The Cosmic X‐Ray Background and the Population of the Most Heavily Obscured AGNs

Frontera, F.; Orlandini, M.; Landi, R.; Comastri, A.; Fiore, F.; Setti, G.; Amati, L.; Costa, E.; Masetti, N.; Palazzi, E.

doi:10.1086/519985

Cited by 92 publications

(113 citation statements)

References 47 publications

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“…Converting this log(N)−log(S ) into the 17−60 keV band, our results are in full agreement with those reported by Krivonos et al (2007) for the INTEGRAL survey. The integrated flux of this extragalactic sample is ∼1.4% of the cosmic X-ray background in the 14−150 keV range (Gruber et al 1999;Churazov et al 2007;Frontera et al 2007;Ajello et al 2008c).…”

Section: Discussionmentioning

confidence: 92%

The PalermoSwift-BAT hard X-ray catalogue

Cusumano¹,

Parola²,

Segreto³

et al. 2010

A&A

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Aims. We present the Palermo Swift-BAT hard X-ray catalogue obtained from the analysis of data acquired during the first 39 months of the Swift mission. Methods. We developed a dedicated software to perform the data reduction, mosaicking, and source detection of the BAT survey data. We analyzed the BAT dataset in three energy bands (14−150 keV, 14−30 keV, 14−70 keV), obtaining a list of 962 detections above a significance threshold of 4.8 standard deviations. The identification of the source counterparts was pursued using three strategies: cross-correlation with published hard X-ray catalogues, analysis of field observations of soft X-ray instruments, and cross-correlation with SIMBAD databases. Results. The survey covers 90% of the sky down to a flux limit of 2.5 × 10 −11 erg cm −2 s −1 and 50% of the sky down to a flux limit of 1.8 × 10 −11 erg cm −2 s −1 in the 14−150 keV band. We derived a catalogue of 754 identified sources, of which ∼69% are extragalactic, ∼27% are Galactic objects, and ∼4% are already known X-ray or gamma ray emitters, whose nature has yet to be determined. The integrated flux of the extragalactic sample is ∼1% of the cosmic X-ray background in the 14−150 keV range.

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Section: Discussionmentioning

confidence: 92%

The PalermoSwift-BAT hard X-ray catalogue

Cusumano¹,

Parola²,

Segreto³

et al. 2010

A&A

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“…A number of models have been developed to reconstruct the spectrum of the X-ray background (Comastri et al 1995;Gilli et al 2007;Treister et al 2009;Ballantyne et al 2011;Akylas et al 2012;Ueda et al 2014). All these models require a substantial number of Compton-thick AGN to reproduce the peak of the spectrum between 20 and 30 keV (Marshall et al 1980a;Gruber et al 1999;Revnivtsev et al 2003;Frontera et al 2007;Ajello et al 2008;Moretti et al 2009;Türler et al 2010). However, the exact number is still unconstrained with the various models predicting a fraction of Compton-thick AGN between 10 and 35% of the total AGN population.…”

Section: Introductionmentioning

confidence: 99%

Compton-thick AGN in the 70-monthSwift-BAT All-Sky Hard X-ray Survey: A Bayesian approach

Akylas

Georgantopoulos

Ranalli

et al. 2016

A&A

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The 70-month Swift/BAT catalogue provides a sensitive view of the extragalactic X-ray sky at hard energies (>10 keV) containing about 800 active galactic nuclei (AGN). We explore its content in heavily obscured, Compton-thick AGN by combining the BAT (14-195 keV) with the lower energy XRT (0.3-10 keV) data. We apply a Bayesian methodology using Markov chains to estimate the exact probability distribution of the column density for each source. We find 53 possible Compton-thick sources (probability range 3 -100%) translating to a ∼7% fraction of the AGN in our sample. We derive the first parametric luminosity function of Compton-thick AGN. The unabsorbed luminosity function can be represented by a double power law with a break at L ∼ 2 × 10 42 ergs s −1 in the 20-40 keV band. The Compton-thick AGN contribute ∼17% of the total AGN emissivity. We derive an accurate Compton-thick number count distribution taking into account the exact probability of a source being Compton-thick and the flux uncertainties. This number count distribution is critical for the calibration of the X-ray background synthesis models, i.e. for constraining the intrinsic fraction of Compton-thick AGN. We find that the number counts distribution in the 14-195 keV band agrees well with our models which adopt a low intrinsic fraction of Compton-thick AGN (∼ 12%) among the total AGN population and a reflected emission of ∼ 5%. In the extreme case of zero reflection, the number counts can be modelled with a fraction of at most 30% Compton-thick AGN of the total AGN population and no reflection. Moreover, we compare our X-ray background synthesis models with the number counts in the softer 2-10 keV band. This band is more sensitive to the reflected component and thus helps us to break the degeneracy between the fraction of Compton-thick AGN and the reflection emission. The number counts in the 2-10 keV band are well above the models which assume a 30% Compton-thick AGN fraction and zero reflection, while they are in better agreement with models assuming 12% Compton-thick fraction and 5% reflection. The only viable alternative for models invoking a high number of Compton-thick AGN is to assume evolution in their number with redshift. For example, in the zero reflection model the intrinsic fraction of Compton-thick AGN should rise from 30% at redshift z∼ 0 to about 50% at a redshift of z=1.1.

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“…The peak of the X-ray background at 20−30 keV (e.g. Frontera et al 2007;Churazov et al 2007;Moretti et al 2009) can be reproduced only by invoking a significant number of Compton-thick sources at moderate redshifts. However, the exact density of Comptonthick sources required by X-ray background synthesis models still remains an open issue (Gilli et al 2007;Sazonov et al 2008;Treister et al 2009a).…”

Section: Introductionmentioning

confidence: 99%

X-ray observations of highly obscuredτ_9.7μm > 1 sources: an efficient method for selecting Compton-thick AGN?

Georgantopoulos

Dasyra²,

Rovilos³

et al. 2011

A&A

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Observations with the IRS spectrograph onboard Spitzer have found many sources with very deep Si features at 9.7 μm, that have optical depths of τ > 1. Since it is believed that a few of these systems in the local Universe are associated with Compton-thick active galactic nuclei (hereafter AGN), we set out to investigate whether the presence of a strong Si absorption feature is a good indicator of a heavily obscured AGN. We compile X-ray spectroscopic observations available in the literature on the optically-thick (τ 9.7 μm > 1) sources from the 12 μm IRAS Seyfert sample. We find that the majority of the high-τ optically confirmed Seyferts (six out of nine) in the 12 μm sample are probably Compton-thick. Thus, we provide direct evidence of a connection between mid-IR optically-thick galaxies and Compton-thick AGN, with the success rate being close to 70% in the local Universe. This is at least comparable to, if not better than, other rates obtained with photometric information in the mid to far-IR, or even mid-IR to X-rays. However, this technique cannot provide complete Compton-thick AGN samples, i.e., there are many Compton-thick AGN that do not display significant Si absorption, with the most notable example being NGC 1068. After assessing the validity of the high 9.7 μm optical-depth technique in the local Universe, we attempt to construct a sample of candidate Compton-thick AGN at higher redshifts. We compile a sample of seven high-τ Spitzer sources in the Great Observatories Origins Deep Survey (GOODS) and five in the Spitzer First-Look Survey. All these have been selected to have no PAH features (EW 6.2 μm < 0.3 μm) to maximise the probability that they are bona-fide AGN. Six out of the seven GOODS sources have been detected in X-rays, while for the five FLS sources only X-ray flux upper limits are available. The high X-ray luminosities (L X > 10 42 erg s −1 ) of the detected GOODS sources corroborates that these are AGN. For FLS, ancillary optical spectroscopy reveals hidden nuclei in two more sources. SED fitting can support the presence of an AGN in the vast majority of sources. Owing to the limited photon statistics, we cannot derive useful constraints from X-ray spectroscopy on whether these sources are Compton-thick. However, the low L X /L 6 μm luminosity ratios, suggest that at least four out of the six detected sources in GOODS may be associated with Compton-thick AGN.

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The Cosmic X‐Ray Background and the Population of the Most Heavily Obscured AGNs

Cited by 92 publications

References 47 publications

The PalermoSwift-BAT hard X-ray catalogue

The PalermoSwift-BAT hard X-ray catalogue

Compton-thick AGN in the 70-monthSwift-BAT All-Sky Hard X-ray Survey: A Bayesian approach

X-ray observations of highly obscuredτ_9.7μm > 1 sources: an efficient method for selecting Compton-thick AGN?

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The Cosmic X‐Ray Background and the Population of the Most Heavily Obscured AGNs

Cited by 92 publications

References 47 publications

The PalermoSwift-BAT hard X-ray catalogue

The PalermoSwift-BAT hard X-ray catalogue

Compton-thick AGN in the 70-monthSwift-BAT All-Sky Hard X-ray Survey: A Bayesian approach

X-ray observations of highly obscuredτ9.7μm > 1 sources: an efficient method for selecting Compton-thick AGN?

Contact Info

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X-ray observations of highly obscuredτ_9.7μm > 1 sources: an efficient method for selecting Compton-thick AGN?