Three years Swift-BAT Survey of AGN: reconciling theory and observations?

It is well accepted that unabsorbed as well as absorbed AGN are needed to explain the nature and the shape of the Cosmic X-ray background, even if the fraction of highly absorbed objects (dubbed Compton-thick sources) substantially still escapes detection. We derive and analyze the absorption distribution using a complete sample of AGN detected by Swift-BAT in the first three years of the survey. The fraction of Compton-thick AGN represents only 4.6% of the total AGN population detected by Swift-BAT. However, we show that once corrected for the bias against the detection of very absorbed sources the real intrinsic fraction of Compton-thick AGN is 20$^{+9}_{-6}$%. We proved for the first time (also in the BAT band) that the anti-correlation of the fraction of absorbed AGN and luminosity it tightly connected to the different behavior of the luminosity functions (XLFs) of absorbed and unabsorbed AGN. This points towards a difference between the two subsamples of objects with absorbed AGN being, on average, intrinsically less luminous than unobscured ones. Moreover the XLFs show that the fraction of obscured AGN might also decrease at very low luminosity. This can be successfully interpreted in the framework of a disk cloud outflow scenario as the disappearance of the obscuring region below a critical luminosity. Our results are discussed in the framework of population synthesis models and the origin of the Cosmic X-ray Background.

💡 Research Summary

This paper presents a comprehensive analysis of the absorption properties of active galactic nuclei (AGN) detected in the first three years of the Swift Burst Alert Telescope (BAT) all‑sky survey. The authors assembled a complete, flux‑limited sample of 199 non‑blazar AGN (|b| > 15°, S/N ≥ 5σ) covering the 15–195 keV band, and complemented the hard‑X‑ray data with soft‑X‑ray (0.3–10 keV) observations from Swift‑XRT and XMM‑Newton. By performing joint spectral fits for each source, they derived intrinsic column densities (N_H) and photon indices (Γ).

Key findings include:

1. Compton‑thick fraction – Only 4.6 % of the BAT‑detected AGN are classified as Compton‑thick (N_H ≥ 1.5 × 10²⁴ cm⁻²). However, the BAT band is strongly biased against such heavily obscured sources because >50 % of the 15–55 keV flux is absorbed for log N_H > 24.5. Using Monte‑Carlo simulations to correct for this bias, the authors infer an intrinsic Compton‑thick fraction of 20 % +9/‑6 % for the local AGN population, in line with the requirements of Cosmic X‑ray Background (CXB) synthesis models.

2. Luminosity‑dependence of obscuration – The fraction of absorbed AGN (N_H ≥ 10²² cm⁻²) declines sharply with increasing hard‑X‑ray luminosity. At L_X ≈ 10⁴³․⁵ erg s⁻¹ the absorbed fraction is ≈ 70 %, while at L_X ≈ 10⁴⁵ erg s⁻¹ it drops below 20 %. This anti‑correlation is directly linked to the distinct shapes of the X‑ray luminosity functions (XLFs) for absorbed and unabsorbed AGN: the absorbed XLF peaks at lower luminosities and falls off steeply at high L, whereas the unabsorbed XLF remains relatively flat. Consequently, absorbed AGN are on average intrinsically less luminous than their unobscured counterparts.

3. Low‑luminosity behavior – Intriguingly, the absorbed fraction appears to decrease again at the lowest luminosities (L_X < 10⁴²․⁵ erg s⁻¹). The authors interpret this as evidence for the disappearance of the obscuring torus below a critical accretion power, consistent with disk‑wind models where radiation‑driven outflows lift dusty clouds off the accretion disk only when the luminosity exceeds a threshold.

4. Spectral properties – The average photon index in the BAT band is ⟨Γ⟩ = 1.95 ± 0.02. Absorbed AGN have a slightly harder mean (⟨Γ⟩ = 1.92) than unabsorbed AGN (⟨Γ⟩ = 2.07). The modest difference is attributed to the presence of a stronger reflection component in absorbed sources, rather than an intrinsic hardening of the primary continuum.

5. Implications for CXB synthesis – By providing a bias‑corrected intrinsic Compton‑thick fraction and a quantitative absorbed‑fraction‑versus‑luminosity relation, the study offers stringent constraints for population‑synthesis models of the CXB. Adjustments to torus covering factors, opening angles, and the luminosity dependence of obscuration are required to reconcile model predictions with the observed BAT sample.

Overall, the paper demonstrates that the Swift‑BAT survey, when properly corrected for selection effects, reveals a substantial hidden AGN population and a clear luminosity‑driven evolution of obscuration. These results support a physical picture in which the dusty torus is a dynamic, radiation‑driven structure that can vanish at low accretion rates, thereby shaping both the observed AGN demographics and the integrated X‑ray background.