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

Abstract

ABSTRACT 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 $\sim 10^{42} {\!-\!} 10^{46}\, \rm {erg}\, \rm {s}^{-1}$ in the $2 {\!-\!} 10\, \rm {keV}$ energy band, redshifts up to 2.66, and $\sim 10\,000$ X-ray photon counts in the $0.3{ \!-\! }10\, \rm {keV}$ energy band. Our analysis confirms the ‘Iwasawa–Taniguchi effect,’ an anticorrelation between the X-ray luminosity ($L_x$) and the Fe–k$\alpha$ equivalent width (${\rm EW}_{\rm Fe}$) possibly associated with the decreasing of the torus covering factor as the AGN luminosity increases. We investigated the relationship among black hole mass ($M_{\rm BH}$), $L_x$, and X-ray variability, quantified by the Normalized Excess Variance ($\sigma ^2_{\rm rms}$). Our analysis suggest an anticorrelation in both $M_{\rm BH} - \sigma ^2_{\rm rms}$ and $L_x- \sigma ^2_{\rm rms}$ relations. The first is described as $\sigma ^2_{\rm rms} \propto M^{-0.26 \pm 0.05}_{\rm BH}$, while the second presents a similar trend with $\sigma ^2_{\rm rms} \propto L_{x}^{-0.31 \pm 0.04}$. 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 ($\lambda _{\rm Edd}$), the hard X-ray photon index ($\Gamma$), 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 $\log (\lambda _{\rm Edd})-\Gamma -\log (A)$ plane could arise naturally from the connection between the accretion flow and the hot corona.