Cosmic X‐ray surveys of active galactic nuclei: The synergy between X‐ray and infrared observations

Alexander, D. M.

doi:10.1002/asna.201713326

Cited by 2 publications

(1 citation statement)

References 60 publications

(64 reference statements)

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“…X-rays penetrate deep into material with high column densities and therefore are nearly unaffected by moderate obscuration (N H < 10 24 cm −2 ) (Brandt & Alexander 2015;Alexander 2017). Thus wide-field X-ray imaging constitutes the most common and efficient technique to identify AGN.…”

Section: Introductionmentioning

confidence: 99%

Robust identification of active galactic nuclei through HST optical variability in GOODS-S: comparison with the X-ray and mid-IR-selected samples★

Pouliasis

Georgantopoulos

Bonanos

et al. 2019

Monthly Notices of the Royal Astronomical Society

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Identifying Active Galactic Nuclei (AGNs) through their X-ray emission is efficient, but necessarily biased against X-ray-faint objects. We aim to characterize this bias by comparing X-ray-selected AGNs to the ones identified through optical variability and mid-IR colours. We present a catalogue of AGNs selected through optical variability using all publicly available z-band Hubble Space Telescope images in the GOODS-South field. For all objects in the catalogue, we compute X-ray upper limits or discuss detections in the deepest available ∼7 Ms Chandra Deep Field South images and present the Spitzer/IRAC mid-IR colours. For the variability study, we consider only sources observed over at least five epochs and over a time baseline of up to ten years. We adopt the elevated median absolute deviation as a variability indicator robust against individual outlier measurements and identify 113 variability-selected AGN candidates. Among these, 26 have an X-ray counterpart and lie within the conventional AGN area in the F X /F opt diagram. The candidates with X-ray upper limits are on average optically fainter, have higher redshifts compared to the X-ray detected ones and are consistent with low luminosity AGNs. Out of 41 variable optical sources with IR detections, 13 fulfill the IR AGN colour selection criteria. Our work emphasizes the importance of optical variability surveys for constructing complete samples of AGNs including the ones that remain undetected even by the deepest X-ray and IR surveys.

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Section: Introductionmentioning

confidence: 99%

Robust identification of active galactic nuclei through HST optical variability in GOODS-S: comparison with the X-ray and mid-IR-selected samples★

Pouliasis

Georgantopoulos

Bonanos

et al. 2019

Monthly Notices of the Royal Astronomical Society

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The NuSTAR Serendipitous Survey: Hunting for the Most Extreme Obscured AGN at >10 keV

Lansbury

Alexander

Aird

et al. 2017

ApJ

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We identify sources with extremely hard X-ray spectra (i.e., with photon indices of  G 0.6) in the 13deg 2 NuSTAR serendipitous survey, to search for the most highly obscured active galactic nuclei (AGNs) detected at >10 keV. Eight extreme NuSTAR sources are identified, and we use the NuSTAR data in combination with lower-energy X-ray observations (from Chandra, SwiftXRT, and XMM-Newton) to characterize the broadband (0.5-24 keV) X-ray spectra. We find that all of the extreme sources are highly obscured AGNs, including three robust Compton-thick (CT; >Ń 1.5 10 H 24 cm −2 ) AGNs at low redshift ( < z 0.1) and a likely CT AGN at higher redshift (z = 0.16). Most of the extreme sources would not have been identified as highly obscured based on the low-energy (<10 keV) X-ray coverage alone. The multiwavelength properties (e.g., optical spectra and X-ray-mid-IR luminosity ratios) provide further support for the eight sources being significantly obscured. Correcting for absorption, the intrinsic rest-frame 10-40keV luminosities of the extreme sources cover a broad range, from »5 10 42 to 10 45 erg s −1 . The estimated number counts of CT AGNs in the NuSTAR serendipitous survey are in broad agreement with model expectations based on previous X-ray surveys, except for the lowest redshifts ( < z 0.07), where we measure a high CT fraction of16 . For the small sample of CT AGNs, we find a high fraction of galaxy major mergers (50% ± 33%) compared to control samples of "normal" AGNs.

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Cosmic X‐ray surveys of active galactic nuclei: The synergy between X‐ray and infrared observations

Cited by 2 publications

References 60 publications

Robust identification of active galactic nuclei through HST optical variability in GOODS-S: comparison with the X-ray and mid-IR-selected samples★

Robust identification of active galactic nuclei through HST optical variability in GOODS-S: comparison with the X-ray and mid-IR-selected samples★

The NuSTAR Serendipitous Survey: Hunting for the Most Extreme Obscured AGN at >10 keV

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