XMM-Newton Ultra Narrow Deep Field survey II: X-ray spectral analysis of the brightest AGN population

In this work, we present the results of a detailed X-ray spectral analysis of the brightest active galactic nuclei (AGNs) detected in the XMM–Newton 1.75 Ms Ultra Narrow Deep Field. We analysed 23 AGNs that have a luminosity range of ~ 10⁴² \!-\! 10⁴⁶\, erg\, s⁻¹ in the 2 \!-\! 10\, keV energy band, redshifts up to 2.66, and ~ 10\,000 X-ray photon counts in the 0.3 \!-\! 10\, keV energy band. Our analysis confirms the ‘Iwasawa–Taniguchi effect,’ an anticorrelation between the X-ray luminosity (Lₓ) and the Fe–kα equivalent width (EWFe) possibly associated with the decreasing of the torus covering factor as the AGN luminosity increases. We investigated the relationship among black hole mass (MBH), Lₓ, and X-ray variability, quantified by the Normalized Excess Variance (σ ²ᵣₘₛ). Our analysis suggest an anticorrelation in both MBH - σ ²ᵣₘₛ and Lₓ- σ ²ᵣₘₛ relations. The first is described as σ ²ᵣₘₛ ∝ M-0.26 ± 0.05BH, while the second presents a similar trend with σ ²ᵣₘₛ ∝ Lₓ-0.31 ± 0.04. These results support the idea that the luminosity–variability anticorrelation is a byproduct of an intrinsic relationship between the BH mass and the X-ray variability, through the size of the emitting region. Finally, we found a strong correlation among the Eddington ratio (λ Edd), the hard X-ray photon index (Γ), and the illumination factor log (A), which is related to the ratio between the number of Compton scattered photons and the number of seed photons. The log (λ Edd)-Γ -log (A) plane could arise naturally from the connection between the accretion flow and the hot corona.