obscured AGNs that have optical classifications are labeled as broad-line AGNs, suggesting that, at least for part of the AGN population, the heavy X-ray obscuration is largely a line-of-sight effect, i.e., some high column density clouds on various scales (but not necessarily a dust-enshrouded torus) along our sight line may obscure the compact X-ray emitter. After correcting for several observational biases, we obtain the intrinsic NH distribution and its evolution. The CT/highly obscured fraction is roughly 52% and is consistent with no evident redshift evolution. We also perform long-term (≈17 yr in the observed frame) variability analyses for 31 sources with the largest number of counts available. Among them, 17 sources show flux variabilities: 31% (5/17) are caused by the change of NH, 53% (9/17) are caused by the intrinsic luminosity variability, 6% (1/17) are driven by both effects, and 2 are not classified owing to large spectral fitting errors.

Original languageEnglish (US)
Article number5
JournalAstrophysical Journal
Volume877
Issue number1
DOIs
StatePublished - May 20 2019

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piercing
active galactic nuclei
x rays
occultation
erg
spectral analysis
hardness
dust
visual perception
completeness
line of sight
spectrum analysis
emitters
luminosity
effect

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Li, Junyao ; Xue, Yongquan ; Sun, Mouyuan ; Liu, Teng ; Vito, Fabio ; Brandt, William N. ; Hughes, Thomas M. ; Yang, Guang ; Tozzi, Paolo ; Zhu, Shifu ; Zheng, Xuechen ; Luo, Bin ; Chen, Chien Ting ; Vignali, Cristian ; Gilli, Roberto ; Shu, Xinwen. / Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. I. X-Ray Spectral and Long-term Variability Analyses. In: Astrophysical Journal. 2019 ; Vol. 877, No. 1.
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abstract = "We present a detailed X-ray spectral analysis of 1152 active galactic nuclei (AGNs) selected in the Chandra Deep Fields (CDFs), in order to identify highly obscured AGNs (NH > 1023 cm-2). By fitting spectra with physical models, 436 (38{\%}) sources with LX > 10 erg s-42 1 are confirmed to be highly obscured, including 102 Comptonthick (CT) candidates. We propose a new hardness ratio measure of the obscuration level that can be used to select highly obscured AGN candidates. The completeness and accuracy of applying this method to our AGNs are 88{\%} and 80{\%}, respectively. The observed log N?log S relation favors cosmic X-ray background models that predict moderate (i.e., between optimistic and pessimistic) CT number counts. Nineteen percent (6/31) of our highly obscured AGNs that have optical classifications are labeled as broad-line AGNs, suggesting that, at least for part of the AGN population, the heavy X-ray obscuration is largely a line-of-sight effect, i.e., some high column density clouds on various scales (but not necessarily a dust-enshrouded torus) along our sight line may obscure the compact X-ray emitter. After correcting for several observational biases, we obtain the intrinsic NH distribution and its evolution. The CT/highly obscured fraction is roughly 52{\%} and is consistent with no evident redshift evolution. We also perform long-term (≈17 yr in the observed frame) variability analyses for 31 sources with the largest number of counts available. Among them, 17 sources show flux variabilities: 31{\%} (5/17) are caused by the change of NH, 53{\%} (9/17) are caused by the intrinsic luminosity variability, 6{\%} (1/17) are driven by both effects, and 2 are not classified owing to large spectral fitting errors.",
author = "Junyao Li and Yongquan Xue and Mouyuan Sun and Teng Liu and Fabio Vito and Brandt, {William N.} and Hughes, {Thomas M.} and Guang Yang and Paolo Tozzi and Shifu Zhu and Xuechen Zheng and Bin Luo and Chen, {Chien Ting} and Cristian Vignali and Roberto Gilli and Xinwen Shu",
year = "2019",
month = "5",
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Li, J, Xue, Y, Sun, M, Liu, T, Vito, F, Brandt, WN, Hughes, TM, Yang, G, Tozzi, P, Zhu, S, Zheng, X, Luo, B, Chen, CT, Vignali, C, Gilli, R & Shu, X 2019, 'Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. I. X-Ray Spectral and Long-term Variability Analyses', Astrophysical Journal, vol. 877, no. 1, 5. https://doi.org/10.3847/1538-4357/ab184b

Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. I. X-Ray Spectral and Long-term Variability Analyses. / Li, Junyao; Xue, Yongquan; Sun, Mouyuan; Liu, Teng; Vito, Fabio; Brandt, William N.; Hughes, Thomas M.; Yang, Guang; Tozzi, Paolo; Zhu, Shifu; Zheng, Xuechen; Luo, Bin; Chen, Chien Ting; Vignali, Cristian; Gilli, Roberto; Shu, Xinwen.

In: Astrophysical Journal, Vol. 877, No. 1, 5, 20.05.2019.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. I. X-Ray Spectral and Long-term Variability Analyses

AU - Li, Junyao

AU - Xue, Yongquan

AU - Sun, Mouyuan

AU - Liu, Teng

AU - Vito, Fabio

AU - Brandt, William N.

AU - Hughes, Thomas M.

AU - Yang, Guang

AU - Tozzi, Paolo

AU - Zhu, Shifu

AU - Zheng, Xuechen

AU - Luo, Bin

AU - Chen, Chien Ting

AU - Vignali, Cristian

AU - Gilli, Roberto

AU - Shu, Xinwen

PY - 2019/5/20

Y1 - 2019/5/20

N2 - We present a detailed X-ray spectral analysis of 1152 active galactic nuclei (AGNs) selected in the Chandra Deep Fields (CDFs), in order to identify highly obscured AGNs (NH > 1023 cm-2). By fitting spectra with physical models, 436 (38%) sources with LX > 10 erg s-42 1 are confirmed to be highly obscured, including 102 Comptonthick (CT) candidates. We propose a new hardness ratio measure of the obscuration level that can be used to select highly obscured AGN candidates. The completeness and accuracy of applying this method to our AGNs are 88% and 80%, respectively. The observed log N?log S relation favors cosmic X-ray background models that predict moderate (i.e., between optimistic and pessimistic) CT number counts. Nineteen percent (6/31) of our highly obscured AGNs that have optical classifications are labeled as broad-line AGNs, suggesting that, at least for part of the AGN population, the heavy X-ray obscuration is largely a line-of-sight effect, i.e., some high column density clouds on various scales (but not necessarily a dust-enshrouded torus) along our sight line may obscure the compact X-ray emitter. After correcting for several observational biases, we obtain the intrinsic NH distribution and its evolution. The CT/highly obscured fraction is roughly 52% and is consistent with no evident redshift evolution. We also perform long-term (≈17 yr in the observed frame) variability analyses for 31 sources with the largest number of counts available. Among them, 17 sources show flux variabilities: 31% (5/17) are caused by the change of NH, 53% (9/17) are caused by the intrinsic luminosity variability, 6% (1/17) are driven by both effects, and 2 are not classified owing to large spectral fitting errors.

AB - We present a detailed X-ray spectral analysis of 1152 active galactic nuclei (AGNs) selected in the Chandra Deep Fields (CDFs), in order to identify highly obscured AGNs (NH > 1023 cm-2). By fitting spectra with physical models, 436 (38%) sources with LX > 10 erg s-42 1 are confirmed to be highly obscured, including 102 Comptonthick (CT) candidates. We propose a new hardness ratio measure of the obscuration level that can be used to select highly obscured AGN candidates. The completeness and accuracy of applying this method to our AGNs are 88% and 80%, respectively. The observed log N?log S relation favors cosmic X-ray background models that predict moderate (i.e., between optimistic and pessimistic) CT number counts. Nineteen percent (6/31) of our highly obscured AGNs that have optical classifications are labeled as broad-line AGNs, suggesting that, at least for part of the AGN population, the heavy X-ray obscuration is largely a line-of-sight effect, i.e., some high column density clouds on various scales (but not necessarily a dust-enshrouded torus) along our sight line may obscure the compact X-ray emitter. After correcting for several observational biases, we obtain the intrinsic NH distribution and its evolution. The CT/highly obscured fraction is roughly 52% and is consistent with no evident redshift evolution. We also perform long-term (≈17 yr in the observed frame) variability analyses for 31 sources with the largest number of counts available. Among them, 17 sources show flux variabilities: 31% (5/17) are caused by the change of NH, 53% (9/17) are caused by the intrinsic luminosity variability, 6% (1/17) are driven by both effects, and 2 are not classified owing to large spectral fitting errors.

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UR - http://www.scopus.com/inward/citedby.url?scp=85069695062&partnerID=8YFLogxK

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